Flywheel balance SBC 1-piece rear main
Posted: Fri Jun 15, 2018 4:32 pm
I have three flexplates (flywheels) for a SBC 1-piece rear main seal engine. The engine is a '98 Vortec 5.7L, if that makes a difference. These guys have a big welded-on weight of unusual shape. I dropped them off at my favorite machine shop to have him compare the balance.
Short story: A broken GM flywheel, and two non-broken never-been-installed aftermarket replacements show WILDLY different offset weight.
First flywheel is a Genuine GM, with the center broken. It's cracked 360 degrees around the flywheel bolt pattern--a common problem. The center is loose, can be wiggled around, but won't actually come free from the rest of the flexplate. My friend says the flexplate wobbles around some on the balancer, but shows "about" 180 grams of out-of-balance. Given that the flexplate is broken, I'm not going to trust that reading to be accurate.
Second flywheel is an unknown brand, with lovely gold-colored plating and a SFI sticker that may be genuine, or may be a Communist Counterfeit. The guy I bought it from says it came included as part of a SCAT stroker-crank kit. Welded-on weight looks about the same as the Genuine GM, but not identical. This one shows 130 grams of out-of-balance. Does not appear to have been modified.
Third flywheel is a Pioneer FRA-159 I bought from Summit. Welded-on weight looks like it was carved-out on primitive equipment, the thing is generally the same shape as the others, but it's got "thicker" sections, like the machine cutting it couldn't make tight curves. It appears to weigh MUCH more than the OEM weight, and then the manufacturer drilled a half-dozen lightening holes into the flexplate on either side of the weight. This one shows 210 grams of offset.
https://www.summitracing.com/parts/pio-fra159
The machinist tossed another Genuine GM flywheel from a core 5.7L one-piece-seal engine onto the balancer, it's reading about 187 grams--fairly close to my OEM broken flexplate. My wild guess is that the OEM flexplate is supposed to be 185 grams out-of-balance, plus-or-minus 5 grams for production tolerance.
I forgot to ask if the offset weight (counterweight) ANGLE is the same relative to the dowel pin hole. Wouldn't surprise me to find that that's different among my three flexplates, too. I'll have to inquire about that next time I talk to him.
WHAT IS THE SPEC FOR THE OFFSET (Counterbalance) WEIGHT FOR A SBC 5.7L ONE-PIECE REAR MAIN SEAL FLEXPLATE/FLYWHEEL? Am I right in thinking it should be about 185 grams off?
Do all one-piece rear main SBC engines specify the same amount of out-of-balance? I know there's a difference in the big-blocks, depending on cast or forged crankshaft. As far as I'm aware, all SBC one-piece engines--5.0L or 5.7L, TBI or Vortec, '86 to current production--take the same amount of flywheel counterweight. Am I wrong?
Short story: A broken GM flywheel, and two non-broken never-been-installed aftermarket replacements show WILDLY different offset weight.
First flywheel is a Genuine GM, with the center broken. It's cracked 360 degrees around the flywheel bolt pattern--a common problem. The center is loose, can be wiggled around, but won't actually come free from the rest of the flexplate. My friend says the flexplate wobbles around some on the balancer, but shows "about" 180 grams of out-of-balance. Given that the flexplate is broken, I'm not going to trust that reading to be accurate.
Second flywheel is an unknown brand, with lovely gold-colored plating and a SFI sticker that may be genuine, or may be a Communist Counterfeit. The guy I bought it from says it came included as part of a SCAT stroker-crank kit. Welded-on weight looks about the same as the Genuine GM, but not identical. This one shows 130 grams of out-of-balance. Does not appear to have been modified.
Third flywheel is a Pioneer FRA-159 I bought from Summit. Welded-on weight looks like it was carved-out on primitive equipment, the thing is generally the same shape as the others, but it's got "thicker" sections, like the machine cutting it couldn't make tight curves. It appears to weigh MUCH more than the OEM weight, and then the manufacturer drilled a half-dozen lightening holes into the flexplate on either side of the weight. This one shows 210 grams of offset.
https://www.summitracing.com/parts/pio-fra159
The machinist tossed another Genuine GM flywheel from a core 5.7L one-piece-seal engine onto the balancer, it's reading about 187 grams--fairly close to my OEM broken flexplate. My wild guess is that the OEM flexplate is supposed to be 185 grams out-of-balance, plus-or-minus 5 grams for production tolerance.
I forgot to ask if the offset weight (counterweight) ANGLE is the same relative to the dowel pin hole. Wouldn't surprise me to find that that's different among my three flexplates, too. I'll have to inquire about that next time I talk to him.
WHAT IS THE SPEC FOR THE OFFSET (Counterbalance) WEIGHT FOR A SBC 5.7L ONE-PIECE REAR MAIN SEAL FLEXPLATE/FLYWHEEL? Am I right in thinking it should be about 185 grams off?
Do all one-piece rear main SBC engines specify the same amount of out-of-balance? I know there's a difference in the big-blocks, depending on cast or forged crankshaft. As far as I'm aware, all SBC one-piece engines--5.0L or 5.7L, TBI or Vortec, '86 to current production--take the same amount of flywheel counterweight. Am I wrong?