OEM iron castings were production based, which means that they’re made in production batches. Because of mass-production techniques, inconsistencies and variations of design tolerances were common, such as core shifts that may have resulted in variances of cylinder wall thickness, cylinder bore on-centers, lifter bore on-centers, and machining tolerances that may have resulted in uneven deck heights and block deck squareness. By and large, these inconsistent mass-production tolerance issues were nothing that prevented the engine from running, or even running extremely well, but the path of mass production didn’t always optimize the power (and durability) potential of the wonderful design.
Many aftermarket blocks feature splayed outer main cap bolts at the number 2, 3, and 4 cap locations, taking advantage of the increased mass toward the side of the block as opposed to entering the main web area.
The mass-production tolerance issues aside, the majority were built using cast-iron main caps; some had a two-bolt design and higher-performance models featured a more robust four-bolt main cap clamping.
I am by no means trying to disparage the use of factory blocks. They served many generations of performance enthusiasts well both on the street and track. However, in an effort to make engines more durable in the process of attaining more power, performance and racing engine builders continually strove to improve and enhance the blocks by adding steel billet main caps accompanied with stronger, higher tensile strength main cap bolts or studs. Remachining factory blocks to improve the geometry involved careful align honing of the main bearing bore, oversizing cylinder bores to increase displacement, accurizing lifter bore centerlines by overboring, correcting casting-shift flaws, and installing bronze bushings back to required lifter bore diameter, etc.
In the quest for added displacement, large-stroke crankshafts were fitted and required relieving the cylinder bore edges and pan rails for clearance while being limited to stroke by potential clearance issues between camshafts and connecting rods. When chasing the demons of power, factory blocks posed other limitations in terms of block deck thickness, main web strength, cylinder wall thickness, and moderately efficient oiling circuits.
In addition to the evolutionary improvements on the original design, aftermarket blocks provide the option of starting the build with a brand-spanking new block as opposed to dealing with an aged, worn, and pitted original GM mass-produced block. Keep in mind, however, that Chevy Performance offers high-quality cast-iron racing blocks that are a cut above blocks that are produced for mass vehicle builds.
Enter the world of aftermarket performance engine blocks. Manufacturers, such as Dart, Brodix, Donovan, World Products, and Chevrolet Performance, offer small-block Chevy platform blocks in versions that range from stock-spec to racing-application-specific. These accommodate extremes of bore diameter and stroke to lightweight alloy construction and more, all featuring a vastly superior level of precision and strength compared to OEM mass-production blocks.
Today’s aftermarket block makers offer variations that suit any application. No longer are you stuck with OEM designs that limit build potential. For example, today you can obtain blocks that feature tall-deck designs and raised-cam versions, both allowing the use of an increased crankshaft stroke. Raised-cam blocks, depending on the specific block, can feature the cam bore raised by about 0.391 inch beyond the OEM crank-to-cam centerline, resulting in a crank-to-cam centerline distance of 4.912 inches.
Moving the camshaft higher relative to the crank provides additional rod big-end to cam lobe clearance, which is especially critical when running fatter aluminum rods. Performance aftermarket blocks also typically feature longer cylinder walls that extend farther toward the crankcase, providing superior piston skirt support during bottom-dead-center transition. Typically, aftermarket blocks also feature enlarged lifter bosses, which allow the builder to machine the lifter bores to accommodate the lifter diameter of choice.
These are features that builders back in the day wish they had access to. Currently available aftermarket blocks are designed with all of the features and potential tweaks that allow builders to achieve the performance parameters that they want instead of being limited to what’s feasible with an aged production block. Today’s blocks unleash the builder’s creativity to levels unheard of even a decade ago.
This aftermarket small-block features a 0.134-inch raised cam tunnel, providing extra clearance between a healthy larger-diameter cam core and the rod’s big ends when an increased crank stroke is planned.
In addition to thicker cylinder walls, beefy pan rail and main web areas, aftermarket blocks usually feature cylinders that extend toward the crankcase to offer additional stability for the pistons, reducing piston rock during the transition from bottom dead center.
In essence, the Generation I small-block Chevy platform has been around for so many years and has been the basis for countless race-build variations that aftermarket block designs have evolved to reach an unprecedented level of strength and durability with superior casting processes and CNC machining. In short, today’s blocks far surpass the capabilities and virtues of the original production blocks.
When it comes to small-block Chevy builds with aftermarket blocks, a common choice is between 350 or 400 main sizes. The 350 mains (2.45 inches) are fine for the street and high-performance builds, while the 400 mains (2.650 inches) are generally a better choice for extreme-duty builds because there’s more cross-sectional mass between main and rod pin areas. The 400 main size provides a bit of added insurance against crank breakage.
Aftermarket blocks tend to feature thicker pan rail areas for both added strength and to permit notching for stroker clearance. Blocks are usually pre-notched by the block maker in anticipation of the stroke that the builder has in mind. For instance, if the block was ordered for a 4.000-inch stroke, the factory clearance notches are usually sufficient. If additional clearancing is needed, it involves minor material removal.
When ordering most aftermarket blocks, lifter bores are usually slightly undersized, allowing you to bore to accommodate the desired lifter diameter. While 0.842-inch lifters are considered stock size, high-performance and race-engine builders tend to go larger; for example, 0.905-, or even 0.937-inch-diameter, lifters could be used to obtain a larger roller wheel diameter. While many iron-block builders use bronze lifter bushings (requiring overboring beyond the lifter diameter, installing the bushings, and sizing the bushings to provide the desired lifter oil clearance) when dealing with an aluminum block, bronze bushings really aren’t needed since many builders prefer the lifters to ride against the parent aluminum. It should also be noted that many aftermarket blocks are available to accommodate either one-piece or two-piece rear main seals, which depends on the crankshaft selection.
Advantages of an Aftermarket Block
Today’s aftermarket blocks offer many distinct advantages compared to mass-produced factory-original blocks. The most notable differences are as follows.
OEM blocks were designed with an oiling circuit that delivered oil from the pump to the filter, then to the cam bearings, to the lifters and rockers, and finally to the main and rod bearings. For high-stress and high engine speeds, this is simply not an efficient system. New aftermarket blocks as well as Chevy’s line of performance Bowtie blocks reverse this with the main bearings receiving oil delivery first. This is referred to as priority main oiling, which has become the standard in performance blocks.
Block