Help! 454 flex plate does not fit TH350

[DirtyDYT]

‘85 454 from C30 TH400 into ‘79 TH350 truck.

I forgot to measure the hole spacing on the flex plate after being assured by every chevy guy I know that it’s a drop-in.

Well, it’s not.

It only took an hour and a half to get the engine in and bolted to the trans. I was busy patting myself on the back until I noticed the torque converter does not fit the flex plate. There are no “small” patterns, just six “large” pattern holes. Shiiiiiii—

What flex plate do I need? because I only have 6 1/2 hours left to get this thing torn apart again and put back together.

Edit: went to a local trans shop and bought what is most likely a replacement for $30. Better to try and fail than not to try!

Now how does one remove a flex plate with the engine in the truck? Unbolt driveshaft, unbolt trans, back off trans, and crank on the bolts? I couldn’t break them free outside the truck the one time I gave it quick go. I just ended up turning over the engine instead of loosing them. Advice on that?

 

[QBuff02]

Just order or try and find a dual pattern flex plate for your engine. Also, torque converter size makes a difference. if it's a stock converter, it would use the stock hole locations, but if it's a larger stall speed it's likely it will use the smaller diameter bolt pattern. Your best bet is to measure center to center of the mounting lugs across the torque converter itself and that will tell you the flywheel you need. No matter where you go or who you call, that's going to be about the first question you get asked regarding flex plates..

 

[climb-101]

back the trans out a little. so you can get in and work, get the right sized socket and use a dead blow hammer to hit the ratchet, this will help keep the engine from turning and loosen up the bolts. if you used red loctight then you will need to heat them up a little.

or just use an impact gun to make things easy.

 

[DirtyDYT]

Amazingly, Gorby’s transmission shop in town sold me the correct dual pattern flywheel sight unseen. That was a gamble but it worked!

Trans is pulled out completely but still under the truck. New flex plate is on. Going to pick up my son then hopefully reinstall the trans, cross member, drive shaft, and bolt things down with plans for them to stay there.