sbc to th400 flexplate issue

[87ck10]

long story short i had this old timer do a trans swap on my truck from a sm465 to a th400, he did evrything right aside i was getting a rattle in between gears i figured he coudnlt have screwed up the flexplate bolts, but i took the cover off just to see whats up. what do you know theres only 3 bolts out of 6 i think, and the other holes dont line up, wth is wrong here. the flexplate looks new bronze colored with made in US stamped in it. i need some help because hes definitely going to be putting the right one in if someone can give me a summit part number whatever from the right flexplate that would be great, the trans came from a big block its now on a good wrench 350, the part that pisses me off the most is he told me himself he bought the right flexplate and it fits great... i turn wrenches for a living on newer cars but to my knowledge if theres 6 bolts on your torque converter, you put 6 bolts in it lol right?

 

[paratrooper307]

Depending on the flexplate, many of them, particularly aftermarket ones, have a dual pattern drilled to accomodate different torque converters. Some torque converters (such as the one on my 350/700R4) have only 3 mounting points. The replacement Summit flexplate I put in there was drilled for two different converter diameters, and either 3 or 6 hole mounting. Diesels (and probably big blocks as well) usually mounted with 6 bolts.

The flexplate is engine dependent, not transmission dependent. Is your Goodwrench a pre-86 or post 86 motor? Pre-86 is the two-piece seal, post-86 is a one-piece. Each one requires a different flexplate.

 

[4X4HIGH]

There are two different convertor bolt patterns. You can buy a flexplate that has BOTH patterns on the flexplate but only 3 bolts will be used (sounds like this is what's going on).

 

[Greg Ducato]

You don't need to run all six, three bolts is plenty, the other three can go along for the ride, most 400's, all 350's, 700R4's etc only used three bolts. The bronze color you noted means it is a heavy duty flywheel, maybe even an SFI type, it is made of thicker material and has more welds holding the ring gear in place. It is much less prone to flex, one more reason the three bolts will suffice.