Any good-running small-block Chevy begins with the cylinder block. It’s the foundation from which all important engine functions initiate. Cylinder block selection and preparation has a substantial influence on an engine’s performance potential. Block selection is primarily dependent on the final application and desired displacement. While nonperformance applications can succeed with just about anything, high-performance engines should make use of selected cylinder blocks offering increased strength and reliability. For our purposes, we will only be discussing blocks that lend themselves to highperformance applications.
This Tech Tip is From the Full Book “SMALL-BLOCK CHEVY PERFORMANCE: 1955-1996“. For a comprehensive guide on this entire subject you can visit this link:
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Standard factory cylinder blocks have, for the most part, been available with three basic bore sizes and three main bearing sizes. For convenience they can be referred to as 283 blocks (37⁄8-inch bore, 2.3-inch mains); 302, 327, and 350 blocks (4-inch bore, 2.3- inch or 2.45-inch mains); and 400 blocks (41⁄8-inch bore, 2.65-inchmains). Some of these blocksmay be purchased fromperformance centers or located at engine shops and salvage yards, but the core supply of blocks—other than 350 and 305 blocks—is rapidly dwindling. Mail order aftermarket blocks have also become very popular.
Chevrolet Bow Tie cylinder blocks are built in the same basic configuration for all current high-performance applications. These blocks feature revised coolant passages and further improved rigidity, thanks to beefier main webs, siamesed bores, and greater overall metal content. They weigh up to 15 pounds more than a standard small-block casting.
Standard four-bolt replacement blocks are currently available as PN 10066034 (two-piece rear seal) or 10105123 (one-piece rear seal). They feature the thicker, fully machined main webs, but are cast with the standard alloy. Large-bore 41⁄8-inch blocks are also available, as shown in the cylinder block selection chart at the end of this chapter.
An important thing to remember with any of these new blocks is that they are “green” or unused castings, which may experience dimensional changes unless they are heat-treated or subjected to considerable “run-in” prior to final assembly. Production engines are subjected to repeated heating and cooling and normal operating stress during their service life, causing the blocks to take a permanent set that cannot be duplicated ina greenblock.Once a casting has set or stabilized, dimensional changes are minimized, and the engine canmake consistent power.
For certain racing or vintage buildups, used blocks will be required. For example, since all new factory blocks incorporate large-journal main bearings, if you need small crank saddles for a specific application, locating a suitable used block may be the only option.
The process of identifying and selecting the best possible seasoned block is often time consuming, but the results can be worth the effort. Casting numbers and engine identification numbers are used to identify blocks, but the accuracy of these numbers is often questionable. A Chevrolet source informed us that casting numbers only reflect general usage and that engine identification numbers are oftenmore specific. Engine IDs use a letter andnumber code to indicate the year, engine size, and type of transmission. Combined with the casting number, theywillusuallyprovideanaccurate indicationof theblockorigin.Manyracers check only the last three digits of the casting number, because they feel that certain castings are especially desirable.
Searching for a specific casting number is oneway of selecting a block, but successful engines have been built from virtually every casting number around. It ismore important to select a block that possesses all the correct physical characteristics. When you know what to look for, you can make an intelligent choice based on your actual requirements.
Inspecting a Used Block
Once you’ve selected a likely candidate, give it a preliminary inspection. If the block is dirty youwon’t be able to locate cracks, but you can still check certain features. See if the cam bore is centered on the front cam bore boss and if the lifter bores are centered on each individual lifter boss. If they are centered, there is probably only a little core shift and the block will probably have good cylinder walls. If the block looks good thus far, it’sworth having it hot-tanked for further inspection. Be sure to select a shop with a tank that uses rotating jets or a rotating table, instead of a simple vat. It takes agitation to clean out all the debris, and soaking in a vat just isn’t good enough.
After the block is thoroughly cleaned, carefully examine the deck surfaces for cracks and any deep gouges or scratches that can’t be removed with normal decking. Check the main bearing webs for signs of cracks or previous damage. The main bearing caps should fit snugly into the main cap guide slot. They should exhibit no evidence of side play, and the parting lines should match smoothly when the caps are torqued in place. Examine the bottom of each cap for scouring, an indication that the cap has been moving around during operation. Also, examine the outside of the block for possible freeze cracks. Throughout this examination, you should keep in mind the ultimate application of the engine. For a high- horsepower package, everything should be right on, but a moderate performance engine in a street car is not going to suffer appreciably if the block exhibits aminor amount of core shift.Make your selection intelligently and don’t pay for something you don’t need. The final step is to have the block pressure tested and inspected for cracks. If it passes this test, you’re ready to get serious with it.
Getting serious means more cleaning, deburring, more cleaning, block prepping, and more cleaning to get the block as near perfect as possible. If the block wasn’t completely stripped of hardware for the first cleaning, do it now. Everything that can be unbolted, knocked out, or pulled out must be removed. It should be mentioned here that an engine stand is an absolute prerequisite to proper block preparation and engine assembly. It makes every part of the engine easily accessible and the cost (generally about $75) is not prohibitive. There’s no point in trying to keep everything clean if you’re rolling the engine around on the garage floor.
These guidelines apply to both new and used cylinder blocks. As I mentioned earlier, new blocks are “green” and may experience some amount of dimensional shifting until they take a set. Fortunately, modern technology has brought new methods of stress relieving blocks and other components so that the dimensional stability of new components remains consistent. Blocks can be vibrated at specific frequencies on a special table designed to ease stress slowly out of the part. Dimensional stability doesn’t usually present a problem for basic engine applications, but racing powerplants often benefit from these hightech, stress-relieving processes.
Start cylinder block preparation by completely deburring the casting. You don’t have to get carried away here, but you should break all the sharp edges and radius all the corners. Remove casting flash that might break away under stress and smooth over areas where potential stress cracks might develop. Deburring can be done with a small drill motor and some sanding drums or even small files if you’re really on a budget. Larger sections of flashing will require using small grinding stones. A high-speed grinder makes the job really easy. Work carefully and be sure not to gouge any machined surfaces. Small files are excellent for deburring the edges of main bearing webs and other short, straight edges. Be sure to tackle every remote part of the block including the distributor pocket, camshaft cavity, timing cover cavity, and lifter gallery. It takes some time to properly deburr a block so don’t get impatient. The only place you can skip is the edge of the deck surfaces, since they will be rendered sharp again during the decking procedure.
Now you should spend some time running a tap or a thread chaser through each threaded hole in the block. If the hole is a blind hole and it carries some torque (e.g., main-cap bolts), it should be tapped with a bottoming tap to insure that threads reach all the way to the bottomof the hole. A bottoming tap can be made from an ordinary tap by grinding off the tip, which is normally used for starting the tap into an unthreaded hole. If any of the head-bolt holes are pulling out, wait until after the decking operation to repair them with Heli-Coil thread repair inserts.
A popular modification at this point is to tap the front three main oil galleries and install threaded pipe plugs. This procedure is unnecessary when the standard plugs are properly installed and staked in place, except in a NASCAR or long-distance application where vibration is severe. However, tapping the passages only takes a few minutes, so you may want to perform this modification even for street use. If you decide to give it a try, make sure you do not tap the gallery passages too deep. If the plugs screw in too far, they can restrict the oil feed passages to and from the front main journal.
There are two other small modifications that some experienced engine builders make in this area. One is to drill a tiny 0.030-inch hole in the face of the front cam bore boss, directly above the cam bore. It should be centered directly above the cam bore centerline and pass into the short oil passage leading from the main oil gallery to the front cam bearing. This hole will provide a constant source of lubrication for the cam drive gear thrust surface, reducing the chance of scoring the block or damaging the backside of the cam gear. It will also allow some air trapped in the oil galleries to escape, improving oil flow upon engine startup. Another modification is to drill a second 0.030- inch hole in the center of the middle oil gallery plug. This bleeds trapped air and ensures rapid front main bearing oiling. However, a second bleed can reduce idle oil pressure and may lead to front cover leaks on street engines, so restrict the second bleed to engines used for competition only. Finally, make sure that the bleed hole(s) is no larger than 0.030-inch or too much oil pressure will be lost.
You may also wish to give some consideration to the oil drain-back holes in the lifter valley. There are several holes located along the length of the valley and two large openings at the front and rear. The small holes should be drilled, tapped, and fitted with screw-in plugs even in a street application. In addition, for racing, the two large drain holes at the rear of the block should be gently rounded with a grinder and then tapped and plugged with pipe plugs. The left side will generally take a 3⁄4-inch pipe plug, with a 5⁄8-inch pipe plug on the right. This eliminates the large volume of oil draining onto the large rear crank throw spinning directly below. Many racers carefully open up the drain holes at the front of the valley so that oil will drain more easily. With the rear holes plugged, a certain amount of oil will build up in the valley during a run, but it will quickly drain when the engine is shut off. The extra oil in the valley has no discernible effect on the operation of the lifters and pushrods, and oil draining at the front of the block never touches the rapidly spinning crank. It works!
There is, of course, no reason to install any of the plugs until final assembly, when all the major machining operations have been completed and the block has been thoroughly cleaned.
Align boring is a highly touted procedure to straighten the alignment of the main bearing saddles. Align boring should be reserved for special applications where the block has become warped or some unique modification absolutely requires realignment of the crank bores. Most slight irregularities can be cleaned up with a simple align-hone operation, but even this is not generally necessary on the average used cylinder block. Main-saddle straightness can be checked with a machinist’s straightedge and with a simple check that you can perform yourself. By laying a straight crank (make sure you check crank run out with a dial indicator before you perform this procedure) into the block with properly clearanced bearings, you can check for binding by spinning the crank by hand. If it spins smoothly, the block is in great shape. Align boring is a valuable machining procedure when properly applied, but don’t do it if it isn’t necessary.
Align boring is a fairly straightforward procedure, but it requires carefulc and very accurate machine setup and operation to achieve a first-class job. The standard procedure is to take a slight cut off the bottom (the partingline face) of eachmain cap and to then mount them in place on the block. Then a carefully aligned boring bar is spun through the main bore, cutting new bearing saddles that are perfectly aligned, one to the other. It is important that aminimumamount ofmaterial be removed fromthe block saddles because the operationmoves the crank centerline closer to the cam, creating extra slack in the timing chain, which may subsequently cause valve-timing problems. It is also absolutely essential that the boring fixture is exactly aligned on the block, otherwise the crank bore may be higher or lower at one end or itmay not be centered sideto- side in the block.
In addition to correcting a warped block, align boring is required for the installation of specialty four-bolt caps or when bearing spacers are being used to install a small-journal crank in a big-journal block (sometimes desirable to achieve unique bore/stroke combinations). Special four-bolt main caps can be added to any small-block case. They are available from many sources but all come with rough-cut bearing saddles. When a block is modified to accept such caps, it is essential that the main bores are accurately align bored.
Special extra thick main bearings are also available to fit small-bearing cranks into large-journal blocks without align boring. However, these bearings are expensive. Therefore, some engine builders prefer to use spacers for installing standard smalljournal bearings.
Deckingisaprocedurewherebythe deck surfaces aremachined perpendicular (90 degrees) to the centerlines of the cylinder bores and equidistant from—and dead parallel to—the centerline of the crankshaft. This establishes the distance fromthe deck to the crank centerline, an important dimension that must be carefully controlled to gain the desired piston-to-cylinderhead clearance.
Ordinary street engines rarely receive this treatment, but there is some power to be gained fromequalizing compression and cylinder volume through decking. When teamed with the use of assorted head gasket thicknesses, deck height and piston-tocylinder- head clearance can be closely controlled. The most common procedure is parallel decking, which involves removing the minimum amount required to get both deck surfaces parallel to the crank centerline. Many engine builders use parallel decking as a starting point fromwhich they can establish desired piston-deck heights through additional block machining or piston-top machining. (Remember that deck height is the distance from the piston flat to the deck surface of the block. It is not the clearance between the cylinder head and the top of the piston. True piston-tohead clearance is established with a combination of head gasket thickness anddeckheight.) If you’re rebuilding a usedengine, itmay behelpful tocheck existing deck heights before the engine is disassembled. By determining where the deck surface is in relation to the pistons, youmay be able to avoid decking if things are close. As a last resort, some amount of piston top machining may be necessary to achieve the desired clearances.
Many engine builders like to push everything to the absolute limit, and with that in mind, a good rule of thumb for small-block Chevy engines is to maintain a minimum of 0.035- inch piston-to-head clearance with steel rods and 0.060-inch with aluminum rods. How you arrive at these figures is not particularly important except in all-out, high-compression racing engines where you may be concerned with the amount of quench in the combustion chamber.
The importance of straight, perfectly round cylinder bores cannot be overstated. You can’t make power if you don’t seal the cylinders, and you can’t seal the cylinders without precision machining. It has become commonplace to simulatenormal operating stress on the block during all boring and honing operations, the theory being that the cylinder needs to be round while the engine is running, and every effort should be made to duplicate normal operating conditions. Installing and torquing all the main caps simulates the stress imparted to the cylinders. Some engine builders also feel that a dummy oil pump should be torqued in place on the rear main cap to truly simulate distortion of the rear cylinders and the rear main bearing saddle. This is an important point to remember when align honing the block.
Torque plates have become de rigueur and the difference theymake is measurable. They are used to simulate stress placed on the block when the heads are torqued in place. Head bolts cause a great deal of cylinder distortion, especially near the deck surface where the piston rings operate. To fully simulate normal operating conditions, you also need to use the proper head gasket under the torque plates. Many engine builders have their own pet theories on how thick a proper torque plate should be. Some like a very thick plate, on the order of four inches or more, while others are happy with a plate between two and three inches thick. Obviously, you can reach a point of diminishing returns with all of this, especially when you consider that the head bolts normally clamp a head that is made of cast iron and is full of ports and water passages. The clamping force exerted by a steel plate may (and probably does) cause a slightly different distortion pattern. Regardless of the subtleties, torque plates produce good results and all experienced engine builders use them. The best advice we can give you is to use the plates when building a racing engine and exercise your own good judgment for a street engine. If you can afford it, do it; if not, you may never notice the difference.
Since cylinder wall shape and condition significantly impact power output, thoughtful boring and honing procedures should be exercised. First, you have to ensure that the cylinders are bored 90 degrees to the crank axis. If the deck surfaces have already been cut parallel to the crank axis, a standard deck-referenced boring bar may be used. Otherwise, the block should be bored with a bar that references directly off the crank centerline. For honing, you should seek out a shop that uses a Sunnen CK-10 Cylinder King or similar power hone. Hand honing has long been considered the path to maximum horsepower, but even the most prominent racers have come to love the CK-10.
The final honing procedure depends on the type of rings you are using. In nearly every case except a full race engine, there is little reason to use anything other than a standard-width ring set (with a molyfaced top ring) from a major manufacturer like Speed Pro or TRW. All-out racing engines may use thin, plasma-moly, stainless steel, or chrome rings, but average street or bracket racing engines live quite happily with a standard ring set. For either application, the cylinders should be finished with a 400–500- grit hone and plenty of clean, filtered honing oil. A good shop will leave the block coated with honing oil when they are finished to prevent surface rust from forming. If you’re not going to assemble the engine immediately, be sure to check the cylinders periodically and relubricate them if necessary.
Installing cam bearings is normally relegated to a machine shop (some shops even offer the service at no charge when they are performing other cleaning and machining operations for you), but you may wish to tackle the job yourself if you have access to the tools. Machinists install these bearings all the time, so they’ll probably get them in correctly, and doing it yourself can be a frustrating experience if you’re a first-timer.
First of all, you can encounter real problems if you aren’t aware that the cam bearings come in different sizes. When you put the bearings in the block, they must be installed in 1, 2, 3, 3, 2 sequence. Check the chart in this section to determine which bearing part numbers are intended for specific bearing bores. These numbers are standardized on all blocks since 1957. Earlier blocks use a different bearing set with wider rear (number 5) bearing.
A cambearing installation tool can be purchased or rented depending on your particular requirements. Install the rear cam bearing first and work your way forward. Before you drive any bearings into place, be certain the impact edge of the bearing has a sufficient chamfer to allow its easy installation in the block and to prevent raising a burr on the bearing surface that can interfere with camshaft rotation. Drive the rear bearing in carefully until it rests just ahead of the rear camplug housing. The bearing oil holes and block passages must align on early 1955–1957 blocks, and this is good practice on all engines. The intermediate bearings will fit evenly into their housings and they should not hang out on either side. The front bearing should be driven in until it is even with the inner edge of the chamfer in the front bore. If you damage a bearing during installation, most machine shops will sell you a single replacement.
Core (Freeze) Plugs
Anytime you have an engine completely torn down for a rebuild, it’s a good idea to replace all the freeze plugs and the rear cam bearing plug. This is a simple task that is easily accomplished right on the engine stand prior to final assembly. These plugs are reasonably foolproof, but it’s possible to get a leaker if you’re not careful. A large-diameter socket is ideal for tapping in the plugs. Each plug should be coated with sealer before you install it and it should be carefully driven straight into the block. Take particular care when driving the cam plug because it is thinner and shallower and prone to leaking if it’s distorted. Wipe off the excess sealer around each plug, but try to leave a nice bead to help give a good seal. Do not drive plugs in too far or they will fall into the water jackets. They should only be driven in until flush with the block surface. Most applications will make use of cup plugs in either steel or brass that may last longer when corrosion problems are anticipated. You’ll also find specialty bolt-in plugs that have rubber seals, but they are not significantly better than a standard plug (and the rubber may deteriorate). They are, however, easier to replace when the engine is in the car, and this may be something to consider.
Final Block Prep
Once the block has been decked, the edges of the deck surfaces should be deburred and a chamfer should be cut at the top of each of the head-bolt holes. (This prevents the top threads from contacting the head surface when the bolts are torqued.) Wash the block down with clean solvent and scrub it thoroughly with warm water and laundry detergent (warm water promotes less rust than hot water but still cleans effectively). Scrub brushes and a rifle-bore cleaning kit are entirely permissible for this operation and you should wield them with a vengeance. Plan on getting yourself good and wet and try to use a source of pressurized warm water. One of the most convenient methods of cleaning a block is to cart it down to your local self-service car wash and have at it with a handful of quarters. In any event, be sure to have plenty of clean lint-free towels for drying the machined surfaces. If at all possible, take along a small air tank so you can blow the block dry immediately. Even before the cylinders are dry, apply a thin film of lubricant to protect them. Lightweight engine oil, automatic transmission fluid, or WD-40 are good choices.
Cylinder block preparation is as vital as any other step in high-performance engine building. It can make a vast difference in performance, one way or the other. Before you begin final assembly of your engine, check off the items on our block prep checklist. And you may wish to refer to the S-A Design book How To Rebuild The Small-Block Chevrolet for complete step-by-step engine prep and assembly instructions.
Factory Block Choices
Chevrolet has implemented many cylinder block revisions, with the end result being a full lineup of specialized Bow Tie blocks designed to meet the specific needs of a broad range of racing and high-performance applications. Following is a current list of Chevrolet high-performance block features and availability.
Written by John Baechtel and Posted with Permission of CarTechBooks