There is no magic involved in building great race motors. It’s not voodoo, and there really is very little mystery in building race-winning Chevrolet motors. What is required is the temperament of a perfectionist (I won’t call it anal, but it doesn’t hurt), an obsession with cleanliness, a modicum of organization, and a willingness to learn. Oh, and a handful of specialized tools, but that’s the easiest part. The goal of this book is to help you, at least a little bit, with all of those things.
This Tech Tip is From the Full Book “HOW TO BUILD KILLER BIG-BLOCK CHEVY ENGINES“. For a comprehensive guide on this entire subject you can visit this link:
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When it comes to stock car racing, in any class from Street Stock to NASCAR Nextel Cup, no engine package has enjoyed the success of Chevrolet’s venerable small block. Because of its success and popularity, it enjoys a broad base of support from both General Motors and aftermarket manufacturers. Performance parts for the small block are abundant, and because of competition from many different manufacturers, the cost of building a Chevy race motor is generally less than for any other brand.
The Chevy small block was first introduced in 1955 and remains in production today in the form of the LS-series motors. For the purposes of stock car racing, we are concerning ourselves only with the 350-ci design, produced from 1968 until 1996. These engines used a 4.00-inch bore with a 3.500-inch stroke. In 1968, General Motors increased the main journal size to 2.45 inches, and the rod journal size was standardized at 2.100 inches. These are commonly referred to as “large journal” blocks. The connecting rod length remained at 5.7 inches. Though stock car race motors can be constructed from the 305- and 400-inch motor designs, they are rare, so I won’t discuss them in this book.
There are a few additional variations among the 350-ci engines over the years. In 1986, Chevrolet replaced the two-piece rear main seal design with a one-piece seal in order to reduce oil leaks and subsequent warranty repairs. The onepiece rear main seal design required changes to both the block and the crank, as well as minor changes to the oil pan, but none of the changes affect power output.
The one-piece design has never really taken hold in racing circles because almost all performance cranks are manufactured for use with two-piece rear main seals. Stock twopiece rear main blocks, however, are becoming more difficult to find. Thankfully, it is relatively easy to convert a block from a one-piece rear main seal design to the older twopiece in order to work with many aftermarket performance crankshafts. No major machining operations must be performed on the block, but it may require drilling and tapping a couple of holes. In addition to the adapter plate, performing this conversion also requires a small fixture to simulate crankshaft location.
Another important feature for race engine builders is the four-bolt main block, introduced in 1968. It was used mainly in trucks and a few high-performance applications. Most people refer to it simply as a “truck block.” Other than the four-bolt mains, there are no other differences, meaning that the crankshafts are interchangeable. A four-bolt block is preferable over a block with two-bolt mains because the extra bolts add rigidity to the main caps as well as the structure of the block itself. In lower-horsepower Street Stock racing classes, an engine with two-bolt mains generally should not be a problem. If you are racing in highhorsepower classes, however, it is relatively easy for your machine shop to upgrade a two-bolt block to one that will accept four-bolt main caps.
No matter how well you assemble your race engine, you cannot overcome the hurdles created by a poorly prepared engine block. Find an engine machine shop capable of properly machining your block so that you will have the perfect foundation for your race engine.
The analogy that the machine work performed on a block before a build is like laying a solid foundation for a new house has been overused for years. It’s a tired comparison that I hesitate to mention except for the fact that it’s very true. It doesn’t matter how much thought you put into parts selection or how much effort you dedicate to assembly—it’s all wasted time if the block isn’t cast correctly and machined within the proper tolerances.
A quality block is critical because it houses and serves as the attachment point for every other engine component. Likewise, every moving part is designed to work best in a precise location relative to one or more of the block’s three most critical reference centerlines: The crankshaft centerline, the camshaft centerline, and each of the eight cylinder bore centerlines.
Of course, this doesn’t mean that you must have a PhD in geometry to build race engines, but you must understand how one part of the engine can affect many other areas in ways you may not expect. For example, if the camshaft centerline is in the wrong position, the lifter bores won’t allow the lifters to contact the cam lobes correctly, which can lead to a destroyed cam.
Because the purpose of this book is to help you build your own Chevrolet racing engines, the working assumption is that you will perform as many of the steps as you possibly can. The one area where most racers have to hand off the work is the machining that must be performed on the block and cylinder heads as well as the crankshaft, rods, and other parts of the engine. This is because the machinery required to perform these processes is very specialized and quite expensive. This equipment also requires a great deal of expertise to operate properly. Unless you are already building engines for a living, or have a best friend who does, it’s really a much better idea to hire a machine shop for this work.
The first step is to find an engine machine shop capable of doing the machine work you need and doing it well. If you don’t already have a good working relationship with an engine builder, the best place to gather good recommendations is the racetrack. Nobody has more experience with racing engines and engine builders than racers. If you aren’t already racing, visit a local track or two and ask around. Most racers will be more than happy to help anyone who is looking to go racing.
While you are gathering information, your main question should be “Who does good machine work?” But you can also use the opportunity to find out more. You may also wish to ask about prices, if the machinist is easy to deal with and if he normally turns around work in the time promised. After all, the best machine work in the world won’t do youmuch good if you can’t get it back in time to build your engine and make the race. When you do find a machinist you want to work with, don’t be afraid to discuss exactly what machining operations he recommends for your engine and the reason why each operation is necessary.
Finally, before hiring out work, you should be aware that most raceengine builders only guarantee complete engines will crank and make it to the first turn. If they are providing the machine work and you are completing the assembly, no one will even guarantee that much.
Before beginning assembly you should be able to check the work with your own measurements. We’ll show you how to do that in future chapters so that you can spot machining problems before the engine is assembled, and before it’s too late.
The first step is to select your engine block. If you race in more restrictive classes or in a spec engine class, you likely are very limited in which engine blocks you can use. If the rulebook is more forgiving, you have a few options available. In entry-level racing classes with lowerpower engines, the basic choice is a stock block. Chevy’s first-generation V-8 was produced in massive quantities between 1968 and ’96 and they are still relatively easy to find. You can purchase a stock 350 block as a used junkyard core or new from GM Goodwrench.
Stock blocks are available in a few variations. The most important difference is that you can find blocks with either two- or four-bolt main bearing caps. They generally cost a little more, but if you can afford it, a block with four-bolt mains is definitely the way to go. The two extra bolts help hold the caps more securely and provide stability for the crank and bearings, which becomes critical as the horsepower increases.
You should be aware that General Motors made a relatively significant alteration to the engine block in 1986 when it changed the rear main seal from a two-piece design to a singlepiece style. This change was made to reduce oil seepage from the back of the block. The one-piece seal required changes to the block and crank, among other things. Most racers prefer the two-piece design because it eases disassembly for rebuilds and, more importantly, most high-performance cranks available are built for that style engine. If you have a newer block, adapters for the two-piece seal are readily available.
In higher-horsepower applications, a thin-walled stock block becomes a liability. If your rules allow it and you are exceeding 500 horsepower, you will want to consider a block based on the classic Chevy small-block parameters but constructed with racing in mind. Chevrolet labels their high-performance pieces “Bow Tie” blocks, but aftermarket race blocks are available from other manufacturers as well.
Companies such as Dart and World Products have been producing their own Chevy-based block and cylinder head castings for years and boast fantastic quality. The differences between a stock block and a race block are normally tighter tolerances for machining and core shift, thicker cylinder walls, improved oil and water passages, and reinforced areas to address traditional weak spots in the stock block. They are also available in either cast iron or aluminum with iron cylinder sleeves.
The aftermarket race blocks also have additional features to differentiate them from Bow Tie blocks. For example, the sides of World Products’ Chevy block are “bulged” out at each of the cylinders. This was done to allow even larger bore diameters and still provide an adequate water jacket for sufficient cooling. Some blocks are also designed to allow stroker cranks without the need for grinding for clearance on the interior of the block. The block that best suits your needs will largely be determined by the engine’s estimated power, your rulebook, and your budget.
Identifying Core Shift
Core shift is a by-product of the casting process and is most common in older, mass-produced blocks.When blocks are cast, there is the possibility that the two sides of the mold aren’t lined up perfectly. This becomes more of a problem as the molds become older because the locating dowel pins can wear, allowing misalignment of the two halves. In low-volume Bow Tie or aftermarket high-performance blocks, this is rarely a problem.
If the camshaft housing bore isn’t centered in the cam boss on the front of the block, it doesn’t necessarily mean that the block is junk. But it does mean that further measurements should be made before using the block. Engine builders like Automotive Specialists have instruments that can be used to measure water jacket thickness throughout the block to make sure that core shift hasn’t weakened it in a critical area. A sonic tester will also help you determine whether the cylinder walls have been made too thin as a result of core shift.
Installing Freeze Plugs
Racing engines endure extreme vibration and stress that an over-theroad engine will never be subjected to. Because of that, it’s a good idea to pin the freeze plugs in place so you know they won’t blow out at an inopportune time and spill coolant all over the track. When this happens, your back tires will likely roll right though it, which often leads to a spin and wreck. Plus, it’s a relatively simple procedure.
Boring the Cam Tunnel
Decking the Block
Stress Relieving the Block
For years, many racers preferred to race blocks that had already seen use on the road. This is because the repeated heat cycles stress relieved or “settled” the material in the blocks and made them more stable. Shot-peening the metal does the same thing to an extent, but today there are even better methods to stress-relieve a new block.
Automotive Specialists uses a Meta-Lax machine on all new blocks before performing the final honing processes. A Meta-Lax machine is actually a vibratory table. The block is bolted in place and left to vibrate for a few minutes. It may not seem like much, but it does help settle new blocks so that the critical dimensions will remain stable after several races. A block can also be thermally stress relieved by essentially baking it, but this must be done before any machining processes have taken place.
Proper honing leaves a crosshatch pattern of tiny grooves in the cylinder bore. These grooves are important because they allow a thin film of oil to remain on the cylinder walls after the rings have moved past. This improves cylinder sealing as well as ring and cylinder bore life. The trick is getting the correct texture in the crosshatch to suit the rings you plan to use; too deep or too shallow and you will compromise both engine power and life.
Keith Dorton recommends leaving the bores 0.005 inches undersized after the boring process, and removing the remaining material by honing. At least three honing steps will be used to bring the bore to final diameter. Each step will use progressively finer stones so that the crosshatch will be perfect just as the cylinder reaches the correct size.
Dorton starts with a 525 or 518 stone and hones the cylinders until they are 0.003 inches undersized. Next, he moves to a 625 stone until it is 0.001 inches under. The last thousandth is finished up with an 820 stone. You cannot start with an 820 stone because it would take all day to cut the excess metal and would generate too much heat in the process. Finally, Dorton says he finishes off each cylinder with a cork-type stone, which works well with most types of rings.
Currently, engine builders for professional racing teams are experimenting with different honing processes, which may improve the process even further. Some, like diamond honing or hot honing (which heats the engine block before honing to simulate the expansion of a running engine) show promising results, but these processes are still too expensive to be common among hobby level racers.
The importance of cleanliness when building a race engine cannot be overstressed. You will be cleaning the block again once you receive it from the machine shop, but a good session in a hot tank goes a long way toward getting all the accumulated crud and metal shavings out of the block.
Written by Tony Huntimer and Posted with Permission of CarTechBooks