TC to flexplate issue

[Daddy Bartholomew]

Swapped engines between my 1971 and a 1978 4x4.

1971 beginning setup: 350 cid, 700R4 (originally TH350).

1978 beginning setup: 400 cid, TH350.

400 went into 1971 with no issues, bolted onto 700R4, have been driving it for a week or two with no troubles.

Just dropped the 350 into the 1978 yesterday, bolted onto TH350 pretty easily.

Went to bolt the torque converter to the flexplate today, and discovered the torque converter is jammed tight to the flexplate or the crankshaft. Lookly closely, there appears to be a very narrow gap between the block and the transmission on one side. Turning the flexplate by hand causes the torque converter to rotate.

I haven't run across this issue before. Both engines are for automatic transmissions, they both were for 1970s era TH350s. I didn't remove the flexplates from either engine. As near as I can tell, the nose of the converter is jammed against the crankshaft. The converter is seated against the pump fully, there is space between the converter and the flexplate at the bosses.

Is there a substantial difference in the 4x4 converter? What else might be the problem?

[gchemist]

The problem is different flexplates. The 400 is externally balanced. It has weights added to it. The 350 is internally balanced. The flexplate is neutrally balanced.
Torque converters also come in different sizes. Stall for a 4X4 is higher than a 4X2.

[ont_guy_38]

sounds like your converter has slipped out of the grooves , do not force it , take the engine back out and reset the convertor back in and hopefully the tabs on the tranny pump are not broken already,




K read it again and are the tabs that the bolts go through touching the flywheel?

[Daddy Bartholomew]

Originally Posted by ont_guy_38

sounds like your converter has slipped out of the grooves , do not force it , take the engine back out and reset the convertor back in and hopefully the tabs on the tranny pump are not broken already,




K read it again and are the tabs that the bolts go through touching the flywheel?

No, there is a small gap between the tabs and the flexplate (or it looks like there should be, since they actually aren't even lined up. I had just finished bolting the trans to the engine last night and quit for the night without bolting the TC up.). Shining a flashlight between the TC and the flexplate, the only point of contact is at the crankshaft. The TC is seated fully - any further back, and the body of the TC would come into contact with the body of the transmission. There is only an 1/8th of an inch or less free space between the body of the TC and the transmission.

The nose is just too big, as near as I can see. The engine used to be mated to a TH350 before I swapped in the 700R4, so it can't be the crank.

I do recall seeing a yellow number marked on the TC when it was out - perhaps the PO put in a junkyard converter, so who knows what I've got now...

[80SPORT]

I think you are going to have to pull the engine back out and check your crankshaft hub;could be some corrosion causing a tight fit...is there any possibility there was a pilot bearing for a standard trans in your crank?

Pull the torque converter out and check it on the crank/flexplate;also change the torque converter front pump seal if you have not done so already

[Daddy Bartholomew]

I pulled it out again, and it appears to me that the lip on the hub of the TC could be ground down just a bit, eliminating the interference.

It is definitely a junkyard converter - there's an "81" stenciled on it as well as "GM3". It appears to be the converter NAPA lists for a 350/TH400 combination. On the other hand, NAPA doesn't list a 350/TH350 application...

Anyway, can anyone think of a reason why grinding the lip slightly could be a problem?

[78maliburat]

looks like a pilot bushing to me

[Daddy Bartholomew]

Those are terrible pictures. I blame my wife, who was napping at the time.

The inner diameter on the crank mics at 1.698 or so, the outer diameter of the TC at 1.701. There's variance, it's not as accurate as I'd like, but coupled with the scratches around the inner bevel of the crank, it sure looks to me like I just need to clean up the surfaces a bit so the TC will draw into the crank. Right now, it's just catching at the outer edge of the opening.

**********

And, it worked. I used some heavy grit sandpaper around the outer lip and inner bevel, and smoothed it to the point where the TC will slide into the crank with little difficulty, whereas before I started it just wouldn't slide in at all. It should bolt up smoothly now.

Thanks, all. I was just really hoping I didn't have to pull it out again, but no such luck. I will be looking for this type of situation in the future, though.