M20 "Crunches" While Shifting into 4th

[Mark E.]

1970 M20: To avoid crunching, I double clutch into 4th. Sometimes it won't crunch if shifted very slowly.

1st through 3rd gears shift great going up or down the gears.

I heeded Joe Lucia's advice about carefully adjusting the clutch on these cars due to their limited travel. The clutch is adjusted for as much disengagement as possible with the pedal pressed, while still providing a minimum of free play when engaged (pedal at rest).

It has the proper level of fresh GM Goodwrench Gear Lubricant 80w-90.

Any tricks which may help 4th gear? Additives?

[Patrick T.]

Re: M20 "Crunches" While Shifting into 4th

I believe I may have had the same problem in my Muncie M-22. It was the slider sleeve which was the problem. I replaced it and the transmission no longer crunched.

[Chuck G.]

Re: M20 "Crunches" While Shifting into 4th

I recently had the same problem with the 65 Muncie wide ratio in my 56 Chevy. A friend and I had rebuilt this trans a few years ago with new synchros, sliders, etc.

Started grinding shifting into 4th gear.

Took it out and took it to a transmission shop. Turned out it was the input shaft, which is 4th gear. It was badly worn, but the other gears were equally worn, so I replaced them all. Works like a dream now.

Cost me $1000.00 labor and parts. I did the removal and reinstallation myself.

[Chuck G.]

Re: M20 "Crunches" While Shifting into 4th

PS: I tried synthetic oil, dinosaur oil, additives, etc. and nothing helped.

[Tom P.]

Re: M20 "Crunches" While Shifting into 4th

NOT always, but quite typically, this condition is a result of worn brass syncro rings. In time, the grinding (or crunching) is a result of the engagement teeth/slider splines clashing and increasing the wear. Depending on the amount of wear on the engagement teeth of the gear(s), a rebuild, to include new brass rings and new sliders (I almost always go back with the Tork Loc sliders) should be done. I've been building 4spds (mostly Muncies) for about 50yrs, and I have to admit that these old Muncies are becoming quite tired and well worn. And sometimes (depending on abuse and wear), the best solution is a complete gear set replacement.

[Mark E.]

Re: M20 "Crunches" While Shifting into 4th

Thanks guys. The tranny has over 103k miles and six owners under its belt, so it may be time for an overhaul. Not that expensive as Corvette projects go.