Which flexplate

wont work · Oct 9, 2013

I have been trying for a week now to find which flexplate I need. What I have is an 8.1 496 motor and mating it to a 400 trans. I have looked up part numbers from all over the place and ordered one from summit that did not work. Now I have one from the parts store that is the same problem. Neither will bolt up to the crank but both say they fit the 8.1 motor. Both plates in the center are to small to fit around crank.

Anybody have a good guess on which flexplate I need?

al1tonyota · Oct 9, 2013

Stock flex plate that is for a 4l80 will work some have to use a spacer(washer) to get correct tc spacing

wont work · Oct 9, 2013

al1tonyota said:
Stock flex plate that is for a 4l80 will work some have to use a spacer(washer) to get correct tc spacing

Washer on converter bolts or crank side?

leeman1790 · Oct 9, 2013

If its the same concept as bolting a th350 to a 5.3, GM parts sells a machined spacer that works to support the center of the TC.

al1tonyota · Oct 9, 2013

wont work said:
Washer on converter bolts or crank side?

convertor bolts but you need to check the amount of play first if there isn't more than 3/8" no new for a spacer

wont work · Oct 10, 2013

I hope I have found one that works this time. A GM flexplate is $370 and only rated for stock motors. Finally talked to a guy today at the place pasted below that seemed like he knew what he was talking about. Even learned a few new things. Even asked me to measure items to make sure I was getting the correct part. Its ordered and we will see if about a wk if it will fit. Also only cost $140 shipping and is rated for 600 or more Hp and is drilled to accept any type GM tranny setup.

http://www.prwonlinestore.com/

95limitedzj · Oct 11, 2013

http://www.summitracing.com/parts/hup-hp3795
What I used to get the TH350 TC to fit with the 5.3 crank/flexplate. If it's the same on the 8.1... I'm not sure...

al1tonyota · Oct 11, 2013

8.1 has th flywheel pattern of an old big block

monster · Oct 17, 2013

is the 8.1 internaly balanced or externaly balanced?

wont work · Oct 21, 2013

monster said:
is the 8.1 internaly balanced or externaly balanced?

Internally

Got this together finally and didnt need a spacer.

al1tonyota · Oct 21, 2013

What did you end up going with?

wont work · Jan 13, 2014

al1tonyota said:
What did you end up going with?

This is still trying to be fixed.

What I have learned is the 496 and 454 Gen V share the same block just different everything else. What we did was built a motor with the Gen V block, 496 head, 454 internal parts, and 454 intake and distrubutor.
What all the amounts to is a big block with lots of low end power using part from this and that. It also amounts to a nightmare when finding parts at the parts store because I dont know what color the truck is is cam out of.
To make matters worse GM has 6 flexplates that fit a 454 depending on year, crank, and rear main seal. I know mine is 91-96 and 1 piece rear main but it has still been a nightmare to find this. Also with the 454 internals it makes it externally balanced. Would not have known it for how smooth it ran even being all solid mounted drivetrain.

I have ordered another plate and hope to have this back going in the next week or so.

For future you cannot bolt a factory flexplate for a 8.1 or 454 to a turbo 400. The wholes are not the same and only aftermarket dual patterns will fit.

ALLJ · Jan 13, 2014

I have a friend that has one these motors its been a nightmare for him getting parts to work on it he said they are not worth all the trouble he said to try Rylar Gas they may be able to help you