Flywheels and Flexplates

Chevy Small-Block V8 | Flywheels and Flexplates

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.

Quick Reference Charts


10105832	Flywheel
Outside diameter is 14" for years 1986-up. Crank Flange bolt pattern: 3.00"; clutch dia.: 11", 11.85"; starter ring gear teeth: 168. For one-piece crank seal.



12554824	V8 & V6 / 90 degree Automatic Transmisison Flexplate
Outside diameter is 14" for years 1986-up. Crank Flange bolt pattern: 3.00"; Converter Bolt Pattern.: 11.50"; starter ring gear teeth: 168. Heavy-duty flexplate with increased thickness for one-piece crank seal. Torque converters for TH350 and TH400 transmissions are produced with both 10.75" and 11.50" diameter converter bolt patterns. To identify the correct pattern, measure from the converter center to an attaching bolt hole and then multiply this dimension by 2.



14085720	Flywheel
Outside diameter is 12 3/4" for years 1955-85. Crank Flange bolt pattern: 3.58"; clutch dia.: 10.4"; starter ring gear teeth: 153. Lightweight nodular iron flyweel. Weighs approximately 15 pounds. For two piece crank seal.



14088646	Flywheel
Outside diameter is 12 3/4" for years 1986-up. Crank Flange bolt pattern: 3.00"; clutch dia.: 10.4", 11"; starter ring gear teeth: 153. Lightweight nodular iron flyweel. Weighs approximately 16 pounds. For one-piece crank seal.



14088650	Flywheel
Outside diameter is 12 3/4" for years 1986-up. Crank Flange bolt pattern: 3.00"; clutch dia.: 10.4"; starter ring gear teeth: 153. Standard weight flywheel for one-piece crank seal.



14088761	V8 & V6 / 90 degree Automatic Transmisison Flexplate
Outside diameter is 14" for years 1986-up. Crank Flange bolt pattern: 3.00"; Converter Bolt Pattern.: 10.75", 11.50"; starter ring gear teeth: 168. For one-piece crank seal. Torque converters for TH350 and TH400 transmissions are produced with both 10.75" and 11.50" diameter converter bolt patterns. To identify the correct pattern, measure from the converter center to an attaching bolt hole and then multiply this dimension by 2.



14088765	V8 & V6 / 90 degree Automatic Transmisison Flexplate
Outside diameter is 12 3/4" for years 1986-up. Crank Flange bolt pattern: 3.00"; Converter Bolt Pattern.: 10.75"; starter ring gear teeth: 153. For one-piece crank seal. Used with 5.7-liter HO engine. Torque converters for TH350 and TH400 transmissions are produced with both 10.75" and 11.50" diameter converter bolt patterns. To identify the correct pattern, measure from the converter center to an attaching bolt hole and then multiply this dimension by 2.



3986394	Flywheel
Outside diameter is 14" for years 1970-80. Crank Flange bolt pattern: 3.58"; clutch dia.: 11"; starter ring gear teeth: 168. For externally balanced 400ci small-block V8 only. Use with balancer P/N 6272225.



3991469	Flywheel
Outside diameter is 14" for years 1955-85. Crank Flange bolt pattern: 3.58"; clutch dia.: 10.4", 11"; starter ring gear teeth: 168. For two-piece crank seal. This 14" diameter manual transmission flywheel accepts an 11" diameter heavy-duty clutch. It can be used with all internally balanced Chevy big-block V8 engines.



471529	V8 & V6 / 90 degree Automatic Transmisison Flexplate
Outside diameter is 12 3/4" for years 1969-85. Crank Flange bolt pattern: 3.58"; Converter Bolt Pattern.: 9.75", 10.75"; starter ring gear teeth: 153. For internally balanced engine with two-piece crank seal. Torque converters for TH350 and TH400 transmissions are produced with both 10.75" and 11.50" diameter converter bolt patterns. To identify the correct pattern, measure from the converter center to an attaching bolt hole and then multiply this dimension by 2.



471578	V8 & V6 / 90 degree Automatic Transmisison Flexplate
Outside diameter is 14" for years 1970-850 Crank Flange bolt pattern: 3.58"; Converter Bolt Pattern.: 10.75", 11.50"; starter ring gear teeth: 168. For externally balanced 400ci small-block only. Use with balancer P/N 6272225. Torque converters for TH350 and TH400 transmissions are produced with both 10.75" and 11.50" diameter converter bolt patterns. To identify the correct pattern, measure from the converter center to an attaching bolt hole and then multiply this dimension by 2.



471598	V8 & V6 / 90 degree Automatic Transmisison Flexplate
Outside diameter is 14" for years 1970-85. Crank Flange bolt pattern: 3.58"; Converter Bolt Pattern.: 10.75", 11.50"; starter ring gear teeth: 168. For internally balanced engine with two-piece crank seal. Torque converters for TH350 and TH400 transmissions are produced with both 10.75" and 11.50" diameter converter bolt patterns. To identify the correct pattern, measure from the converter center to an attaching bolt hole and then multiply this dimension by 2.

*Flywheels Quick Reference Chart*
Part Number	Outside Diameter	Year	Crank Flange Bolt Pattern	Clutch Diameter	Starter Ring Gear Teeth	Notes
14085720	123/4"	1955-85	3.58"	10.4"	153	Lightweight nodular iron flywheel. Weighs approximately 15 pounds. For two-piece crank seal.
3986394	14"	1970-80	3.58"	11"	168	For externally balanced 400ci small-block V8 only. Use with balancer P/N 6272225.
3991469	14"	1955-85	3.58"	10.4", 11"	168	For two-piece crank seal.
10105832	14"	1986-up	3.00"	11", 11.85"	168	For one-piece crank seal.
14088646	123/4"	1986-up	3.00"	10.4", 11"	153	Lightweight nodular iron flywheel. Weighs approximately 16 pounds. For one-piece crank seal.
14088650	123/4"	1986-up	3.00"	10.4"	153	Standard weight flywheel for one-piece crank seal

*Chevrolet Small-Block V8 and V6/90 Automatic Transmission Flexplates Quick Reference Chart*
Part Number	Outside Diameter	Year	Crank Flange Bolt Pattern	Converter Bolt Pattern*	Starter Ring Gear Teeth	Notes
471598	14"	1970-85	3.58"	10.75", 11.50"	168	For internally balanced engine with two-piece crank seal.
471578	14"	1970-80	3.58"	10.75", 11.50"	168	For externally balanced 400ci small-block only. Use with balancer P/N 6272225.
471529	123/4"	1969-85	3.58"	9.75", 10.75"	153	For internally balanced engine with two-piece crank seal.
14088765	123/4"	1986-up	3.00"	10.75"	153	For one-piece crank seal. Used with 5.7-liter HO engine.
12554824	14"	1986-up	3.00"	11.50"	168	Heavy-duty flexplate with increased thickness for one-piece crank seal.
14088761	14"	1986-up	3.00"	10.75", 11.50"	168	For one-piece crank seal.