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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

Large Cat. Image
Modifier      
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.   Small Image
 

Modifier      
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.   Small Image
 

Modifier      
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.   Small Image
 

Modifier      
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.   Small Image
 

Modifier      
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.   Small Image
 

Modifier      
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.   Small Image
 

Modifier      
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.   Small Image
 

Modifier      
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.   Small Image
 

Modifier      
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.   Small Image
 

Modifier      
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.   Small Image
 

Modifier      
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.   Small Image
 

Modifier      
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.   Small Image
 


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.

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