Is a Buick Grand National Flexplate Any Better Than, and Can It Replace, a 1981 231 V6 Standard Flexplate?
QUESTION:
I have a normally aspirated 1981 Buick 231 V6 in my hot rod and was wondering what flexplate to use. It seems there are internal balance and external balance applications. Is there any reason I can’t use a Grand National flexplate that is rated for high rpm? The trans I am using is a TH200-4R. My hope is to eventually turbocharge this motor.
Steve Helwig
Via email
ANSWER:
Aftermarket rotating assemblies with forged cranks are usually internally balanced. All production Buick V6 cast-iron crankshafts are externally balanced. The unbalance factor for 1977-and-earlier odd-fire Buick V6s is different than the factor for 1978–1987 even-fire Buick V6 engines. According to TA Performance, the even-fire production unbalance factor is 36.6 percent.
During the even-fire era, Buick still used different stock flexplates depending on the specific model application. The late-model Buick V6 flexplates I’ve seen all have a 13.42-inch outer diameter with 160 ring-gear teeth. Torque-converter bolt patterns are the same, too. The major difference seems to be the number of lightening holes drilled in the flexplate, which correspond to different rotating-assembly weights. A 1986–1987 Grand National/Regal turbo 3.8L V6 (VIN-7, RPO LC2) came with heavier pistons, so the corresponding flexplate for this app had more lightening holes in it than a normally aspirated V6 (VIN-A) of the same model years. The original 1981 flexplate for your present normally aspirated 3.8L (also VIN-A) is different than either of the 1986–1987 flexplates. There’s no difference between any of these in terms of strength or high-rpm capability.
| Common Buick V6 3.8L V6 Even-Fire Flexplates | |||
| These are the most common numbers that may pertain to Mr. Helwig’s specific inquiry, but there are a number of additional flexplates not specifically listed here. | |||
| Year | VIN Code | GM PN* | Pioneer PN |
| 1981–1982 | A | 25512346 | FRA-134 |
| 1983–1987 | A | 25512348 | FRA-137 |
| 1986–1987 | 7 (turbo) | 25512350 | FRA-147 |
| *Listed for information only; all parts in this column have been discontinued. | |||
Bottom line: If your 1981 bottom end is still stock and has never been rebalanced, use the original flexplate for your application, or there’s a chance there may be a slight vibration. When rebuilding for turbo use (you aren’t really going to use the stock normally aspirated, unreinforced, cast pistons with a turbo, are you?), if retaining the production crank and rods but using different pistons, the machine shop should adjust the internal rotating assembly balance but with the externally balanced flexplate and harmonic balancer mocked up on the balancing rig. In other words, you don’t necessarily need to change to a so-called Grand National flexplate. Note that up front, the turbo GN’s harmonic damper is different than your existing damper because it needs to work with the SFI (sequential fuel injection) crank trigger.
Now if you are using an aftermarket complete rotating assembly that already comes internally balanced, you’d need an aftermarket neutral-balance flexplate.TA Performance is one source for SFI-approved, heavy-duty Buick flexplates: internal-balance 231 V6 and 350 V8, PN TA_2032INT; external-balance even-fire 231, PN TA_2032EXT. When using “generic” aftermarket externally balanced flexplates (not direct replacement for stock), it may be necessary to adjust the engine’s internal balance as discussed above.
Contacts
Duttweiler Performance; Saticoy, CA; 805.659.3648; kduttweiler@earthlink.net
Pioneer Automotive Industries LLC; Meridian, MS; 601.483.5211; PioneerAutoInd.com
TA Performance Products; Scottsdale, AZ; 480.922.6807; TAperformance.com
Ask Marlan A Tech Question: pitstop@HotRod.com
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