Preparing the block is a long and sometimes tedious process. Once all the checking andmachining processes are finished and the block has been completely cleaned of any debris, grease, and other contaminants, you are ready to begin building your short block.
Of course, the process of measuring, pre-fitting, and cleaning has only just begun. As mentioned earlier, actual engine assembly is only a small portion of the process of quality engine building. Now that the block is correct, you will also need to ensure that each component that goes into it is correct as well. The best place to begin is with the crankshaft.
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As you advance to higher-level classes, rules generally become less restrictive about what crank designs you can use. A stock crank has very limited usefulness in racing, especially if it is cast iron. Generally, you will only want to use a stock crank in classes with cheap claimer rules that prevent you from spending any real money on the engine build. The problem isn’t only increased horsepower, but the shock a grippy racing tire can impart upon the crank can cause problems as well. Every time the driver brakes to slow the car through the corners, the driveline creates significant tension on the crank. Also, a shock is sent through the driveline and forward to the crank each time the tires break and then regain traction.
There are several factors to consider in a quality racing crank. Almost all performance cranks are forged and hardened from 4340 steel. Ideally, a crank needs to be very stiff in order to resist flex caused by eight connecting rods pushing and pulling along its length (which can wipe out bearings if the crank is too flexible). The crank should not be so stiff that it becomes brittle and cracks or shatters .
Most lower-cost racing cranks hover around 50 pounds because that’s the lightest many rulebooks allow. A 50-lb crank can be forged without many lightening measures. Lightening holes cut only in the rod journals leave plenty of material in critical areas, meaning the crank will be quite strong. A crank can be lightened further by undercutting the counter-weights and gun-drilling some of the main journals. Somewhere around 44 to 45 pounds is the limit for a lightweight crank that will stand up to oval-track abuse. Billet cranks are also available, but they are very expensive and usually the choice of drag racers with engines capable of well over 1,000 hp.
A good way to free up some power is by reducing the size of the rod journals. This is done to lower bearing speed, which reduces friction. The more RPMs the engine turns, the greater the power savings. The standard Chevy rod journal size is 2.100 inches, but popular sizes for racing are also 2.00 and 1.889 inches. The 1.889 size is commonly referred to as a “Honda journal” because this is the stock size for Honda four-cylinder engines.
Be aware that running Honda journals can cut the useful lifespan of a crankshaft because the smaller journal size reduces the amount of overlap area between the rod journals and main journals. This arrangement also requires a specially designed crank to make the oil passages work correctly. It can be costly, but in many cases it’s a worthwhile investment.
In contrast to the rod journals, many engine builders prefer cranks with the larger main journals. Standard main journal size for the 350 is 2.45 inches. The Chevy Bow Tie block is available with both the “350 Main” size and the “400 Main,” which is 2.65 inches. You can even find aftermarket blocks with undersized 2.30-inch mains. A smaller main journal also helps reduce bearing speed, but the loss of overall strength is often not worth it.
A decade ago, a connecting rod failure was a common problem in racing. Today, these components are so well designed and manufactured that you rarely hear of a true rod failure. Quality racing rods have become so affordable that there is no reason to run a stock rod unless your rules require it.
The trick is to use the lightest rod that will withstand your projected HP. Powdered metal rods designed for sportsman-level performance applications are relatively new on the scene and boast surprisingly low prices for the budget-minded engine builder. Howards Racing Components is one company leading the charge in this area.
For more strength, use forged rods. On a very high-end build, you can go with a fully machined forged rod. This is a rod forged with extra material that is then cut away to the rod’s final dimensions in a CNC machine. This machining process cuts away any surface irregularities that can cause weaknesses in a standard forging. Regular forged rods, however, are so advanced that this is only necessary if you are pushing the absolute limits of your engine package.
Most rod designs fall into one of two categories, I-beams or H-beams. I-beam rods are the classic design you see with stock rods. A newer offshoot of that design is Carrillo’s Abeam connecting rod, which is essentially a narrowed I-beam. It is very light and since it requires fewer machining processes, it is also relatively inexpensive. A-beams are a great candidate for low- to mid-level engines of up to 450 to 500 hp. Hbeam rods are much more resistant to the twisting forces exerted upon them and, when manufactured correctly, are a stronger design.
The aftermarket selection of Chevy rods is practically endless, but most circle track builders stick with a few proven standards. The best rod lengths for small blocks vary, from the stock length of 5.700 inches to 6.00 inches and slightly longer. Bushed pin ends are popular to help prevent galling, and cap-screw fasteners, which thread into the body of the rod and eliminate the nut, are also widely accepted.
One trend that has fallen by the wayside in terms of popularity is the pin-oiling rod. This design uses a small oiling hole cut through the beam of the rod to provide pressurized oil to the wristpin. This was done to prevent galling the pin in dry sump engines. Using a combination of thicker, less flexible wristpins and diamond-hard coatings solves most of these problems. You may still be able to find pin-oiling rods, but they are usually a band-aid solution for bigger problems.
Finally, it’s often quite easy to find a good deal on a set of used rods from touring race teams or a highend race engine builder. Teams racing for big money will often discard a set of otherwise good rods after a certain number of engine cycles rather than risk a failure. These are usually high-quality pieces and many engine builders have benefited from getting these rods at a fraction of their original cost.
Do be very careful when exploring this route. You need to know as much as possible about the pieces you are considering. How many races (or laps) did these rods run? Was there any type of engine failure when these rods were in the engine? If the engine was run hot or experienced detonation, the rods can be slightly damaged in ways that are difficult to detect. At the very least, each rod should be Magnafluxed before installing them into your engine.
If you are a veteran of building stock motors, you will be glad to know that the age-old task of lightening and balancing a set of rods is essentially no longer necessary. Dorton says that almost all race rods arrive balanced to within a gram. If he comes across a set that isn’t balanced to his liking he simply sends them back. Grinding on a purposebuilt race rod isn’t wise, since they do not have balance pads on each end like a stock rod. A race rod is designed to be as light as possible and has zero extra material that can be ground away. Don’t even risk it.
Working with Rod/Stroke Ratios
The rod/stroke ratio is simply the length of the connecting rod (from center to center) divided by the stroke. For example, a stock 350 Chevrolet with a 5.7-inch rod and a 3.5-inch stroke has a rod/stroke ratio of 1.629. Many race motors squeeze a 6.0-inch rod into the same package, bumping the ratio up to 1.714. The longer ratio creates more “dwell time,” or a greater percentage of time the piston stays near TDC. This can be an advantage because more cylinder pressure is created by the extended piston dwell time, once the spark plug ignites the mixture.
Using the longest connecting rod possible also has a secondary advantage: By increasing the rod length, you can raise the pin height and reduce the piston’s compression height. The compression height is the distance between the center of the pin and the piston face. Although the piston is aluminum and the connecting rod is steel, lengthening the rod in order to reduce the compression distance will almost always reduce overall reciprocating weight by allowing a smaller, lighter piston.
Many engine builders believe a large rod/stroke ratio is helpful because it reduces piston side loading on the cylinder walls, but Dorton says this really isn’t much of an issue for stock car racing. There are limits, however, to how much rod/stroke ratio you want. If you increase the ratio too much, you can create so much dwell time that detonation becomes a problem. Combining increased dwell time with a very aggressive, high-lift cam can also create piston-to-valve clearance issues. The key is to find the best piston available with the shortest compression distance and then match it with a connecting rod that gets you the proper deck height.
Excellent attention to detail is critical when it comes to preparing your rods for installation in the engine. Achieving the correct amount of clearance between the bearings and the crank’s rod journals is critical to maximizing the life and performance of your engine. The same holds true for the pin end of the rod.
The popular rule of thumb when it comes to finding the correct bearing clearance is 0.001 of an inch between the bearing and the crank journal for every inch of journal diameter. Given that a small journal Chevy small-block is 2.00 inches, and the popular Honda journal size is just 1.889 inches, clearances between the crank journal and the rod bearings are incredibly tight. Dorton likes to hold the clearance between 0.0018 and 0.0022 inches.
The difficulty isn’t accurately measuring to such levels of precision, but knowing exactly where to measure. Most cranks, no matter the manufacturer, tend to have predictable variances in the crank journals. Even on a high-end crank, you can see as much as 0.0002 inches more thickness in the diameter of the journal at the oiling holes. This extra material doesn’t extend all the way around the journal; it is only at the oiling holes. So as long as the “bump” measures 0.0002 inches or less, the crank is acceptable.
Also, most crank journals will be thicker at both ends (at the beginning of the fillet radius), get thinner, and then thicken up again in the middle. Again, I am only talking about a couple tenths of one onethousandth of an inch, but if you are trying to hold your tolerances to plus or minus 0.001, it is something you should be aware of. When measuring journal size, find the largest diameter, which is usually in the middle (disregard the extra thickness at the oiling holes). If your clearances are too loose, the only damage will be a loss of some oil control; too tight might mean a spun bearing.
For this engine build, I am using 1.889-inch crank journals and Carrillo rods with the housing bores sized at 2.015 inches. That means the bearing thickness should be approximately 0.062 of an inch for each shell to give us 0.0019 of an inch of bearing clearance. Simply measuring the bearing shells with a micrometer won’t give you the results you are looking for. There is actually little predictability between bearing shell thickness and the final bearing ID diameter once the bearings are installed in the rods.
The only reliable way to determine that your bearing clearance will be correct is to install a bearing shell inside the rod and measure the ID with a dial bore gauge. When measuring the inside diameter of the rod journal, make sure to always measure the bearing diameter 90 degrees from the parting lines (the seam where the upper and lower bearing shells meet). Bearings are thinner near the parting lines to allow greater clearance because the big end of the rod will stretch when changing directions at TDC.
To get the true inside diameter of the bearing in the rod, measure in line with the beam of the rod. This method of measurement requires assembly of all eight rods with bearings, but it is definitely worth the effort. It also means that from this point forward you will need to keep each bearing with the specific connecting rod it has been fitted to. It’s a good idea to number each bearing and rod so they won’t get mixed up later on.
If the clearances are found to be either too tight or too loose with standard bearing shells, you can use a shell that’s manufactured and marked either 0.001 inches thicker or thinner than standard. (Each shell is thicker or thinner by 0.0005 inches, making the total change 0.001 of an inch.) Usually, a full 0.001 of an inch is too much, but you can mix and match shells to achieve the clearance you desire. If you need slightly more clearance you can use half of a standard shell and half of a shell that’s 0.001 of an inch under. The result is a bearing ID that is 0.0005 of an inch larger than standard, creating slightly more clearance. If you do go this route, it doesn’t really matter if the smaller shell is on the top or bottom; just try to be consistent across all of your rods. Because the bearings are thinner at the parting lines, a ridge between the thicker and thinner shells isn’t a problem.
Although there is no bearing, the pin end of the rod should be honed to the same clearance guideline. So for the 0.927-inch diameter pin I am using, the pin bore should be just over 0.928 of an inch, or between 0.001 and 0.0017 of an inch larger
Rod Bolt Stretch
One of the keys to making sure your rods survive under the worst racing conditions is to take care of the connecting hardware. It’s the two rod bolts that provide all the clamping pressure between the rod and the cap, and they are all that keeps the rod from flying apart under the tremendous inertial forces to which it is subjected. Bolts provide clamping pressure by stretching a small amount as they are twisted into place. Too much stretch and the bolt’s tensile strength will be reduced dramatically; too little and the clamping force will be inadequate. This is true for every bolt in every application, but when it comes to connecting rods, the safety zone is razor thin.
The best way to determine bolt stretch is to measure it with a gauge. Fortunately, it is easy to measure stretch with a dedicated rod-bolt stretch gauge. Most manufacturers recommend between 0.005 and 0.007 of an inch of stretch.
You don’t have to manually check the stretch of every single bolt. As you pre-fit the rods, check to see how much torque is required to stretch several of the bolts. Each time you fit the rod, the burnishing action between the threads in the bolt and the rod will reduce the amount of torque required to get the same amount of stretch. So each time you fit the rods, check the stretch for the first couple of bolts and note how much torque is required to get the proper amount. After that you can safely torque the rest of the bolts with relative certainty that they are properly stretched.
Many people believe that torque and stretch are the same thing, but they are not. The quality of the thread lubricant used dramatically affects torque. If you use a relatively poor lubricant, like thin motor oil, a given amount of torque may not stretch the bolt enough, and it won’t provide enough clamping load. On the other hand, a better lubricant, like a moly-based lube, will make the threads so slick that the same amount of torque may stretch the bolts to the point that they will eventually fail. If you are measuring stretch, it doesn’t matter what lubricant you use, because the proper amount of stretch will provide the correct amount of clamping load no matter what.
Like connecting rods, there is practically no reason to use a stock piston these days. For racing, where sustained high temperatures and detonation are common, there is really no reason to use any cast or hypereutectic piston at all, unless your rulebook requires it. There are forged aluminum pistons specifically designed for every class you can race, and the weight savings and increased durability are well worth the price.
Race-designed performance pistons can help you shed a lot of weight compared to stock pieces. One of the greatest advantages is the modern slipper skirt design. Compared to stock designs, it looks like it is barely large enough to keep the piston from flopping over inside the bore. The small skirt not only shaves precious weight, but it also reduces friction as it slides up and down inside the cylinder bore. It does allow more “slap” as it rocks inside the bore at BDC (Bottom Dead Center) and at TDC but noise is hardly an issue in racing.
Other big areas of weight savings you should look for in a quality piston are the wristpin size and the location of the pin towers in the piston. Moving the pin towers inboard toward the center of the piston reduces the length of the wristpin and cuts weight. The wristpin, however, must still maintain enough wall thickness (usually 0.165 of an inch on a 0.927-inch-diameter pin) to keep pin flex to a minimum.
Almost all stock car racing rules require a flat-top piston, so that is what I will concentrate on. Of course, even with this limitation, not all flattop pistons are built the same. Dorton says that dealing with a reputable manufacturer such as JE, Mahle, or Wiseco is key because of the technical knowledge they bring to the table.
No matter how much time is invested in R&D, no engine builder can be thoroughly knowledgeable in every facet of every part that goes into his race engines. Some of this you simply have to leave to the manufacturer and its experience. Manufacturers that are deeply involved in motorsports use what they learn at the highest levels, such as Nextel Cup, to advance the performance of products built for lower racing levels.
A perfect example is the valve pockets in the piston. Just a few years ago, it was standard practice for many engine builders to fly cut valve pockets in the piston tops in order to get the perfect amount of valve clearance for their specific engine package while maximizing compression. The problem is that when a piston manufacturer doesn’t know what the final valve pocket size will be, it has to add in extra material to the underside of the piston top. If the valve pocket the engine builder cuts isn’t very deep, this extra material is simply excess weight traveling up and down the cylinder bores at 7,000- plus rpm. Also, the heat generated by any machining processes performed on a piston after the ring lands are cut can cause the lands to distort and harm ring sealing.
Instead, Dorton works with the piston manufacturer to design a custom set of pistons with the correct valve pockets and the minimal amount of aluminum required to withstand the horsepower generated. It is a more expensive option, but one with a large payoff. On a highrpm racing engine with solid lifters, minimum piston-to-valve clearance should be 0.040 of an inch on the intake and 0.100 of an inch on the exhaust. If you suspect the team you are building the engine for will be experimenting with valve lash or may not lash the valves as often as they should, it is a good idea to open up the minimum clearance even more.
Forged aluminum pistons are almost always constructed from one of two alloys: 2618 or 4032. Pistons forged from 2618 alloy are generally stronger and more durable because of the reduced silicon content.These pistons require bore clearances between 0.008 and 0.010 of an inch because of the material’s greater expansion rates. You will normally only see 2618 alloy in very high-end pistons.
The 4032 aluminum has a higher silicon content. Because of the silicon content, it exhibits less thermal expansion than the 2618 alloy. It is strong enough for racing applications and probably more common in most pistons because it is less expensive. With 4032 pistons, Dorton recommends a cylinder bore clearance between 0.004 and 0.005 of an inch.
While there are many advantages to running a lightweight reciprocating assembly (crank, rods, pistons, and wristpins), it also requires extra diligence. If one piston/rod combination is either over or under weight relative to the others, if only by a small amount, the relative percentage is much greater than if you are running heavier, stock components. The problem is compounded because your race engine regularly sees RPM levels, for extended periods, that a stock motor cannot even reach. This means that properly balancing your rotating assembly is even more critical. An engine with even a small balance problem operating at extreme RPMs can create vibrations that will destroy bearings, shear flywheel bolts, or worse.
There are two methods of balancing an engine: externally and internally. An externally balanced engine uses weights on the damper and flywheel to bring the entire rotating assembly into balance. The problem with this, however, is if you ever replace either the damper or flywheel, the engine must be rebalanced. That’s why almost all race engines are internally balanced. Weight is either added to or removed from the crank’s counterweights at specific locations to bring the entire rotating assembly into balance. Balancing the crank requires expensive machinery, and there are several things you need to know to make sure it’s done correctly.
A crank is balanced when the weight of the rotating portion of the assembly (the crank, the rod bearings, and the big end of the rod) matches the weight of the reciprocating portion (the pistons, wristpins, locks, rings, and the small end of the rod) plus a couple grams thrown in to account for clinging oil. As already noted, the most common method for balancing the assembly is either to remove weight from the crank’s counterweights by cutting material away or by adding weight in the form of Mallory metal plugs. Mallory metal is a tungsten alloy that is several times denser, and therefore heavier, than the steel a crank is made from.
A 50 percent balance is when the bob weights used on the crank balancing machine are equal to 100 percent of the rotating weight and 50 percent of the reciprocating weight. (For more information on bob weights and how to calculate them, see the “Calculating Balance Weights” sidebar.) This will eliminate most engine vibrations but isn’t always effective for high-rpm applications.
Overbalancing is when you balance the crank with 100 percent of the rotating weight and more than 50 percent of the reciprocating weight. Typically, 51 or 52 percent of the reciprocating weight works best for short-track racing engines. You don’t want too much overbalance percentage because a circle track engine will sweep through a wide RPM band from the corner to the end of the straight. The goal is to find the balance percentage that most effectively eliminates vibrations in your racing RPM range.
The act of gapping rings is one of the most tedious aspects of building any engine, and most engine builders will tell you it’s their least favorite part. Still, it must be done. Most professional engine builders use a powered ring filer—Goodson sells a popular one. They are the quickest and most accurate option available. But they are a significant investment and likely are too costly for people who aren’t building engines for a living.
A crank-style ring filer is a more economical option and works well if you don’t mind working slowly. You must be careful to only make small cuts on the rings because you can’t put material back if you grind away too much. It may take a little practice to get a smooth, perpendicular cut on the ring.
Most performance packages use 1/16, 1/16, 3/16 ring sets. For all-out racing applications, consider moving to thinner rings. Generally, these were originally designed for smaller import motors and are measured in metric sizes. You can get good oil control with less friction using .043- inch first and second rings with a 3- mm oil ring. When using a moly top ring with a ductile iron second ring, you will want your rings to be gapped at 0.022 of an inch for the first ring and 0.016 of an inch for the second.
There really isn’t much power to be gained from going tighter, and these gaps protect you from scrubbing the rings against the cylinder walls in the event of overheating. Dorton says he has dyno tested top ring gaps as open as 0.032 of an inch and not seen much of a power loss on the dyno, although you probably will notice excess smoke from the tailpipes.
Dorton normally gaps oil rings to 0.010 of an inch. Like the top ring, you can live with as much as 0.025 of an inch gap without significant performance loss. And again, too loose is better than too tight. An oil ring gapped too tight means cylinder wall damage and potentially micro welding the ring to the piston’s ring land. As before, too loose just means a little less oil control.
Measure the ring gap by placing the ring in the bore it will wind up in upon final assembly and use a feeler gauge to determine the width of the gap. Before you can accurately measure ring gap with a feeler gauge you have to be able to make sure the ring is parallel to the deck. The easiest way to do this is to place the ring in the bore, near the top. Then, place another ring in the top or second ring land of a piston and insert the piston in the bore upside down. This will push the ring you are fitting down the cylinder bore. When the ring on the piston catches on the deck of the block, the face of the piston— and the ring inside the bore— will be parallel to the deck.
Once you have the correct gap, use a very fine file to remove any burrs created by the grinding process from the ends of the rings. Be very careful not to round off the corners of the rings, as this will allow combustion gasses to leak past. All you want to do is knock off any rough edges.
Installing the Crank
Written by Jeff Huneycutt and Posted with Permission of CarTechBooks
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