833 popping out of 4th

I have '68 A body 4 speed in my Mirada the last few years. Last summer it started popping out of forth occasionaly only on decelleration and has gradually gotten worse. It isn't quite so bad when the trans is warmed up though.

From what I've read this is likely caused by the center bore of bellhousing being out of alignment with the crank. (I didnt check it for run out at the time, just put it together) WHich may put the input shaft at a slight angle causing wear on the engagement teeth for 4th on the input shaft and/or 3-4 shift collar.

I plan to pull the gearbox and check and fix the bellhosuing alignment if necessary. My question is it likely I'll need parts in the gearbox too? input shaft etc? ANy other thoughts on what it could be? I checked the shift rods alignemnt and it seemed fine, I even tried adding a little more length on the 4th gear shift by taking some from 3rd. but it didn't make a difference that I could tell.

What I have seen when they are way off is the end of the output shaft wears where the needles ride, the needles, the input bearing, and if ran long enough the input gear and cluster gear polish up pretty good too. You will also see a heavy polish where the pilot bearing rides. A trans that is dialed in looks like new at the pilot bearing. It used to be a big problem with blowproof bells, and early hemi swaps.

How is the shifter linkage, sloppy? Have you tried adjusting it? Before you pull the trans I would try that.

Most common cause is worn clutching teeth on the 4th (input shaft) and the 3-4 slider. I would first check the obvious shifter linkage adjustments but if it goes in solid but pops out as soon as you let of the gas it probably needs a new input and slider
Gus

Now I'm NOT an engine/trans guy BUT back in 1976-77 my old 340 Cuda 4-speed was doing this. I had a trans guy go through it & said it was the synchros.

^^^ That was always the deal back in my T-10 days, usually showed up in 2nd gear first...

This, if the shifter is correctly adjusted the dog teeth are most likely worn.

I agree

I have a photo to back this up. My transmission (in my 71 Challenger) kept popping out of 4th and I finally got it into my mechanic and this is what he found. I was fortunate in that it didn't damage the trans case but it did require an entire rebuild because of all the metal in the tranny. In my photo, you can see the teeth were wearing against the case when the input shaft was shoved forward. I don't remember all the details as this happened in 1973-74. It cost me almost $200 back then. I now have a great clutch alignment tool though. Get it taken care of as soon as possible. Note on the gear teeth how it was rolling over the edge of the teeth. It is cut back into the gear about 1/4".
Terry

I have also seen the shifter body bolts come loose and cause issues like this, including not being able to shift.

Just to update on this...

A few weeks back I jack the car up to pull the gearbox out and see whats going on. After I pull the driveshaft I look up and see one of the trans to bellhousing bolts is missing, and the others are loose.

There was almost a 1/4" gap opened up between the bell and trans. I find another 7/16" bolt, tighten them up, stick the driveshaft back in and test drive it. No more popping out of forth.

I should have checked but it didnt even occur to me that it would be something like that.
Now I'm sure the input shaft being at an angle like that didn't do anything any good, but I've been driving it since then a lot and it seems fine so far.