First off, it's not for a Camaro but I know someone out there knows.

I was trying to install a 700r4 on my brothers '83 C20. He blew up the TH400 that was on it a week or two after he bought the truck. When we removed the 400 we noticed a few things:

1) The flexplate (168 tooth, externally balanced, single dowel-hole) mounting flange had completely sheared off and the flexplate was basically spinning around it.

2) The rear main seal was a one piece. The block turned out to be an 86-88 350 and none of the parts that my brother bought fit (we had assumed it was pre-86).

3) There was no evidence of any weight on the harmonic balancer.

Keep in mind I test drove this truck before he bought it and I didn't notice anything wrong with it. But, my brother mentioned that after he brought it home, he noticed a lot of vibration in the drive-train. The tranny puked out shortly thereafter.

So my question is, did 86 and up sbc's come externally balanced and I just can't see the weight on the balancer, or is it possible that the guy who sold it to my brother (who has since magically disappeared) swapped out the tranny and flexplate before my brother picked up the truck (it took him a while to pay the guy in full)? Should he buy an internally balanced flexplate or externally balanced to install the 700r4?

hardline,,
the bolt flange nub/hub and bolt circle are different for the one piece rear seal blocks compared to the older 2 piece seal blocks.

the one piece rear seal blocks use a weighted flexplate/flywheel and a neutral balanced damper up front as stock. the 2 piece is neutral in front and rear for the 350's and smaller.

you'll need to use the correct starter also,, for the flexplate teeth count that you plan on using. 153 tooth flexplate with the straight across bolt pattern starter,,, or the diagonal bolt pattern starter with the 168 tooth flexplate. hope it helps.

hardline,,
the bolt flange nub/hub and bolt circle are different for the one piece rear seal blocks compared to the older 2 piece seal blocks.

the one piece rear seal blocks use a weighted flexplate/flywheel and a neutral balanced damper up front as stock. the 2 piece is neutral in front and rear for the 350's and smaller.

you'll need to use the correct starter also,, for the flexplate teeth count that you plan on using. 153 tooth flexplate with the straight across bolt pattern starter,,, or the diagonal bolt pattern starter with the 168 tooth flexplate. hope it helps.
Thanks Rat, that's exaclty what I needed to know. The original flex plate is definitely the 168 tooth with the dual TC pattern so I'll stick with that.

your welcome. good luck with it.

It was the "fan" on the back of the crankshaft's that allowed them to not be externally balanced is all..

265 and early 283 were round, 302 had a notch out, 327 had a small fan and finally the 350 had a full fan on the flange is all.

The differences in all the flanges are real apparent(Sp?) when you look all the cranks over!!

pdq67