How to Build Killer Chevy Small-Block Engines. Mike Mavrigian
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.
CHAPTER 3
CRANKSHAFTS
Over the years, the evolution of performance crankshafts has experienced a high level of development in terms of materials, manufacturing processes, dimensions, weight reduction, windage concerns, oil delivery, surface finishes, counterweight aerodynamics, and quality control. Gone are the days when a racer is forced to make do by modifying a factory OEM crankshaft.
Crankshaft Stroke
Crankshaft stroke refers to the distance from the crankshaft main centerline to the centerline of the rod journal. Published crankshaft stroke refers to the total sweep of the rod journal from top dead center (TDC) to bottom dead center (BDC). For example, a crankshaft that features a 4.000-inch stroke indicates that the rod journals will move 4.000 inches from TDC to BDC.
However, when we are planning a stroke, rod, and piston combination to determine the crank stroke, rod length, and piston compression distance (CD) relative to the block deck height, we consider only half of the total stroke of the crank. With the rod journal at TDC, the half-stroke distance plus the rod length plus the piston CD will dictate where the piston dome is located relative to the deck at TDC. If our goal is to achieve a zero deck, we refer to the deck height as our target.
This is an example of a Scat lightweight forged crank with a scalloped flywheel flange, fully gun drilled, and with bullnosed and knife-edged counterweights.
Deck height is the distance from the main bore centerline to the block’s head deck surface. If our deck height is 9.000 inches, the combination of half-stroke plus the connecting rod length plus the piston CD must equal the target 9.000-inch deck height. Connecting rod length refers to the distance from the centerline of the rod’s big end to the centerline of the rod’s wrist pin bore. Piston CD refers to the distance between the centerline of the wrist pin bore of the piston to the piston’s top dome edge.
As an example, again referring to a deck height of 9.000 inches, if our crank features a crank stroke of 4.000 inches, we use half of the total stroke, which in this case is 2.000 inches. If our connecting rods feature a center-to-center length of 6.000 inches, our piston CD needs to be 1.000 inch. If our deck height is 9.025 inches, along with a 2.000-inch half stroke and 6.000-inch rod, piston CD would be 1.025.
Factory Stock Crankshaft Stroke
When building a high-performance small-block engine, we typically take advantage of changes to the stroke to obtain increased performance. Simply as a reference, the following tables show the factory-original stroke, rod length, and piston CD found in original Chevy small-block engines.
Popular Aftermarket Bore/Stroke Combinations
When planning a build to deliver increased horsepower and torque, we’re certainly not going to adhere to factory specs. Depending on the limitations of the block at hand, a wide range of cubic inch displacements is possible. Listed on this page are a few examples. Other limiting factors involve the crankshaft strokes available from specific manufacturers.
Aftermarket