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I just got my TH400 rebuilt, I've put it back under the car but I'm having trouble putting back the yoke. The yoke has a key alignment, but on the transmission there is none. I tried so an hour to see where the key is on the transmission but could not find one. I don't feel like hammering it in, cause I don't really think that's how it goes.
Can someone tell me if in fact there is a key on the rear end spline of the transmission please.
Yoke with the alignment key at the top. transmission spline.
Shouldn't be a index on the transmission but the yoke needs to slide as it will move around on the shaft during operation (a little)
Is it a new yoke or was it on the car?
Check for burrs on the trans or yoke, should just slip on without issue (within reason) definitely not need a hammer or anything
M
H-m-m-m-m-m. I have never seen that. Not that I ever looked real close.
Alignment keyway. I don't see the purpose unless it has something to do with balancing from the factory. That would insure its put back the same way every time.
Is that a TH400 only thing?
Shouldn't be a index on the transmission but the yoke needs to slide as it will move around on the shaft during operation (a little)
Is it a new yoke or was it on the car?
Check for burrs on the trans or yoke, should just slip on without issue (within reason) definitely not need a hammer or anything
M
it's the original yoke that came off the car, it slides in only about 1 inch then it's as if it bottoms out but it seems only half way in.
If it slips in that far I'm gonna say it's probably getting caught up on the bushing
There's a bushing in the tail that's about that deep, it might just be going into the bushing but catching on the rear of the bushing, make sure the edge of the chamfer on the yoke where it meets the OD is smooth, put some trans oil or a little grease and give it a little more of a shove and see if it doesn't go (still shouldn't need a hammer but give it a good thump with your hand. If you really have to start banging on it then stop, something is wrong and you may need to remove the tailshaft to have a look
M
Is it possible the rebuild shop put one of their splined shafts in your transmission. Does the stamped numbers on the transmission they returned to you match the partial number sequence of your car (did they give you a different transmission?)?
I've never had to align a "key" on a GM automatic trans. The end of the splines on the output shaft look burred/damaged. to me.
Some yokes have to be precisely aligned with the splined shaft and then tapped on with a dead-blow (runner) mallet, but sliding on an inch or so is unusual.
You could buy a tube of Prussian Blue Dye (paste) to lightly coat the inside of the yoke splines to see where the metal-to-metal rubbing happens.
If the mating parts are shown in your two photos above, there should not be any problem with them going together. However, as mentioned above, your yoke journal may not be piloting into the rear bushing as it should. Use a bright (LED) penlight to inspect the entry edge of that bushing to see if it has been damaged, causing a 'high metal' condition which would interfere with smooth yoke assembly. If no problem is noted, you may just have to LIFT (push upward) the yoke as you install it to make sure the trans output shaft is on-center with that bushing (which is pressed into the output housing). The output bushing puts the yoke/shaft on center...but with the yoke removed, that shaft will drop off-center.
Yup that did the trick, tapping it lightly with a rubber mallet. I got the same response for the shop that rebuilt my transmission.
Thank you all for your help, cheers
I just got my TH400 rebuilt, I've put it back under the car but I'm having trouble putting back the yoke. The yoke has a key alignment, but on the transmission there is none. I tried so an hour to see where the key is on the transmission but could not find one. I don't feel like hammering it in, cause I don't really think that's how it goes.
Can someone tell me if in fact there is a key on the rear end spline of the transmission please.
Yoke with the alignment key at the top.
transmission spline.
I happen to be readying for dropping the BB engine and TH400 trans back in my '72. Today I looked at both the original yoke, after installing a new u-joint, and the transmission splined output shaft. If I had taken photos they would be duplicates of what you posted above: same "keyway" in the yoke and same lack of one on the shaft. Curious. Perhaps the yoke is somewhat universal for other applications.
That (vendor-supplied) yoke is likely used on many applications OTHER than for GM. Perhaps some of those others required the indexing feature. Having it on the yoke does not impact GM's non-indexed shaft, whatsoever.
C3 TH400 yoke issue
