Depending upon your approach, the crankshaft of your small-block Chevy can be either one of the cheapest or the most expensive power products you’ll ever buy. Confused? Let me explain. Remember that nothing smaller than 350 ci is worth building. Unless cubes are restricted by class limits, you shouldn’t use a crank of less than 3.48-inch stroke (stock 350). Correctly utilized, a crank with more stroke is cheap power, but if it breaks it will be expensive. The key is obtaining cubes reliably at a price reflecting a limited budget. With that in mind, let’s look at our options.
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If regularly serviced, Chevys are slow to wear. If, when torn down your core unit’s crank is in good order, then, assuming a very tight budget, a crank journal is all that’s needed. If it’s less than near perfect, the crank journals can be reground an undersize but the cost effectiveness of such other options is marginal. But first it should be understood a measurably “out of round” or tapered journal in a high-output engine will survive full-throttle operation only a short time. If the crank looks good and is sized up correctly as per the on page 35, a regrind isn’t necessary.
It can be used confidently at power levels of 350 to 375 hp for early cranks to about 425 hp for one-piece rear main seal cranks. If the crank journals are out of tolerance, then you have some options. The cheapest is to have existing crank reground. This will cost about $100 to $125 when done properly—ground with as large a fillet radius between the journal and the crank cheeks as possible. In this respect, has advantages Many racers are concerned about undersized crank journals, reasoning that the crank must be weaker. But cranks don’t break through the journals. A correctly reground crank can offer the racer a stronger part than a standard-size crank.
Why? Because it can be ground leaving larger fillet radii in the corners between journals and crank cheeks. This is where 99 of 100 cranks break, and the bigger the radius, the smaller the chances of breakage. I’ve ground cranks with perfectly sized standard journals, but with what I considered too small a fillet radius, to 0.030 undersize for no other reason than to increase the fillet radius and hence reliability. For power levels above 300 hp and near continuous wide open throttle usage, check to see that there’s a well-defined radius in every journal corner. If there isn’t, your crank could break!
Now consider this: after possibl spending $125 on a crank regrind, what are you left with? Answer: Nothing more than a stock reground crank. Although it has acceptably sized journals, it is still of stock material and you may not know what sort of abuse it has already had and how much of its fatigue life is already used up.
The aftermarket in general and Scat in particular has much to offer the engine builder needing a rotating assembly at rebuilder prices. Using one of Scat’s stock replacement cranks means getting a new crank with larger fillet radii, and better material for about $50 to $75 more. Scat’s entry-level crank is cast steel (not cast iron) and almost as tough and strong as a factory forged crank, which needs to be born in mind before deciding to re-grind the stock crank.
Regardless of the crank’s regrind or new status, new bearings will be needed. This presents some cost-effective options we will get to later.
Considering the tight tolerances involved, the cost of grinding a crankshaft at about $100 or so is not that using quality production equipment. Be aware, however, that most shops are more interested in turning over quantities for regular street use. To avoid grinding-wheel changes, high-volume shops will often grind as many cranks as possible using the same wheel.
When you ask for as large a fillet radius as can be had, you’re seen not as a customer will help their bank balance bu as a potential pain in the neck. They may smile nicely and say what you want to hear, but you won’t, in most cases, get anything from whatever radius the wheel had on that day. If you must use a tock crank rather than a new-stock replacement crank, get your crank ground at a performance-oriented machine shop and expect to pay accordingly. Such shops will take the time to change or dress a wheel to do the job right. They appreciate that you’re knowledgeable if you’re aware that a 0.030-under crank with big radii is stronger than a 0.010 under with small fillet radii.
The method used to clean a crank prior to machine work can also influence its reliability under high-stress usage. The oven/shot-peening process should be your method of choice. If the shot used is kept in good order rather than allowed to fragment, the process goes part way toward emulating a regular $125 shot peening process.
Choosing your machinist carefully provides you with a crank that will serve as well or better than a new crank in all respects but one. It may break due to fatigue from previous long, hard service, of which you’re unaware. Let’s see how best to guard against such a situation.
A Little Money, a Lot More Torque and Horsepower
It’s a fact that 400-ci small-block Chevys wear out their bores far faster than their cranks. Built by Chevrolet in great numbers during the early and mid- 1970s, the bulk of these engines became due for overhaul in the early 1980s. Bore wear plus cooling problems (fixable if know how) did not endear these engines to many, so blocks with overly worn bores were scrapped by the thousands. This left an equally large number of good 3.75-inch-stroke 400 cranks around, along with a lot of people wondering what to do with them. The solution to that problem didn’t take long to figure out.
The 400 crank has a larger main bearing at 2.650 inches diameter than all other small-block Chevys. By grinding this down to the 2.450 diameter of the stock 350 crank, the 400 crank will pop right into a 350 block, adding some 28 cubes in the process.
Although the crank installation is total simplicity, installing the rods and pistons can bring about a number of minor problems. This, and the ways to avoid such when dealing with rods, is discussed later in this chapter.
Because it makes such a strong street motor out of a regular 350, the 383 conversion became the most popular hardcore engine-rebuilding hop up move of the 1980s. Demand nearly outstripped the supply of cheap 400 crank cores. By the early 1990s, this left the would-be stroker 350 builder with a dilemma. By this time, all 400 cranks were old and had seen a lot of use, so the fatigue life was at the wrong end of the graph.
Finding a decent low-mileage crank to grind was becoming increasingly difficult, and prices for a good piece were escalating. Since so many 400 cranks had been ground to fit in 350s, the used market had quite a few to offer. But buying one needs to be tempered by the thought that no one puts a stroker crank in Granny’s shopping car. A used 350 stroker crank will be just that—used and, likely as not, used up.
The beauty of a 3.75-inch stroke in a 350 is that it allows improvements to be made in other areas and, if not compromised (as is often the case) by too short a rod, proves to be the way to go when hopping up a 350 on a budget. If you have or can get a 400 crank that’s had an easy life and checks out crack free, you can grind it to suit your 350 block for about two and a half times more than the cost of a regular regrind. These days, the big problem with going the reground-400-crank route is that it’s becoming next to impossible to locate a good core, and they’re typically 50- to 75-percent more expensive than a 350 core. On top of this, you can never be sure of the crank’s history.
The only factor working in your favor is that 400 motors were never used in a performance application. If you’re going to build a shorter-cammed street torquer that will never see more than 5,500 rpm, a thoughtfully selected reground 400 crank is not likely to be a problem. However, the purpose of this book is to show how to build some real power, and a totally street-drivable, low-cost 383 can be built that really will put a reground stock 400 crank of unknown history to the test. Since building performance is the goal this raises the question as to why we would even consider the use of a reground 400 crank here when Scat has such a good cast-steel crank that can often be had on sale from discount parts houses for as little as $199! My advice is to be sure of what you’re doing; a crank breakage will be a huge negative impact to your finances.
Keeping that in mind, if you adopt the information within these pages, you could be looking at a 700-hp motor. I suggest only considering a reground crank if you’re targeting no more than 450 hp or operating on the slimmest of budgets. Keep in mind that if you spend money on a stout bottom end, you can always add more power-producing parts later when finances allow. For a little more than twice the cost of a regular regrind on a stock-stroke crank, you can step up to a considerably stronger, longer stroke, Scat cast-steel crank. Since most builds will be based on 350s, let’s look at crank performance and economics for this situation.
Using the cost of a regrind as a yardstick, if you have to acquire a stock cast 350 core, your crank cost will double that of regrind on a core already owned. If you have a 400 crank, converting it will run 250 percent of a regular regrind. If you have to buy a 400 core and have it reground to suit a 350 block, it will cost about 375 to 400 percent of the cost of regrinding a stock 350 crank. Always remember that at the end of the day you still have a used crank. If you buy a Scat cast-steel crank to build a 383, it will have zero time on it, and from the right source it will cost about 300 percent of the cost the economics are shaping up.
In 2009 dollars, the least expensive Scat crank, on sale, costs about $140 more than a stock crank regrind. This additional outlay in a 0.030-over block delivers 28 more cubic inches. If you build to the book (this book, that is), you are unlikely to see less than 1.1 ft-lbs of torque per cube with 1.2 to 1.25 being more common. This means the additional outlay of $140 delivers between 30 to 34 extra ft-lbs, with most of this increase showing up in the usable rev range. That works out to about $4.40 ft-lbs increase. To put that into perspective: not only do you get a new and stronger crank, but also the torque increase per ft-lbs is about 40 percent of the cost, of that delivered by a typical bolt-on blower installation. Assuming a moderate cam that allows peak power at 6,000 rpm, the extra inches provided by the crank are worth between 34 and 40 hp. If it’s a race application, the advantages of a longer-stroke crank in a 350 don’t stop there, especially if your budget means using production-type heads with a low set plug position.
Bigger displacement means the ability to achieve a higher compression ratio without compromising the chamber shape with excessively high piston crowns. Often it’s possible to achieve the desired ratio with cheaper and lighter flattop pistons. Having a better chamber in a 13:1 motor can alone account for a minimum of 10 additional horsepower. In terms of performance, the Scat 383 crank looks like a winner, but you may be asking: Just how good is a crank when it costs appreciably less than a new Chevy crank?
Back in the early 1990s I asked that question of a number of engine builders before I put one of these cranks into any of my motors. This was not because I thought the cranks might be of less strength than the stock cast crank, but because I’ve built low-cost engines that made four-figure horsepower numbers and broke stock parts. Scat has built a reputation based on quality over a period of more than 40 years, and the boss, Tom Lieb, isn’t about to throw that away.
Unlike many cheap cranks, every Scat crank goes through an inspection routine to check bearing finishes and sizes similar to that of the $2,500 Scat Cup Car race cranks. As a supplier for professional Top Fuel, Cup Car, and Trans Am among others, this company knows what it takes to make a crank live. I needed to know at what level I might expect failures. To this end I spoke to Bill Smith at Speedway Motors in Nebraska. At the time, Smith had used more than 3,000 of cranks in budget/claimer race engines at power levels up to 550 hp and RPM levels to about 8,000, and had not seen one failure attributable to a Scat cast-steel crank. At the time of this writing, I’ve built more than 700 hp with a -injected motor on a Scat cast crank. I’m sure it is approaching the limit but I’m still no closer to knowing exactly where the line should be drawn.
One thing about redesigning something that’s been around 40 years is that there’s plenty of opportunity to document any problems. When Scat went to the drawing board, the intent wasn’t to simply produce a copy crank, but to build a superior crank. This isn’t too difficult if the price tag is open ended, but only half the street rodders out there have a budget for forged-steel or billet stroker cranks. In the low budget category, this limited Scat’s production method to casting. Stock-cast Chevy cranks are made of Detroit’s wonder metal—cast iron—but in a seriously upgraded form.
Chevrolet engineers have their act together when it comes to building a tough, cost-effective crank. This meant anyone following in their footsteps had their work cut out for them if improvements were to be made. Keeping costs reasonable and strength up, Scat opted to drop into that narrow window between cast iron and forged steel—cast steel. After heat treating, Scat’s current 9000 series cranks are in excess of 100,000-psi yield with 6-percent elongation. Although a little more costly to cast, these cranks have a marked advantage in terms of strength and, equally important, are tougher and more ductile than the cast iron used for stock Chevy cranks.
The cast-iron alloy used for Chevy cranks produces, in a tensile test, some 3- percent elongation before failure. The Scat material, at 6 percent, doubles the stock Chevy figure and, although not proportional, is a good indication of just how much tougher it is.
There is, however, more to the Scat cast-steel crank than stronger material. It’s designed from the outset to use a significantly more desirable 5.7- or 6.0-inch connecting rod. When Chevrolet opted to go shorter on the 400 rod, the result was a lighter rod/piston assembly than the 350. The 400 crank’s counterweights reflect this because they’re only intended to balance this lighter weight. Since it’s a good move to install 5.7 or even the lighter variety of 6-inch rods, a crank designed from scratch should reflect these requirements. The Scat crank does this and a little more.
To appreciate the counterweight advantage of the Scat crank, you need to understand that a 400 crank is significantly short of counter-balance weight for the center four cylinders. As we turn up the RPM on our hopped-up stroker motors, the lack of sufficient counterweight mass in the center of the engine results in an increase in center main bearing loads. This is already the highest loaded main bearing and we could be making it worse. The external balance weights of the 400 serve to smooth things out but do little to reduce internal main bearing loads. Indeed external balance mass puts additional loads on the front and rear main bearings. An external balance crank damper at the front also puts a severe bending load on the crank snout. This may be okay for an engine turning a maximum 5,000 rpm, but with the heads I describe in Chapter 6 you could be building a motor that will see as much as 7,700 rpm. If an external balance damper is used at this RPM the crank better be strong enough for the job, otherwise the snout will eventually part company from the rest of the crank!
In essence, external balance is a convenient production-line Band-Aid fix for insufficient counterweight in and around the center of the crank. So, as RPM increases, there’s a tendency to bend the ends of the crank up and down. This, as has just been said, overloads the front, center, and rear main bearings at the expense of the two intermediate bearings. The problem is—and always has been—getting enough counterweight in the center of the crank. The only real cure for fixing a stock crank, and a costly one at that, is to install of heavy (Mallory) metal in the counterweights.
But, when starting from a blank sheet of paper it is something of a different story. In practice, there’s little room to maneuver with counterweight design within the confines of a conventional crank. Here Scat took the opportunity to include as much extra counterweight as possible, consistent with other production and usage constraints. Because the Scat crank has hollow big-end journals and slightly larger internal counter weighting, the main bearing loads are a little more evenly distributed than the stock Chevy crank. Also, if rods and pistons toward the lighter end of the scale are used, it balances internally, allowing you to use a stock 350 damper/ flywheel/flexplate.
So far, we’ve looked at the economies of Scat’s cranks for a 350 stroked to 383. Obviously there are instances where the stroke or displacement is a race-mandated requirement. Excluding for the moment short-stroke exotics, this means using a 3.48-stroke crank. Unfortunately, finding a good used factory forging gets harder every year. The good news is that Scat’s 3.48-stroke crank costs about 25-percent less than the 3.75-stroke.
Here’s my view on the use of a new Scat cast-steel crank as a replacement for the stock factory forged 350 crank: If the factory forging is new or close to it and the price is right, then that’s the right forging. Unfortunately, nearly new forged cranks don’t crop up that often. If I had to choose between a used forged crank of unknown history needing a regrind and a new Scat cast crank, I’d go for the Scat crank because it’s new and has proven reliability at the power levels we are mostly dealing with. It’s also one third the cost of a new Chevy forging.
If you’re building a 400, play it safe. The extra cubes of this larger displacement make it a little easier to break an old and possibly tired Chevy casting. The extra money needed to buy a new Scat 400 crank will look insignificant when the stocker breaks. If you were fortunate enough to pick up a later 360-degree one-piece rear main seal block, Scat has stock- and long strokes available for this application.
Buying a Forged or Billet Crank
While used forged and billet cranks are well within the budget constraints we’re dealing with, new ones, with one or two exceptions, are hardly so. To help avoid catastrophic failure, only buy a used forged or billet performance crank on the basis that it’s proven to be crack free. Remember, all race cranks have a hard life, and if it’s used up before you get it you’ll have nothing more than an expensive doorstop.
When looking for a used crank, you’re likely to see a variety of brands. I haven’t used every brand on the market. However, I have had experience with Callies, Crower, and Winberg for outright race cranks. All are top-notch manufacturers so if you find a used crack-free crank at a good price, it is more than likely a good deal. The least expensive new forgings with assured quality come from the factory in non-heat-treated 1053 steel (110,000-psi ultimate tensile and 100,000-psi yield) and, once again, include Scat in the much stronger 4140. Both run about the same price, but the Scat cranks have the advantage of being better steel and are nitride hardened. The nitriding process, generally common to all cranks, forms a hard, wear-resistant case while producing a softer more ductile core, allowing greater flexure before breakage. Unlike cheaper (and usually off-shore produced) brands of forged cranks, Scat’s budget forged cranks, as with its cast cranks, go through the same inspection given to its Winston Cup or Top Fuel cranks. These cranks are available in 3.48- to 4.00-inchstroke lengths with the 2.45 main journal sized for the 350. To use them in a 400 block, spacer main bearings can be used.
Another brand of crank I have successfully used is produced by K1 Technologies. I have used their 4-inch-stroke crank to build a 408-inch engine out of a 350. This requires the right combination of block, crank, and rods; otherwise, cutting rod clearance into the block will hit water.
Given the budget, I use Vanderbilt, Michigan/Cleavite 77, or Federal Mogul Competition bearings. Admittedly they are more expensive, but still aren’t that much money. If the output of the engine you’re building is likely to be more than 450 hp, I recommend you do likewise. However, we do need to save where possible. For power levels less than this, bearings costing much less will work fine. Some of the “cheap” bearings on the market are low-cost partly because of the quantities sold and partly because they’re cheaper to make. Certainly for power levels of 375 to maybe 400 hp, they’re more than acceptable. In this instance you might want to look at King bearings. I have used quite a few of these in engines up to about 480 hp and the results look fine.
It’s difficult to have a truly cost effective component strengthening/hardenin at the low-cost end of the scale. All those I mention here are proven and are worth the money, but you need to consider the practicality of spending more for a better part rather than having a less expensive part processed. Sometimes another $100 will get you a crank or rods that are heat-treated to the spec you are looking for and are of better material. The aforementioned economies apply big time to used stock parts but there are a few justifications for heat treating a new aftermarket cast crank such as produced by Scat. On occasions, I have had a budget for a little more than a cast steel crank, but not enough for an entry-level forged crank. What I have done here is to detail the entire cast crank with my die grinder. The crank shown above is just such an example. There is about 30 hours work in the grinding and detailing plus the time it took to do a coating job using the electric oven in the kitchen (wife not too impressed here!). Was all the effort worth it? Let’s put it this way—all the detail work on every component that came under scrutiny such as the block, crank, oil pump, etc., totaled about 25 hp. This meant I could use a cam some 10 degrees shorter and have a more streetable motor that still made the top-end output of an engine with no detailing and a bigger cam. A back-to-back test showed the reworked crank work to be good for 7 to 9 hp at 6,800 rpm. This work plus the heat treating meant the crank was about 220 percent of its original cost but that is still only two-thirds the cost of a forged crank.
For heat-treating, there are two processes I have used to good effect over the years. These are proven cost effective and will make whatever part treated that much better.
First is ion nitriding (Nitron, Inc.). This is a relatively common process used in the improvement of mostly ferrous (iron) parts and is popular in the aerospace industry.
It’s a lower-temperature process that results in a hard case, which itself is under a compressive load. This has the effect of significantly increasing fatigue resistance as well as delivering about a 5- percent increase in tensile strength. Wear on crank journals is markedly reduced and fatigue life considerably increased.
Another process, apparently exclusive to Hinterlighter in the Los Angeles area, is called “cold casing” and achieves similar results.
One process, now losing favor for environmental reasons is “tuftriding.” A few companies that have been able to afford the system-clean-up modifications are still doing this. The phone book should steer you to a convenient shop.
The last process is relatively common: shot peening. It must be done right to make the most of it. Find a company that does aircraft cranks and ask for your crank to be shot peened to MIL-S-13165C or any superseding spec.
Crank Vibration Dampers
First let me make it clear that the often-used term “harmonic balancer” is totally wrong. This item does not balance harmonics, nor can anything else for that matter. It is a “crank vibration damper” and, as the name would suggest, damps crank vibrations.
Crank dampers often are viewed as an inconvenient and weighty hindrance to performance when in reality the reverse is true. Having spent many hours damper/crank vibration power testing, I am able to pass on some little-appreciated speed tips concerning dampers.
To see where we’re going with this, you need to understand that a modern, computer-based cam profile is designed on the premise that the crank rotates in a smooth and uniform manner.
Vibrations transmitted from the crank to the cam can have a significant adverse effect on valvetrain dynamics. By installing a damper, the back and forth oscillation that the crank nose experiences is reduced and power is increased. Acceleration tests on a nominally 400-hp engine on my dyno were set up to simulate a 3-speed automatic car making a quarter-mile pass. These tests showed that, for an engine turning up to 6,000 rpm, the best stock damper was the largest and heaviest available. This heavy damper not only damped best but also produced lower ETs than a super-light aluminum hub that provided no damping. As RPM goes up from here, the picture changes and, because the energy involved equals 1/2 MV2, the optimal damper gets smaller.
To achieve the desired results, a damper must be in good condition. This means the rubber between the outer damping ring and the damper hub must be near perfect. A functionally used damper is normally inexpensive at the wrecking yard. If you have to buy new, you’ll find a stock factory one is pricey. Also, if you intend to race your car, be aware that many sanctioning bodies require the damper be made of steel as opposed to cast iron and that it has SFI certification. This means buying an aftermarket damper. Since the first edition of this book the whole damper industry, in the budget to mid-price or “sportsman” end of the market, has changed. To an extent this has simplified my job in terms of making recommendations. Here is how the things currently play out. For my outright race engines I use ATI or BHJ. The ATI damper is a purpose-built item that is definitely for an up market race application. This is what I have on my Cup Car motor. It does a great job and goes back to ATI every thousand miles for a total rebuild. The BHJ damper, though less fancy looking, is also a purpose-built damper and a little less money. I recommend it for the more costly of the budget motors we are dealing with here.
If I figure the price of all the parts is going to be in the $6,000 or so range and peak RPM up toward 8,000, then I usually opt for the BHJ unit. These are custom built for the job, taking into account all the parts used in your engine. It’s a quality damper, and you will pay the price for it. That comment might sound like I have just priced it out of your reach, but sales of the BHJ unit are on the rise and prices are on the way down; so check first to see if you can afford one. Last on the list are the dampers from Professional Products. These dampers are typically priced way less than stock and come with degree markings on them. For applications up to about 500 hp, they are approved by Scat for use with their cranks.
Now take note of this. A crank manufacturer is not going to recommend a poor damper as it impacts the reliability of their product. So the Scat approval here makes these dampers worth serious consideration. At the time of this writing I have had some five years’ experience with these Professional Products dampers and have had no problems. Dyno results look like they work at least as well as a more expensive stock damper. In this context we have to say that makes them at the very least, a super cost-effective replacement for the stock damper.
If it sounds like I am evading the issue of function or performance here, then that’s not the intent. The problem with elastomer dampers is that they are tuned to damp at a certain frequency. The stock dampers are tuned to damp whatever crank, rods, piston, clutch, and flywheel or torque converter combination the factory may have used in the original build. For an aftermarket damper, the manufacturer has to deal with the possibility of the end user having a range of parts. The torsional stiffness, and consequently the frequency of the vibrations, of an aftermarket heat treated forged crank can be quite a bit different from a cast crank. What this means is that your selection of damper size is not quite as simple as just looking through a catalog and arbitrarily choosing a damper. My recommendation here is to buy the damper with the crank and preferably the rotating assembly.This is a service Scat offers and who should know better what dampers will work with a combination than a company that deals with thousands of such a month. Failin that, call Professional Products and buy the one they recommend.
Do not overlook the fact that a damper must be SFI approved if it’s intended use is for a race engine. For the street Professional Products regular dampers are fine, but for race use you will need to get an SFI-certified damper.
Before parting company with the damper subject, I feel I need to better quantify the difference in power an effective damper can make. If you figure on a nominally 500-hp engine as a starting point, then going from a zero damper to having 85 percent of the torsionals damped out, the power difference is typically 12 to 15 hp as measured under accelerating conditions. Any damper that is even half-way effective will allow the engine to outperform a lightweight hub that has zero damping. With that I hope I have convinced you that a damper is a component that should never be overlooked.
There are many things I could say about connecting rods, but there isn’t space in this book. As a result, I’m telling you only what is needed here.
First let us consider the stock rod. When I wrote the first edition of this book, I went into great detail on the selection and reconditioning of stock rods. The advent of the stock replacement rod has made the use of reconditioned rods almost pointless. These days it costs almost as much to recondition a set of rods as it does to buy new and significantly better rods. This does not mean that you absolutely have to buy aftermarket rods to build a hopped up motor but it is close to that.
As far as the stock rods go there are two rod groups from which to make a choice. First there are the old-style forged steel rods that were used up until about 1987.
From there on the rods were a much more high-tech deal. Instead of the regular forging technique, GM adopted a process first pioneered by Porsche. The technique is known as Powder Metallurgy Forging (PMF). This process involves fine powder steel alloy being pressed into an accurately finished form of a rod but at a slight amount larger than the finished size. The pressed powder rod is then heated up to a high forging temperature, placed in a net-sized die, and stamped with a great deal of force. This causes all the particles to mash together to form a solid high-grade steel-alloy finished part. These rods look good and in practice they prove to be significantly better than the old forged steel rods. These rods are far better than any of the previous rods and do make a good choice for a rebuild up to about the 480-hp mark if you do not have the budget for a new set.
All the forgoing on powder metallurgy rods is to steer you away, as far as possible, from the early forging. If you have virtually no money then those early steel forged rods will work. Years of use when nothing else was available have proved that, but be aware that they finally end up being the Achilles heel of any builder capable of finding real horsepower, and after reading this book, that could be you!
Even when prepped out on a cost-no object basis, we still broke stock rods but at higher levels than stock. An engine with a broken rod is not a pretty sight, and it is not cheap to fix!
If you are forced to use stock rods of the older steel forged type, then you can do as I have done in the past. Here there are three scenarios: First, you may have access to a bin full of used rods at your local engine reconditioner. This is the best situation to have, as there is such a big variation in the rods. Second, you may be using what you have from an engine you already own and have stripped. Third, you may be buying all of your parts from a discount house.
If it’s the first instance, where you have a bin of rods to choose from, here is what you need to do to get eight decent stock rods. First select a set of eight rods and maybe a spare based on the following parameters:
- There are no rusty rod cores.
- Choose only those rods with the smallest balance pad as they are typically stronger.
- Only use cores that have a centrally placed pin bushing within the forging.
At this point you will have some decent forgings. If the rod journal bores check out OK and the bolts are in perfect order, you have a set of the most basic rods for a hopped up motor. The minimum op here, if full floating pistons are to be used, is to hone the pin bore to a 0.001-inch clearance fit on the wrist pin.
You now have a set of rods but to make them any good for even a half-way serious build they will need to be reconditioned and have ARP bolts installed. By the time you have paid out for this you will have about 75 to 80 percent of the cost of Scat’s least expensive rod. At this point you should consider this: the Scat rod is made from 4340 steel and delivers a 50-percent increase in strength over the stock rod. It also comes stock with ARP rod bolts, is bushed at the pin end, and can be had at 6inches long for a better rod/stroke ratio. You get all this with a rod that is still about stock weight, and for only about 25-percent-more outlay than a totally reworked stock-length forged rod.
Another option you might want to consider here is the stock powder metallurgy rods. These are sometimes on sale from one or other of the big parts houses at about the same price as a total overhaul of an earlier forged rod. If you are really cramped for cash and $30 to $40 makes a difference, these rods are a good option. Remember, however, they are a press fit at the pin end so you still may end up spending another $30 on having them honed to a full floating spec, which ultimately depends on the style of piston you end up using.
If you are staying with a stock-style piston, which has no means of pin retention, then the press fit pin has to be retained. However, most aftermarket pistons have circlip pin retention built into them so they can be used with full floating pins or press fit pins. It used to be the case that to convert to a fully floating pin end on the rod they had to be bored out to take a thin-wall bronze bushing. That is not the case any more as modern oils have made the bronze bushing a redundant element in this application. If you choose to go to the full float-style pin, have the rods honed to give a clearance of 0.0007 to 0.001 inch.
Let’s sum up where we are at this point. My feelings here are that, except for a regular re-build, it is just not worth even using those early rods. Why? Because at the end of the day you still have an early rod that was poorly made by current standards and can, even with the cheap speed equipment we have today, be overloaded and consequently unreliable. If we are considering the powder metallurgy rods, a whole different scenario applies. If you end up with a set of these rods in obviously good condition, then by all means use them within the limits of their capability. The best guidelines I can offer here shake down like this: If the rods are in perfect condition, have the pin end honed to convert to fully floating. If the spec you are building looks like it will be more than 480 hp and 6,750 rpm, then get used to the idea you will need to go with aftermarket rods.
Lightening and Rod Balancing on the Cheap
In the first edition of this book I went into great detail on lightening and balancing stock rods. I showed how it was possible to take a stock 610-gram rod and reduce its weight down to 560 grams and still have a stronger-than-stock rod. This all took a great deal of work and effort. With these early rods the forgings were all over the place and putting together a set of the best possible rods (based on stock forgings) was nearer a career than part of the job of building an engine. These days the only stock rods I use are the later, powder metallurgy rods. These are so consistent that, other than honing out the pin end, no work on them is required. Their weight from one rod to another is such that you don’t need to worry unduly about balance—it’s good enough as is. That cannot be said of the earlier rods. Because the PMF rods are so much better and the price of entry-level aftermarket rods has come down, re-working the early rods is a waste of time. But before finishing any discussion of stock rods, let’s consider what makes a rod break because it has a distinct bearing on the power levels a stock rod can deal with.
Even at the RPM we’re likely to turn a modest-budget engine, the inertial loads account for more rod-breaking stresses than gas loads. This means, as far as possible, making sure there’s no excess reciprocating component mass along for a free ride. The rule here is to keep piston, pin, and rod weight to a minimum consistent with the RPM involved. The PMF rod does not lend itself to lightening. The factory was able to make these to dimensions that are sufficiently close to what was needed. This means the PMF rod has metal where it is needed so there is little room for any metal removal without incurring a weakness. Bearing in mind that it is RPM rather than just HP that breaks the rods, we find that if power is gained by virtue of nitrous injection, then the rods can be used to significantly higher HP figures. I have used the stock PMF rods to about 575 hp in half a dozen budget nitrous engines and have not broken one in the period from about 1995 to the time of this writing (2008).
In terms of upgrading to a stronger and possibly heavier rod: though safer, it’s as bad to over-rod a performance engine as it is to under-rod it. The target is the lightest rod we reliably can get away with.
The first Chevy aftermarket rods were built with the pro racer in mind and were nothing short of expensive and as heavy as required for life in a high-output, endurance-race motor. Their added weight over stock also called for the best in cranks. At 700 to 750 grams, such rods are heavy enough to warrant, in most cases, expensive heavy-metal balance jobs. Sheer demand and quantity production since the late 1980s has brought about the production of lighter rods for applications less stringent than all-out race.
These rods fall into the “sportsman” category. They are usually made of the same 4340 material that steel rods are produced from for racing, but are lighter forgings and not machined all over. This is good for the budget-constrained engine builder because: A) we do not want to put any more reciprocating mass into the engine than necessary as it will our crank more; and, B) they save on heavy-metal-balance jobs, costing about half the price of a set of race rods.
Rods: The Final Choice
I have used rods from a number of companies over the last twelve years. Experience with such has brought me to a convenient conclusion: namely, that produces among the best bang for the buck for regular and stroker applications for non-nitrous power levels up to about 625 hp and 800 hp with nitrous. Since about 2004, that, with exception of a few K1 Technologies (another company I can recommend) engine builds, is all I use.
At this point I am going to make the assumption you will be using a Scat rod. The question is, now which one is right for the application at hand? The cheapest Scat rod, the 4340 standard I beam thru-bolt is available in both 5.7- and 6-inch length with bushed or press-fit pin. This rod uses the same bolt style as a stock rod, which is commonly referred to as a thru-bolt. This rod is good in a 350, but for a stroker application it needs the shoulder, and part of the shoulder end of the rod bolt grinding to clear the cam. Although this can be done I have seen evidence that over the long haul the rod bolt head starts to fail.
The bottom line is that in a 350 this rod is a great buy and will serve well, but for a stroker motor it is best to step up to, at least, Scat’s next offering. This is its 4340 Premium cap bolt rod. The “premium” moniker makes this sound like an expensive rod, but in actuality it’s only about $20 or so more than the basic rod. What you get here is a cap bolt (ARP) instead of a thru-bolt-style fastener. This means the rod shoulder can be ground the small amount required to clear the cam when a 3.75-stroker crank is used. This rod is also available press-fit or bushed at the pin end and in 5.7- or 6-inch center to center lengths. This rod is a really good choice for an entry-level 383.
Last on the list is my favorite. This is the Premium 7/16 rod. It is a totally different forging to the previously mentioned rods and comes with ARP 7/16 bolts. Although it’s barely any heavier than a stock rod, it is the strongest of the three rods under discussion. The best part of the deal, though, is that this rod is profiled at the shoulder to clear a stock-base circle cam in a stroker application.
Although it is about 50-percent-more money than Scat’s cheapest rod, it is the one to strongly consider if you intend to build a serious high output motor with either a stock or stroker crank. In addition to the normal 5.7- and 6-inch lengths, it is also available in 6.125- and 6.2-inch lengths.
Length: What to Use
The rod lengths available to you range from the short 5.56-inch center-to center length of the stock 400 rod to 6.2 inches of the Scat Premium 7/16-inch rod. So what should you go for?
In essence, this all hinges on achieving the best rod/stroke ratio the definition of which is shown above. Our main goal here is to minimize piston-to-skirt friction when the rod is at its greatest angle to the bore.
In practice this means utilizing as long a rod as possible within the confines of the engines we are dealing with here. A rod/stroke ratio of about 1.8 to 2:1 would be about optimum but with the block heights and stroke lengths we are using here, that is not a practicality. What this means for the most part is making the most of what will go in, and for the most part this will be a 6-inch center-to-center rod.
If you become involved with engine modifications to any real extent, sooner or later you will hear the comment that short rods make more low speed torque. The reason behind this is due to the fact that the shorter the rod is, the longer it takes to move any given distance either down or up the bore around bottom dead center. This in turn means that the piston is less distance up the bore when the intake valve closes, so it traps more charge above the piston and thus delivers more torque. This may be so, but offsetting this is the fact that the rod’s greater angularity pushes the piston into the bore with greater force on the power stroke. This in turn increases the piston-to-cylinder-wall friction. So it looks to be a trade-off here.
You will get plenty of engine builders to tell you that, in their opinion, the short rod is better for an engine that may never see the topside of 5,500 rpm. I am not in the business of opinions so I dyno tested a 383 with a 5.56-, 5.7-, and a 6-inch rod. This took three different rod and piston assemblies to do so and was not a cheap test by any means. The bottom line is that the 5.56 rod produced a lesser output right down to 2,200 rpm, which was as low as I could test. The 5.7 rod was better and the 6-inch rod the best. The differences were not huge by any means but what did show up was worth the effort to make the change. In round numbers, the 6-inch rod was about 7-hp up on the 5.56 and the 5.7 was about midway between the two. One aspect of importance that would not show on a power curve was the fact that the longer the rod, the mechanically quieter the engine ran. For that reason alone it’s worth using only a 6-inch rod in any build.
I have talked as though a 6-inch rod is “it” for all applications. Sometimes that is all that can be squeezed in. An instance here is a 4-inch stroker conversion into either a 350 or 400 block. About the shortest practical compression height for a piston is 1 inch. This means that with a 9.000 block height the longest rod that can be used with a 4-inch stroke is 6 inches. If you are building a 3.75 stroker then, a 6.125 Scat Premium 7/16 rod can be used with a piston having a 1-inch compression height.
Pistons: Type to Use
There are two choices of piston types for an engine rebuild: forged, or cast. For a performance engine, the most popular is the forged piston but that is slowly changing. The basic differences between these two types are related to both function and economies of cost. A forged piston is tougher and often has higher strength at temperature. A cast piston, which usually has a high or very high silicone content, may, depending on the alloy, be as strong but is harder and, as a result, experiences less skirt and ring groove wear. Also a cast piston expands less than a forged one so it can be run at closer clearances for quieter operation. Lastly, in most instances, they are cheaper to make. I intend to walk you through the aftermarket piston maze but before going there it is well worth looking at what your engine may have been equipped with from the factory.
The push to get ever better mileage has lead to many upgrades on the pistons that GM installs at the factory. Discarding these with no thought as to what benefits they may have is not a smart or cost-effective thing to do, unless you know for sure the engine will need new pistons.
In a bid to cut internal engine friction, develop a better gas seal, and last longer, GM revised a lot of the pistons used on post-1987 engines. The pistons you have in your motor should be checked out for the features shown above. My recommendation is that if you are on a really tight budget, then it is OK to stick with the stock factory high-performance piston, the stock crank, and powder metallurgy rods.
When choosing a piston style that fits your budget, you could be confronted with many different piston designs. What is ultimately chosen will depend on the power level being targeted. The requirements of a stock piston are that it is cheap and, at the power level it is intended to run, is reliable. Also, it must operate without any audible piston slap at any temperature from –30 degrees F to as hot as the engine gets during full throttle operation. Such pistons commonly have a slotted oil-control ring groove. Other than to act as a route for oil return from the oil control ring back to the pan, the purpose of this slot is to stop some of the heat that would otherwise expand the skirt from actually getting there. This allows a closer fit to the bore for a quieter operation. As good as this is for quiet operation, it can be the death knell for a performance engine if a piston of this style is used, unless such a piston has increased cross sections elsewhere to compensate. Some forged aftermarket pistons have a slotted heat-dam oil groove, but these pistons have a thicker cross-section joining the pin boss to the skirt in order to compensate.
If you are using a cast production type piston of this style because of cost constraints, then be aware that 5,500 rpm and about 375 hp is typically the limit for these pistons. If the pistons are overused, the top of the piston will part company from the skirt section, rendering it useless other than to give you more practice at fixing blown up engines.
For a performance piston, the usual means to return oil to the pan is via oil holes in the bottom of the oil groove. This does mean that more heat gets to the piston skirt, causing it to expand more. That in turn will call for an increase in skirt clearance and probably a little noise on start up. But that is a small price to pay for a much stronger piston.
Over the years I have used most brands of pistons, but in the last ten years or so I have settled on a few brands that meet my needs on a range of engines from really cheap to mid-price but still pretty serious race stuff. These brands are Speed Pro, KB, Ross, JE, Mahle, and Wiseco. The only Speed Pro I use are the cast hyper eutectic performance-oriented pistons.
The KBs I use are split 50-50 between cast and forged, while for companies other than Speed Pro, it’s all forged stuff. Let’s start with the cast hyper eutectic pistons first.
A question that you may well ask here is, “What does hyper eutectic mean?” Essentially, “hyper eutectic” means an alloy that contains more of an alloying element than will dissolve in the parent metal. The principle alloying element for an aluminum alloy intended for piston usage is silicon, and for that you could read “glass.” Up to about 12-percent silicon will dissolve in aluminum, and any excess above that disperses “as is” throughout the material in the form of crystals. The silicon increases the strength of the aluminum and also makes it considerably harder and to some extent more brittle. Although a hyper eutectic alloy lacks some of the toughness of a forged alloy, it has more than one redeeming factor that allows its use to higher power levels than might normally be expected. The first of these is that as a casting it is easier to put metal exactly where it is needed to hold out against the loads applied during operation. This means thicker cross sections at key points can be employed. The bottom line is: so long as it is not overloaded, this type of piston makes a good choice for many builds up to about 500 hp. I actually got to do some testing on the Speed Pro pistons prior to their introduction. I used them at 510 hp in a naturally aspirated engine for about a month on the dyno, and a nitrous engine with about 50 nitrous pulls on it at as much as 560 hp. Nothing broke.
As for KB’s pistons this company has really embraced the task of producing a near bulletproof hyper eutectic piston at a super low budget. KB’s Claimer race pistons may not be the prettiest you will ever see, but they are tough players and among the cheapest out there.
KB forged pistons also figure high on my list of cost-effective pistons. Although they are more money than the cast variety, they do allow a significantly greater power loading to be employed without the fear of failure. If we are talking in terms of nitrous, and by that I mean nitrous done right, then 850 to 900 hp is fine. One point you should check though, regardless of the piston used, is the pin clearance within the piston—this is especially important if the application is still using a press fit pin. To avoid a pin seizure when the nitrous is used before the combustion heat has brought the pin area up to full operating temperature (often the case at the drag strip), the piston pin bores need to be honed to a minimum of 0.001-inch clearance on the pin. Also when choosing a piston, especially for a stroker motor application, be sure to get the lightest your budget will allow. Ignore this and you could be paying much more for a balance job.
At a little more money, the Ross, and more recently, the German Mahle (Mah–lee) pistons figure strongly when I am watching weight with the intent of achieving internal balance on a stroker motor. There is not a lot I can say about the Ross pistons that I have not said before. Two decades of use in a couple of dozen street/strip engines up to 800+ hp and never a problem.
The Mahle pistons are a fairly new deal and I have experience of only a half dozen or so builds over a two-year period at the time of this writing. That said, I am impressed with the results and the degree of high tech delivered for the price paid. These pistons have a lighter ring package and come with an anti-friction coating on a strut-supported skirt. In short: they are tough, stiff, and light and the efforts at cutting friction during operation appear to have paid off.
For my higher-end pistons, and we are still talking only about $5,500 per motor for total parts expenditure, I use JE and Wiseco. At the time of this writing I am favoring Wiseco here because of the convenience of skirt coatings as a standard with the option of a metallic/ceramic thermal-barrier coating for the crowns. The coating is a type I have had good results with in the past, especially with nitrous motors. In fact, since using this coating (more than 10 years as of 2009) I have never come close to damaging a piston from thermal abuse (i.e., lots of nitrous). Another factor I like about the Wiseco piston is that the turnaround time is good and they are not that much more money for a custom piston. Buying a custom piston allows me to specify what I want for rings and this can prove a worthwhile power asset in itself.
Before you choose a piston, read- Chapter 6. The reason for this is that the piston constitutes half the combustion chamber form. It is, for a novice, all too easy to select a piston/cylinder head combination that is far less than optimal. Here are a few ground rules: First, choose the lightest piston possible consistent with budget and usage. Also, choose a piston requiring as little piston-to-cylinder- wall clearance as possible consistent with use and piston type. Note here that hyper-eutectic pistons usually need less clearance than forged, so, in the majority of cases, they will run quieter. After you have studied Chapter 6 select a piston that, in terms of CR, engine displacement, and cylinder head chamber volume, allows for a flat- or reverse-dome piston crown. If you must use a raised crown piston to get the CR up to where it’s needed, then limit the dome height to no more than 0.150 inch. If you intend to use full floating rods, be sure to get pistons with the appropriate method of pin retention; i.e., not press fit. In order of ease of fitting: the double lock snap rings are the easiest, followed by the round wire clip (Mahle), then the Spiraloc (which can be a pain until you get the knack of installing).
Size 5/64 rings are used on a lot of the cheaper pistons. This is a size that was common on stock pistons up until maybe the early 1970s. This size of ring is ridiculously cheap at about half of the cost of the cheapest 1/16-inch-wide ring. These wider rings are OK for regular use up to about 450+ hp, plus they work OK with nitrous. The down side is that without proper care they do not last as long—but there is a fix, as we shall soon see.
For those on a really tight budget, you can source “no label” 5/64-inch-wide rings intended for the high turnover rebuilding shops. These cheap 5/64 rings will work well for an engine not expected to turn over 6,000 rpm with a 6,500 max. If the build is a stroker motor, drop the rev limits for wide rings by 500 rpm. These wide rings are almost always made of a relatively soft ductile iron and will bed in quickly. They will also wear much more quickly unless suitable steps are taken in the upper cylinder lubrication department (see nearby Sidebar, “Minimizing Engine Wear”). A 1/16- to 3/16- inch ring package is better, especially if the top ring of the set is a moly plasma ring, but these will cost more.
If you are building a stroker motor, then the reduced width of the ring belt available for the rings almost dictates the use of 1/16 wide compression rings. If the budget is sufficient then it is a good idea to consider a Total Seal ring in the top groove. Since the first edition of this book
I have done a lot of back-to-back testing with this style of ring and believe me, they are about the best on the general market. If there is anything better, the F1 guys are keeping it under their hats!
There is not room here to detail all the tests I did over about a four-year period, but you can get a complete report on my website at Motor Tecmagazine.net.
Let’s start with the stock rods here. First, if you have the powder metallurgy rods you will not need to balance them because they are, for the budget builder, close enough as is. If you have the earlier rods, then life won’t be quite as easy. These are selected as a matched (roughly, that is) group and if you selected rods from a parts bin, figure they could be a ways off. Three ways to deal with this are open to you: First, you can dump the old style rods and find some of the new PM rods. It may cost a little more but at least you also get a decent rod in the bargain. Second, you can pay a shop to balance the rods you have, but be sure they warrant putting that money into them. Third, you can scrap the stock rod deal right there and get a set of Scat’s cheapest rods because they are already balanced by selection to within 2 grams. Pistons are also usually close enough to each other that any more-precise balancing is one step away from gilding the lily.
Let’s take a look at the crank now: what we need to know here is whether or not the crank is OK with the factory balance. Here is a way to check that out: First, you need to know the weights of the pin end and the rod journal end (big end) of the rods. If you have the rods I recommended, it is written on the box; if not, you will have to go to a shop with a rod-weighing fixture and have a rod weighed. At this point add 50 percent of the weight of the piston assembly and the pin end of the rod to the rotating weight (the rod journal end). Don’t forget to include the weight of the bearings and 2 grams for oil. Now double the number—if this works out to be between 1,850 and 1,900 grams, then you are OK because the stock 350 crank is balanced with a bob-weight of 1,870 to 1,900 grams. The most important part of balancing is that the piston assemblies are all the same and that each end of the rods are likewise—the same. Last, on the must-do list, is to have the balance factor (over or under) the same at each end of the crank.
The forgoing makes balancing seem like it is an operation without some open ended options. How so? Optimal balance occurs when the bob-weight used is 51 percent instead of the 50 percent normally used to represent the mass hanging on the crank’s rod journals. If the rod/piston assembly is lighter than stock, it follows that the counterweights will be too heavy by a small amount. This can be an advantage. In highly stressed engines the crank assembly is deliberately overbalanced so that, as the piston travels down the bore, the downward force of the gas pressure loading the main bearings is countered a little by the upward force of the heavier-than-normal counterweight. About 50 grams of overbalance can be used before there is any detectable loss of smoothness. Even if the difference between your crank’s theoretical and stock weight is greater than this, all it will mean is that the engine shakes a little more than we might like. Whatever it does, it is still likely to be less than a balanced four-cylinder engine!
Forgoing balancing does not mean that there will be more destructive loads present. What it does mean is that whatever forces are generated internally at one journal may not be fully countered by those at another. The result can cause the engine to shake due to the difference in these loads. The difference between RPM to- failure on a balanced engine and one that is not is very minimal.
So should you balance your rotating assembly or not? If the budget is there to do so, then go ahead. Be aware that there is more to be gained from a balance job on an engine that requires an external balance damper and flywheel. But let me remind you that, at the end of the day, the best deal is to get the entire rotating assembly (pistons, rods, damper, etc.) from your crank manufacturer already balanced. That is the best deal and your safest option in terms of smooth running.
Written by David Vizard and Posted with Permission of CarTechBooks