1970 M-20 Issue

[Gary C.]

Re: 1970 M-20 Issue

To check reverse fork engagement with reverse gear, you could remove the speedometer cable/fitting/gear and put a borescope (android camera) into the hole to look at the fork & reverse gear. Corvette speedometer hole is on right hand side and points toward top of output shaft so you should have space. Use aluminum tubing (home depot) big enough for borescope but small enough to fit in hole to help position borescope. Shift the reverse lever and see if the reverse gear is moving or not and engaged with shift fork.

1. Yes, agree. But you should not be turning the cluster gear and input, 1st, 2nd, & 3rd. Could remove filler plug and see inside main section when you turn output shaft. Also, reverse gear is a helical gear so if driven by engine, it would thrust one way, if driven by yoke spinning in normal direction, it would thrust other way. Also, reverse gear on output shaft has a spiral spline, not straight spline so this would also move reverse gear axially spinning one direction and opposite direction spinning other direction.

2. With all gears truly in neutral and clutch engaged, the output shaft should turn freely. All gears stationary (input, 1st, 2nd, 3rd, cluster, reverse), locked by input gear thru engaged clutch. As you identify, your output shaft does not spin freely so something is engaged. Could check 1st/2nd & 3rd/4th slider position & reverse thru filler plug and speedometer port using borescope.

3. So when shifted to reverse, no movement of output shaft, obviously, this is not kosher. Could be that reverse gear is disengaging when driven by engine side but reverse gear is engaging when driven by yoke side if reverse gear/fork not engaged. On item 2, did you spin it both directions?

[Ray K.]

Re: 1970 M-20 Issue

Stephen,

I have read all these posts for nearly 30 days and the one from Gary most likely describes the problem. The ring on the reverse gear is not engaged with the slot on the reverse shifter fork inside the rear extension housing. The transmission at some point will have to be removed in order to correct this. Then only the extension housing ( and the side cover ) will have to be removed to gain access to the reverse gear and the reverse shifter fork. Careful re-assembly should correct this issue for you - bench test again as you did before and be sure the trans shifts in and out of reverse without lockup and the output shaft rotates in the opposite direction when engaged in reverse.

Ray

[Domenic T.]

Re: 1970 M-20 Issue

Ray,
I think I did one on the car by removing the taper pin and pulling the reverse arm out enough to move the groove that needs to be on the reverse gear ring is in place. It's a magic act, but as you remember that's what we did during assembly. we carefully lined the gear up and pushed the shifter to the gear ring. Then we moved it to make sure the gear moved BOTH ways, then put the taper pin in.
That's to the best of my memory. Had them towed inand made house calls.

Dom

[Ray K.]

Re: 1970 M-20 Issue

Dom,

Yes, I have used that technique before when it was necessary. I found that generally on a Muncie trans I was able to re-assemble without pulling the pin on the reverse shaft. Borg Warner T-10's were a different story in that it generally always required removing the pin to pull the shaft out slightly and the first try many times resulted in the detent ball and spring popping out of the housing. Second or third try OK, and then carefully re-assemble and check the trans for gear operation.

Ray

[Stephen B.]

Re: 1970 M-20 Issue

Update: I was able to pull the pin and properly set the reverse shaft.

Thanks for everyone's help!

Stephen