Notes: Small Block Chevy Power Takeoff

SOLVING THE BOLT PATTERN PUZZLE

GM engineers redesigned the crankshafts used in production small-block V8 and V6/90-degree engines in 1986 to use a leak-resistant one-piece rear seal. This change from a two-piece to a one-piece seal required a change in the bolt pattern on the crankshaft flywheel flange. The bolt circle on pre-1986 production crankshafts is 3.58" in diameter. 1986 and later cranks have a 3.00" bolt circle. In addition, all cranks which use a one-piece seal require a counterweighted flywheel for proper engine balance.
Many of the flywheels listed in Group 0.666 are designed for heavy-duty and pre-1986 production crankshafts with a 3.58" diameter flywheel bolt pattern. This larger bolt pattern is recommended for competition engines. If your engine has a crankshaft with a one-piece rear seal, you must use a flywheel (of flexplate) with a 3.00" bolt pattern. Flywheels and flexplates for these cranks are available in 12-3/4" and 14" diameters.
The manual transmission flywheels and automatic transmission flexplates shown in the accompanying charts can be used with Chevrolet small-block V8 and 90-degree V6 engines. When selecting a flywheel or flexplate, be sure to match the outside diameter, flywheel flange bolt pattern, and clutch/converter diameter to your application.
GM Performance Parts offers lightweight nodular iron flywheels for pre-1985 and 1986-up engines. These lightweight flywheels reduce an engine's rotating inertia and improve throttle response. They were originally installed on many high-performance Chevrolets.
All 400ci small-block Chevrolet V8's are extremely balanced, and must use a counterweighted flywheel (or flexplate) for proper engine balance.

Be sure to specify before or after 1986 if ordering small block Chevy flanges.

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