Whether you're spinning your very first set of wrenches or tuning engines for a Pro Stock team, there's no denying that the beginning of any engine buildup is the cylinder block. Let's review its qualities, as well as some of the things you should consider when selecting a used one. Our discussion centers around the V-8, although much of what we describe applies to virtually any internal-combustion engine, regardless of configuration.
Iron Versus Aluminum
The vast majority of mass-produced American V-8 engine blocks are made from cast iron. Cheap to manufacture and quite strong, cast-iron blocks offer the best value for the enthusiast on a budget. Aluminum blocks, on the other hand, are typically much more expensive but offer several benefits. They usually weigh half as much as an equivalent iron block, and since aluminum can be welded more easily than iron, all but the most severe damage is repairable.
Most aluminum blocks utilize cast-iron cylinder-bore liners that are either pressed into place or cast as an integral part of the structure. Liners are necessary because aluminum cylinder walls would wear quickly. Expect to find iron liners in nearly any aluminum engine block of American manufacture.
Crankcase Considerations
The crankcase is the part of the block responsible for keeping the crankshaft where it belongs. Composed of main-bearing saddles, support webs, and bulkheads—united as part of the overall block casting—the crankcase must be very rigid to withstand a wide variety of operating conditions. Most production designs are capable of handling more than twice the original power output, but if you're considering large doses of nitrous oxide, begin with a beefier aftermarket racing block. Not only do racing blocks have thicker, stronger crankcase areas, they also feature thicker deck surfaces and cylinder bores, which will pay off when output eclipses the 600hp mark.
Depending upon the origin of the block, it will have either a skirted or non-skirted crankcase. The skirted design brings the walls of the block to a level well below the crankshaft centerline, which creates a stiffer, boxlike crankcase capable of accommodating additional webbing for added strength. Skirted blocks are used in Top Fuel, and the results speak for themselves. Most skirted blocks have a flat oil-pan rail, and employ a simple one-piece pan gasket that virtually eliminates oil leaks. Non-skirted blocks, on the other hand, don't extend downward from the crankshaft centerline. Does this make them weaker? Pro Stock racers all use a non-skirted block design with excellent reliability. The non-skirted block is lighter, and generally allows a lower overall profile.
Bearing-Cap Recap
Main-bearing caps hold the crankshaft in the crankcase. Necessary to withstand tremendous forces, they are generally made of cast iron or forged steel. Most production main caps are held in place with two fasteners, and are commonly known as "two-bolt mains." This system is reliable, but shows weakness in extended high-rpm operation—such as that experienced at the Daytona 500. Under such conditions, the normal main-cap movement associated with high-rpm operation begins to take its toll on the bearings as clearances fluctuate. To combat this, an extra set of bolts ("four-bolt mains") can be employed at each main cap to raise the clamping force and help reduce untoward movement. If you have a choice, use the four-bolt main block.
Inspecting A Used Block
Now that we've covered the basics of the cylinder block and its design variations, what are the criteria for a used block worthy of your time and money? There are several areas you must inspect, and remember, some of this detective work requires special equipment found only in a reputable machine shop. If you skip this phase, you run the risk of building an engine around a crack-riddled cylinder case, and that'll cost you plenty more in the long run!
Cracks
Cracks are caused by three things: the expansion of frozen coolant, heat, and levels of stress that exceed the strength of the block. These cracks can usually be detected by the naked eye, though to be absolutely certain, have the machine shop perform a Magnaflux inspection. Stress cracks are most commonly found in the cylinder bores, in the lifter valley, and around the main-bearing webs. Freeze cracks tend to form along the external surface of the block near the core (freeze-out) plugs. Unless you are dealing with a particularly rare block, the presence of any crack is grounds for immediate dismissal.
Threaded Holes and Dowel Pins
Because most engines are rebuilt at least once during their lifecycle, it is common to find some threaded holes that are stripped or at least in need of cleanup work. Small, tapped holes with thread damage in the oil pan or near the timing-chain cover are easily repaired using Helicoils. Should you discover a munched head-bolt hole or main-bearing-cap hole, things will get tricky fast. Although Helicoils can repair them, it is crucial that they be installed by an experienced machinist to ensure perpendicularity.
On most V-8 blocks, dowel pins are used in several places to properly locate the cylinder heads, timing-chain cover, and transmission bellhousing. A dowel pin should extend from the block by an amount equal to its diameter. Remember, the head gasket will "eat up" some of the pin's effective height, so those pins should extend a little farther to accommodate the difference.
Never assemble an engine unless all dowel pins are in place. For instance, if there are no bellhousing dowel pins and only the bolts are used, there is a very strong chance that the end of the crankshaft and the transmission-input shaft will suffer misalignment. At the very least, this will result in vibration and accelerated front-pump-bushing wear in automatic-transmission applications, and unequal input-shaft bearing loading with a manual transmission.
Cylinder Bores
Because they must contain the forces of combustion, and accurately locate the pistons, the cylinder bores must be in tip-top shape. The bore diameter is of primary concern: The closer to the original it is, the better; wall thickness and strength are maximized. But in most cases, used blocks come from high-mileage vehicles. The cylinder bores usually have considerable wear that is correctable by overboring. Overboring removes metal, and should be limited to a bare minimum to preserve strength. Before requesting this modification by the machine shop, make certain that replacement pistons and rings are available.
Although an easy way to gain displacement (a .060-inch overbore typically adds about 12 ci to the total), resist the temptation to bore your way to extra cubes. Once the bore is expanded .060 inch (less in certain cases), it is essentially unrebuildable. A better plan is to make a .030-inch cut (or whatever it takes) to clean up the bore. From the '70s to the present day, many manufacturers have reduced the metal content of their blocks in the interest of shaving pounds and cutting costs. Most notably, the Chevy 305 and Mopar 440 (those cast after 1974) should be sonic-tested if the anticipated overbore will exceed .020 inch.
Lifter Bores
Lifter-bore problems are rare because most engine designs provide plenty of pressurized oil to prevent excessive wear. When a used block has seen service in a racing application, some engine builders enlarge the lifter bores and install bronze bushings to restrict the flow of oil to the valvetrain so that more of it gets to the crankshaft. This practice is viable in certain racing applications, but on the street it often causes rapid valvetrain wear. If you spot bronze bushings in the lifter bores of a used block, move to the next candidate.
Align-Bore Blues
The practice of align-boring the main bearing saddles and main caps is helpful in that it guarantees minimal drag on the crankshaft. On the other hand, it moves the crank closer to the cam by a small amount, which has the net effect of lengthening the timing chain, directly affecting cam and ignition timing. The process of align-honing removes far less material, and is unlikely to cause timing problems. If you suspect that a potential block has been align-bored, be certain to have a qualified machine shop measure the distance from the crank to the cam to see if (or how much) it differs from stock specs.
Thrust-Bearing Wear
If the cylinder block was mated to a manual transmission, check for excessive wear at the thrust-bearing bulkhead. Every time a high-performance pressure plate is depressed, it loads the crankshaft from rear to front. Increased pedal pressure increases the amount of loading on the thrust bearing, which causes the bearing to wear more rapidly. Faithful maintenance and inspection frequently allays this problem, but it involves dropping the oil pan. Therefore, people ignore it until it's worn out and has allowed the crankshaft to ride up against the block, removing metal in the area, and permitting excessive crankshaft endplay. Avoid such blocks.
Been Decked?
Decking is a machining operation that removes metal from the cylinder block where the heads are attached. Primarily, the block is decked to restore flatness and to improve head-gasket sealing. Decking also corrects minute errors in the factory casting, but this is a racer-only operation for the last bit of performance potential. In most cases, decking causes more problems than it solves. When a block is decked, the cylinder heads are brought closer together, causing a misalignment of the intake-manifold bolt holes. It is important to have a competent machine shop check all dimensions, including deck height.
Unexpected Oddities
The vast majority of domestic engine-block designs are completely interchangeable from model to model. When in doubt, seek the advice of an experienced hot rodder. HR
