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CHAPTER 2

THE BLOCK: WHERE IT ALL BEGINS

From the inception of the Gen I small-block Chevy engine to the current LS and Gen V LT variants, they all share a few common basic traits: a 90-degree V-8 platform, 4.4-inch cylinder bore spacing, a single overhead valve camshaft, and a pushrod connection between the lifters and rocker arms.

As most enthusiasts already know, the small-block Chevy engine has been around since 1955. Over the years, displacement, strength, durability, and machinability (in terms of allowing modifications) variations have proliferated. Modern aftermarket blocks are available to accommodate much larger displacements and design variations than in days past in terms of both bore diameter and stroke increase increments as well as the ability to withstand higher cylinder pressures and operating speeds.

The Generation I small-block Chevy engine just keeps getting better with age. By age, I refer to non-stop development to enhance its capabilities. Refinements over the years have resulted in today’s extraordinarily robust and versatile performance platform. Perhaps no other engine platform in the history of the internal combustion engine has experienced more evolutionary enhancement changes than the venerable small-block Chevy engine. A prime example of those improvements to foundation componentry is the engine block. Currently available blocks specifically suitable for high performance include cast iron (of varying nickel content), cast aluminum, and billet aluminum.

Cast Iron Versus Aluminum

For both street and many racing applications, cast-iron blocks are perfectly acceptable. The inherent advantage of alloy blocks is obvious in terms of weight reduction: cast-iron blocks typically weigh in the area of 170 to 240 pounds, while aluminum blocks can weigh as little as 80 pounds, depending on various lightening options offered by some makers. Weight reduction is also necessary for certain forms of racing (sprint car racing, for example). Another advantage of aluminum is the relative ease/ability to be welded to repair certain failures, such as cracks, broken mounting tabs, etc.

Cast-iron blocks can be overbored within the limits of specific designs, while aluminum blocks feature iron cylinder liners that limit bore oversizing. Today’s aftermarket iron blocks typically feature thicker decks for increased rigidity, thicker cylinder walls that permit more overboring than OEM blocks, and improved cooling and oiling passages. Aluminum aftermarket blocks are available for designated bore sizes to suit just about any desired application. One major difference, aside from the weight difference, is cost. Aluminum blocks are more expensive than cast iron. New cast-iron aftermarket blocks generally run in the $2,000 to $3,000 range, while new aluminum racing bare blocks can cost as much as $5,000 to $6,000.

While today’s cast-iron aftermarket performance blocks are superior in strength and rigidity, some companies also offer compacted graphite iron (CGI) blocks, which offer increased material density and strength. (Photo Courtesy Dart)

Unless your racing application demands the use of an aluminum block, stick with cast iron. We won’t waste time by delving into vintage OEM mass production cast-iron blocks. For purposes of this book, we’ll only discuss currently available blocks offered by qualified aftermarket manufacturers.

A cast-iron block’s material makeup can differ depending on the iron mix from gray iron or nodular iron to iron mixed with a level of nickel for increased hardness to CGI (compacted graphite iron). CGI features a molecular makeup that creates tightly interconnected graphite during the casting process that offers increased hardness, increased fatigue strength, superior ductility, and greater tensile strength. CGI is less brittle and more stable than gray iron. Several aftermarket block makers now offer CGI blocks. Due to the increased strength, a block can be machined for further weight reduction without sacrificing strength compared to a gray iron casting.

For instance, cylinder bore walls can be machined thinner without sacrificing wall integrity and strength. CGI is 75 percent stronger than gray iron and approximately 200 percent more resistant to fatigue. This makes CGI a good choice for racing blocks that need to withstand extreme pressures and temperatures. Also, due to the increased strength level, a CGI block can be machined to reduce weight by as much as 22 percent compared to gray iron.

Main Caps

On the subject of main caps, we generally have three choices: ductile iron, nodular iron, and steel billet. While steel billet may initially seem the strongest, there are factors to consider. Some view nodular iron as the best choice for cast-iron or aluminum blocks that will experience substantial dynamic pressures, since nodular iron caps tend to be more ductile and will move before they break. With that said, nodular iron or steel billet caps are considered by most to be the best choice in terms of strength and rigidity, and they are the least affected by thermal and dynamic stress, depending on builder experience and preference. ■

For any high-performance application, a block should feature four-bolt main caps, which offer much greater rigidity to cope with the pressures of high-revving crankshafts. This reduces crank deflection and harmonics and offers superior clamping force for the main caps compared to early non-performance two-bolt main cap designs. All aftermarket performance blocks feature four-bolt main caps, many featuring splayed outer bolts at the number 2, 3, and 4 cap locations. The splayed outer bolts (usually at about a 10-degree angle) enter toward the side of the block where there’s more material thickness as opposed to the bottom of the main web, which increases cap rigidity.

Aftermarket engine blocks feature superior casting materials and techniques. They are cast and raw machined to much tighter tolerances than vintage mass-produced Chevy OEM blocks. A Dart cast iron Little M block is shown here.

OEM Blocks

The use of OEM or stock blocks has served well as the foundation for street performance and racing applications. That was then, and this is now. Today’s performance aftermarket now offers a dizzying array of block upgrade designs.

All performance aftermarket blocks feature four-bolt main caps. Depending on the specific block, the outer bolts may be straight on all caps, or caps number 2, 3, and 4 will be splayed at an angle.

Aftermarket performance blocks feature four-bolt main caps with the outer bolts or studs on caps number 2, 3, and 4 splayed at an angle, providing stronger fastener clamping force at a thicker area of the outer block instead of the bolts engaging into the bottom of the main webs. The example seen here is a Dart Little M block.

Aftermarket blocks (such as the Dart Little M shown here) typically feature relief cutouts at the pan rail sides to accommodate increased stroke. The block maker anticipates that the builder plans to use a longer crankshaft stroke. The block is pre-notched to provide added clearance between the connecting rod’s big end and the cylinder bottom and pan rail. The block maker will specify that the block is planned for a certain stroke, for example 4.000 inches, and that additional clearance may be needed if a longer stroke or thicker aluminum rods are used. If additional clearance is required, the factory relief notch provides a starting point.

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 deck thickness on OEM blocks can vary greatly, sometimes to the point of not providing the rigidity required for high compression and negatively affecting cylinder bore roundness (cylinder bore distortion). Aftermarket blocks tend to feature much thicker decks, as thick as 0.675-inches, that provide superior strength, reduced cylinder bore distortion, and superior head gasket sealing when the cylinder heads are clamped in place. Cylinder wall thickness is also improved in aftermarket blocks in terms of both minimum thickness and improved consistency of wall thickness around the entire perimeter of the cylinder wall. This increased thickness further limits cylinder bore distortion and allows the builder to increase bore diameter further without weakening the cylinder walls.

OEM blocks may feature wall thicknesses as thin as 0.194 inch in the thinnest spots, while aftermarket blocks may provide minimum thickness in the 0.220- to 0.308-inch range even when cylinders are finished to a 4.125-inch diameter. Many aftermarket blocks feature siamesed bores with no water jackets between cylinders. This provides superior rigidity to the cylinder walls with less dynamic deflection, which also helps to minimize cylinder bore distortion. Controlling cylinder wall geometry (in an effort to maintain as much uniform roundness as possible during engine operation) reduces frictional drag and improves piston ring sealing.

While the standard small-block cam bearing bores are 2.000 inches, aftermarket blocks are available in a variety of bore sizes depending on the cam journal and/or the type and size of the cam bearings. A popular move is to use a small-block cam that features big-block cam journals of 1.949 inches in diameter, requiring cam bearing bores of 2.120 inches. This allows the use of an increased-mass cam core for added cam rigidity for less deflection when using with high-pressure springs. Even larger cam bore sizes of 2.250 inches are available for the use of needle roller cam bearings. Choices of cam bearing bores include 50, 55, and 60 mm. The cam bearing bore size can be specified when ordering the block. The aftermarket offers a high level of versatility compared to working within the constraints of an aged factory production block.

Today’s quality aftermarket blocks offer vastly superior strength and rigidity with less metallurgical movement under thermal and dynamic stress. With these superior blocks, coupled with the ability of today’s engine builder to accurize critical dimensions with either specialty blueprinting machining fixtures or multi-capability CNC machining, we have the best that today’s technology offers to produce blocks capable of running and sustaining extremely high engine speeds. They can withstand the extreme cylinder pressures obtained through the use of high compression and/or high-boost forced induction. Unless you’re planning a true historic restoration where you feel compelled to salvage a vintage block, today’s aftermarket blocks are the better choice performance-wise, especially considering the quality offerings from such manufacturers as Dart, Chevy Performance, Brodix, and Donovan.

Dart

Dart provides the following offerings.

SHP Pro Iron

This is an excellent upgrade or stock replacement. Designed for street high-performance or sportsman racing and special high performance (SHP), this block is ideal for hot rodders, drag/circle track, off-road, or high-performance marine applications. For a few dollars more than you’d spend on a 40-year-old core that needs cleaning, machining repairs, and upgrades, this provides a new foundation for your build.

Many features provide superior performance and durability, including a priority main oiling system, Siamese cylinder bores with extra-thick walls, thicker decks for superior head clamping and gasket sealing, blind head bolt holes that don’t enter water jackets, scalloped water jacket walls that improve flow around cylinders, and pre-clearance for a 3.75-inch stroke. Other features include splayed outer bolts on the middle main bearing caps, provisions for OEM stock roller lifters and cam, the use of 0.300-inch-tall stock 1987–1995 roller lifters, acceptance of the 1981–1985 stock-style oil pan and dipstick, and all OEM-location bolt holes for the starter, clutch ball, etc.

Race Series Aluminum

This weight-saving cast-aluminum racing block is offered in deck heights from 8.850 to 9.500 inches. The casting features a premium and proprietary high-strength RMR aluminum alloy. Cylinders are extended at the bottom for better piston support when used with long-stroke cranks. A raised cam tunnel provides added clearance for strokers. The cam tunnel is raised by 0.391 inch with an option for a 0.434-inch raise. Siamesed bores are offered in 4.000 inches or 4.125 inches and can be safely bored to as much as 4.165 inches. Ductile-iron cylinder sleeves and extra-thick walls provide an excellent ring seal.

The oil pan rails have been spread out by 0.400 inch at each side for more stroker clearance. This small-block platform features big-block cam bearings, allowing the use of larger base circle cams to reduce twisting with cam-driven pumps. Rear external oil inlets with crossovers and restrictor provisions located in the center of the lifter valley simplify external oil pump plumbing. This block has no provision for a block-mounted oil filter, so a remote filter is required.

Dart’s SHP Pro Iron block is pictured. (Photo Courtesy Dart Machinery)

Race Series 4.500 BS Aluminum

This enhanced version of Dart’s cast-aluminum racing block is similar to the aforementioned race series aluminum block but features a 4.500-inch bore spacing and permits bore diameters up to a whopping 4.250 inches. Head bolt patterns are offered in either a standard 17-bolt or optional 19-bolt version. Deck height choices include 8.850, 9.025, or 9.075 inches. Tall-deck versions are also available at heights of 9.325 inches or 9.500 inches. The range of deck heights allows greater versatility for the preferred rod angle and ratio. Standard is a 0.391-inch raised cam tunnel, which provides a camshaft-to-crankshaft centerline of 4.912 inches. An optional 0.434-inch raised cam is also available.

Little M Sportsman Iron Block

This block is another offering from Dart that is ideal for a serious street performance build, sportsman racing, or a marine application when the builder wishes to use standard-type Chevy small-block parts, such as the cam, timing chain, oil pump, oil pan, oil filter, motor mounts, mechanical fuel pump, clutch linkage, etc. Improvements over a factory block, in addition to Dart’s stronger casting and higher precision machining, include priority main oiling, where the main bearings are fed first, Siamese cylinder bores with extra-thick walls (a minimum of 0.275-inch thick even with a 4.185-inch bore), scalloped water jackets, an open lifter valley for improved oil return, enlarged lifter bosses that accommodate offset and oversize lifters, blind head bolt holes that don’t enter water, splayed outer main cap bolts on middle caps, a rear external oil feed, and crossover and restrictor provisions.

This is an example of Dart’s cast-aluminum small-block bare block. Cast-aluminum blocks are slightly more expensive than cast iron but cost substantially less than billet blocks. If weight savings is important and you can’t afford billet, this is a great choice. (Photo Courtesy Dart Machinery)

Dart’s Little M Sportsman iron block is shown. Like all Dart blocks, options include the desired bore size, deck height, and standard or raised-cam versions. (Photo Courtesy Dart Machinery)

Little M Iron Block

The Little M is a true cast-iron racing block suitable for street or track. Like the Little M Sportsman block, it accepts standard SBC parts with all of the features of the Sportsman version. The Little M is fitted with billet steel four-bolt main bearing caps and is available in a CGI (compacted graphite iron) version that doubles strength without adding weight, making it a good choice for boost applications (turbo, supercharger, or nitrous). The Little M is beefed up in critical areas and designed from the ground up as a true racing block that can be used with off-the-shelf SBC components.

Little M 305 Water Iron

This block is legal for 305 Racesaver Sprint Series racing and is suitable as a stock replacement or performance upgrade block. To accommodate race classes that require stock displacements, this block features non-siamesed 3.750-inch cylinder bores and also has priority main oiling, thicker decks, blind head bolt holes, and splayed four-bolt main caps. It accepts standard SBC parts and has a rear external oil feed, crossover and restrictor provisions, an open lifter valley, and enlarged lifter bosses.

Iron Eagle 4.500 BS

The Iron Eagle is targeted at advanced engine builders for unlimited late-model or off-road truck maximum competition. The spread-bore spacing of 4.500 inches requires special 4.500-inch cylinder heads and components. The 4.500-inch bore spacing (as opposed to the standard 4.400-inch spacing) allows larger bore diameters and gasket sealing surface between bores. The maximum recommended bore size is 4.250 inches.

This block features a 0.391-inch raised cam tunnel, but an optional 0.434-inch raise is also available. Oil pan rails are spread out 0.400 inch at each side for reduced windage and added stroker clearance. Oil pan bolt holes are in line with the main caps to eliminate interference with stroker setups. Other features include big-block cam bearings, dual starter mounts to provide a choice of left or right location, front and rear external oil inlets, and crossover and restrictor provisions.

Aluminum Billet Block

In addition to the wide variety of iron and aluminum cast blocks offered by Dart, the firm also offers an aluminum billet block that is machined from one huge chunk of dense forged 6061 aerospace aluminum alloy. These full-water-jacket blocks are custom ordered to your specs in terms of bore diameter, bore centerline, standard or 4.500-inch bore spacing, the deck height of your choice, the standard or raised cam bore location, cam tunnel options up to 60 mm, and custom lifter bore diameter and location. They are equipped with either steel or optional aluminum main caps. For a block that can be custom engineered to a builder’s custom specifications, this is the trick pony.

Dart’s Iron Eagle small-block iron block is shown here. (Photo Courtesy Dart Machinery)

This is an example of a Dart billet block that is ready to ship. The builder then final-hone-sizes the cylinder bores, cuts decks to desired height, and sizes lifter bores to accommodate the lifters of choice. (Photo courtesy Dart Machinery)

Brodix

All Brodix blocks are CNC machined from A-356 high-strength virgin aluminum castings and are popular choices for drag, late-model, and sprint car racing applications. A wide variety of deck heights, bore sizes, and main cap materials is available in addition to CNC-lightening options that carve unneeded material from the blocks, effectively skeletonizing to reduce weight where possible.

Some block manufacturers, such as Dart, offer the option of billet aluminum blocks that are CNC machined starting with raw dense aluminum forged billets. Shown here are raw billets and semi-finished CNC machined blocks. (Photo Courtesy Dart Machinery)

Main bore and cylinder bore options include a 400 main/4.125-inch bore, 350 main/4.125-inch bore, 400 main/4.000-inch bore, and 350 main/4.000-inch bore.

Cam bearing choices include a small-block babbit, a big-block babbit, a standard 2.250-inch OD roller bearing or 50-mm roller bearing, and standard or +0.391-inch raised cam bearing locations. Note that the use of babbit-style cam bearings requires the use of wide cam bearings with 1/4-inch locks in the center bearing locations and spiral locks on the two end bearings. Roller cam bearings require spiral locks on all five bearings.

A host of custom-order versions of blocks are offered, including those with either wet or dry sump applications, with or without the fuel pump boss, standard or wide-pan rails, side water reverse cooling, front oil scavenging, lifter bore locations, spark plug holes, 350 or 400 mains, standard light steel or titanium main caps, deck heights of 9.000 to 9.300 inches, 9.000 to 9.500 inches, and 9.020 inches.

Stroker clearance is provided for 4.125-inch crank strokes if the block has a standard cam location. For raised-cam blocks, clearance is already set for a 4.250-inch stroke. The 410 sprint car blocks are clearanced for a 3.800-inch stroke.

Note: As shipped, cylinder sleeves are left slightly above the deck intentionally, allowing the builder the choice of surfacing the sleeves flat with the deck or not. As you can see, Brodix blocks are intended for serious pro-level racing applications with a multitude of options available to suit specific build requirements.

Brodix blocks are cast aluminum and intended for racing applications. Standard platforms are weight relieved via CNC cutouts, but additional weight relief is available as an option. (Photo Courtesy Brodix)

An example of optional block side lightening is shown where material has been removed without compromising block strength. (Photo Courtesy Brodix)

Brodix offers specific head stud kits for their blocks based on the builder’s choice of specific Brodix cylinder heads.

Note that block versions are also available in solid form with no water jackets for extreme drag applications.

Basic order part numbers for blocks, not including the many options mentioned earlier, are as follows:

400 main/400 bore……………8011000

350 main/350 bore……………8011001

Donovan

All Donovan blocks are made from high-strength aluminum castings with incredibly intensive and intricate CNC milling and feature priority-main oiling. Each block is custom made to suit the engine builder’s requirements. All blocks include a head stud kit, core plugs and dowels, ductile iron sleeves, main caps, and main cap studs, washers, and nuts. When first examining a Donovan block, it seems as if it was milled from a chunk of billet aluminum. The entire block (decks, lifter valleys, sides, front, main webs, etc.) is precision milled on CNC stations with the exception of the rear face, which is the only area that gives a hint that the block began as a casting. These are serious blocks intended for racing only (sans fuel pump and filter bosses, lightened exterior sides, etc.). All holes are threaded to accept –AN straight thread O-ring fittings.

Note the as-cast block side. Although weight is reduced, blocks can be ordered with additional material removal for further lightening. (Photo Courtesy Brodix)

Aftermarket racing blocks typically feature added material at the lifter bore bosses to allow the builder to enlarge the bores to accommodate the lifter diameter of choice. When using roller lifters, lifters must stay in plane with the cam lobes and cannot be allowed to rotate, requiring either individual lifters guided in plane with dog bone guides or lifters that are paired together with a link bar. Note that this block features tapped holes between lifter bores, allowing the option of using individual or link-bar roller lifters. (Photo Courtesy Brodix)

Three basic configurations are available, including the HC400, 410, and 350/400.

HC400 Block

The basic features of the HC400 block include a weight of 105 pounds, the cam tunnel raised 0.390 inch, the deck height available to 9.500 inches, the pan rail 1/2-inch wider than stock, a fuel pump boss, an oil filter boss, and standard Chevy-type motor mounts. Custom pan rail patterns are available.

Aluminum engine blocks provide obvious advantages in terms of weight and are mandatory for some racing classes. The Sprint car series, for example, requires a cast block that is finished to a 410-ci displacement. Shown here is a popular Donovan aluminum block.

The Donovan 410 block is fully machined on a CNC machining center, including the exterior sides, which are shaved down to reduce weight. Because of the dedicated racing application, there is no need for side motor mount bosses, fuel pump bosses, a starter boss, filter boss, etc. This is a good example of a racing-only block that is purpose-designed for racing function only, eliminating as much weight as is practical in the process.

HC410 and 410 Block

Popular for sprint car builds, the HC410 block weighs in at a mere 83 to 88 pounds and is designed for dry sump oiling only. The blocks feature a 0.390-inch raised cam location and a fully machined exterior with a standard pan rail width. The 410 version weighs a paltry 80 pounds and features the standard cam location and removal of the fuel pump and motor mount bosses.

350/400 Blocks

This block is available to accommodate either a 350 or 400 main bore and features a 9.025-inch deck height, a fuel pump boss, standard motor mount bosses, and an oil filter boss. It is offered for either a 4.000 or 4.125-inch cylinder bore size. Weight is 100 pounds. Custom options include gear drive accommodation; full, half, or solid castings; and a special cam bore size on request.

The Donovan aluminum 410 block is fully CNC machined except for the rear face, which is the only indication that it’s a cast piece.

Lightweight options are available for all Donovan blocks, such as removal of engine mount bosses and oil filter and/or fuel pump bosses, lifter valley and exterior shaving/profiling, etc., where weight savings is a priority.

Note that the HC designation indicates a raised cam configuration, which raises the cam 0.390 inch to allow for increased stroker crank and rod to cam clearance, which can be sized for either 350 or 400 cranks. Special cam bore sizes are also available. The Donovan blocks are truly custom-order race blocks to accommodate the builder’s preferences.

This Donovan racing block has been CNC machined even in the lifter valley, removing all material that is unnecessary while leaving enough material at the lifter bosses for the builder’s plan to use the lifter diameter of choice.

A close view of the lifter valley in this Donovan aluminum block reveals a series of tapped holes in the center. Since builders of these blocks tend to use needle roller cam bearings, these holes allow the insertion of stopper bolts that prevent the roller cam bearings from walking out of their bores during operation, which is possible due to the thermal expansion rate differences between the aluminum block and the baring cages.

While traditional small-block Chevy main caps provide a mount for a wet sump oil pump, when the builder elects to use a dry sump system, the pump holes in the rear main cap must be plugged. For blocks that are intended for dry sump applications from the very start, block makers provide rear main caps that have no such wet sump oil pump provision.

Main caps for high-performance or racing blocks will feature either nodular iron or billet steel main caps. Some builders prefer steel for its strength, while others prefer nodular iron for its combined strength and ductility, or resistance to breaking. One builder I spoke with, for example, prefers nodular iron because, “They tend to bend instead of break,” he said, citing his experience with regard to extreme-duty engine failures.

When ordering an aftermarket block, in many cases you can specify an option regarding camshaft bore diameter, depending on your planned cam bearing application: standard (2.000 inches), big-block journal (2.120 inches), or roller bearing (50, 55, or 60 mm). In some cases, the block maker will produce the cam bore in a standard size, noting that the bores may be enlarged to accommodate the desired bearings. This example shows a block fitted with roller bearings.

Makers of racing blocks often offer weight reduction options that include the removal of unnecessary material. This Donovan aluminum block is a good example. The block is so extensively CNC machined that it initially appears to have been machined from a solid billet of aluminum.

Aftermarket block makers tend to design their blocks with added mass at the pan rail areas to allow for increased crankshaft stroke. Notice the healthy relief notch on this aluminum block for the big end of the rod’s clearance.

Dedicated racing blocks, such as the Donovan aluminum 410, feature external coolant transfer plumbing due to material removal for weight reduction. The fitting in the center allows the racer to inject hot water into the block before firing the engine.

Chevy Performance Blocks

Chevrolet Performance offers a wide range of blocks specifically intended for racing use that receive more detailed prep compared to the regular street blocks.

Cast-Iron Bowtie Race Block

Features include the use of “premium” castings with thicker deck surfaces, priority main oiling, improved coolant flow, splayed four-bolt steel main caps secured with “premium” fasteners, and precision CNC machining to within 0.005 inch.

Chevy Performance Race Blocks

Racing blocks with applications for NASCAR and NHRA are reportedly CNC machined with closer tolerances than their Bowtie blocks (within 0.001 inch) and are reportedly capable of handling up to around 1,200 hp. Features include priority main oiling, four-bolt splayed steel main caps secured with premium studs, and a minimum cylinder wall thickness of 0.225 inch (at 4.155-inch bore). A sonic bore check data sheet is provided with each block. Nominal wall thickness is 0.340 inch. Extra-thick deck surfaces have blind holes that are not open to water.

Enlarged cam bosses allow custom machining for larger bearings, and extra-thick main bearing bulkheads provide greater strength. Main bearing cap inner bolts are spread 0.210 inch to allow machining for the use of 400 journal cranks. The billet steel rear main cap is designed for wet sump use but can be plugged for dry sump applications. The rear main seals are the two-piece style, requiring pre-1986 style oil pans. Lifter bores are tall, requiring clearancing for some aftermarket solid roller lifters. The oil dipstick boss is not drilled, giving you the option of running a dipstick.

Chevy Performance Iron Bowtie Race Block

The Bowtie blocks, rated at handling 700-plus-hp, are CNC machined to within 0.005-inch tolerance. Designed to accommodate 350-size crankshafts, features include four-bolt splayed steel main caps, a deck height of 9.025-inch, two-piece rear main seals, 2.000-inch OD cam bearings, and 3.980-inch finished bores. Maximum overbore is 4.155 inches; maximum stroke is 3.750 inches.

Race blocks and Bowtie race blocks are available for use with one-piece or two-piece rear main seals and a wide variety of bore sizes and stroke accommodation. All feature four-bolt main caps. Examples include part numbers:

10066034	Two-piece rear seal, 4.000-inch bore
10105123	One-piece rear seal, 4.000-inch bore
88962516	One-piece rear seal, 4.000-inch bore, for 383 stroker
24502503	Bowtie, two-piece rear seal, 3.98-inch unfinished bore
12480175	Bowtie, one-piece rear seal, 4.117–4.155-inch bore
24502572	Two-piece rear seal, 4.500-inch bore
12480157	Bowtie Sportsman, two-piece rear seal, 4.117-inch bore
366300	Bowtie, aluminum, 4.000-inch bore, splayed nodular caps, for 2.45-inch journal cranks
366287	Bowtie, iron, 3.980-inch bores for 4.125 bore max, four-bolt nodular caps

Iron aftermarket blocks are often supplied with bronze lifter bore bushings already installed, requiring finish machining to the necessary diameter size and top trimming. (Photo Courtesy World Products)

World Products

World Products offers a variety of performance small-block Chevy blocks under the Motown II label. All blocks are a high-density cast-iron alloy and feature a deck height of 9.025 inches. Available bore sizes range from 3.995 to 4.120 inches, and some enable as much as a 4.200-inch overbore. Most blocks are pre-clearanced to accept a 4.000-inch stroke. Selections among the 18 variants offered include choices of cam journal sizes of 2.000 inches, big-block Chevy, and 50 mm. The available cam tunnel location is standard SBC or plus 0.134-inch raised. Bronze-bushed lifter bores are offered in either standard or 0.904 inch. Main bore size options include 350 or 400.

Depending on the specific block part number, main caps are ductile iron, nodular, or billet steel. In addition, a series of Motown Pro Lightweight iron blocks are offered with a bare weight of 178 pounds. With 18 different block part numbers from which to choose, the selection offers applications from street to full race.

Aftermarket block makers often use varying oil delivery designs specific to their blocks. This World Products Motown II block features oil crossovers for the lifter oil feed and is tapped for NPT plugs. If solid lifters are planned, deeper-located threads in the holes allow the use of plugs that are drilled to restrict oil delivery. (Photo Courtesy World Products)

This is an example of a World Products Motown II cast-iron block. As with other makers, these blocks are available with standard height or raised cam bores, standard or tall decks, and a variety of raw bore sizes. (Photo Courtesy World Products)

This is an example of a Motown II block with steel billet main caps. (Photo Courtesy World Products)

Depending on the manufacturer, blocks may be ordered with either nodular iron or steel billet main caps, depending on builder’s preference. Shown here is a World Products Motown II block fitted with nodular iron caps. (Photo Courtesy World Products)

Iron blocks are often fitted with bronze lifter bore bushings, which are raw finished on the tight side, allowing the builder to machine the desired lifter clearance based on the diameter and type of lifters being used. Depending on the link-bar lifters, the top exposed bushing material may need to be trimmed to provide link bar clearance.