This is happening with motor out of car on hoist/floor in garage.

Original setup:
70's 350 010 block with stock flexplate, stock converter, and th350.
New setup:
Today switching to th400, Hughes converter, and new flexplate. Same block.

Once installing the new gear the motor will not spin at all...I feel as though the crank bolt will snap before the motor would spin. Upon removal I suspected the flexplate as it was originally intended for a 427BBC. In side by side and back to back comparison they are virtually identical 168 T flexplates. Same bolt patterns etc. and mirror images of each other.
With only the flex on the motor spins easily. Adding the hughes converter still spins easily. With the trans almost snug it spins decently---THEN when I tighten the th400 down all the way--seized completely!
Not to confuse people--I mounted the trans with converter as a package--I did not leave the converter on and "slide" the trans over it--I am not that strong! The hughes has slightly thicker mounting pads where I mount the bolts, but seems that is the only difference.
Am I missing something? I am at a total loss.
I mounted and removed this beast 4 times today and finally gave up when the legs were shaking and lower back was fried!
Tomorrow I am going to attempt to use the stock converter with new th400 and new flexplate to eliminate one of the culprits. Could I be off by thousandths of a millimeter with this flexplate??

Thanks in advance.

Sounds like the converter is not fully seated on the trans input shaft. Spin and push on the converter and it should engage the last set of splines.

Howie,
When the trans is bolted to the engine, you should be able to pull the converter away from the flexplate at least 1/8", usually more. Like Steve said, it sounds like the converter is not all the way into the trans.

I had a TH400 in my race car that the only way I could get the converter all the way in was to stand the whole mess up on end and spin the converter while pushing down on it !

With two of you suggesting the same thing I will wake up and let her rip! I never even thought of having to stand her up like that---but if that's the go I will get a buddy and go to work.
The first time I mounted it I am now certain I DID NOT have it seated well, and now I am guessing I never did.
Thanks for the replies! Will post again tomorrow with news!

Cheers

Howie, make it three of us. You'll get it.

The smallest things bite us on the arse sometimes. I stood the trans up against my thighs...and the converter dropped on snug! Thanks guys for helping me keep it so simple! The motivation is surging now. Motor/trans are back as one:thumbsup: