M20 leaks

[Bill L.]

Vette has 110k miles. Transmission is the original with the VIN# stamped on it. It was shifting fine and making no noise.

Is there anything that can be done to fix the leak that appears to be coming from the input shaft? The inside of the bell housing was covered in gear oil. The clutch disc was dry as a bone and there is no evidence of the rear main seal leaking on the motor.

I understand there is no seal there but cannot imagine that the engineers just assume let it leak. Maybe it was overfilled at one point and came out the vent?

Should I replace the M20 rear seal? There did not seem to be much leakage there. I will be replacing the side cover seal and three shift lever seals.

Any advice is appreciated.

Thanks,


Bill

[Joe L.]

Re: M20 leaks

Vette has 110k miles. Transmission is the original with the VIN# stamped on it. It was shifting fine and making no noise.

Is there anything that can be done to fix the leak that appears to be coming from the input shaft? The inside of the bell housing was covered in gear oil. The clutch disc was dry as a bone and there is no evidence of the rear main seal leaking on the motor.

I understand there is no seal there but cannot imagine that the engineers just assume let it leak. Maybe it was overfilled at one point and came out the vent?

Should I replace the M20 rear seal? There did not seem to be much leakage there. I will be replacing the side cover seal and three shift lever seals.

Any advice is appreciated.

Thanks,


Bill

Bill------


On a Muncie there is no input shaft seal. However, the design does include a front retainer with an oil return "trough" and a hole in the case for the oil to return through. The retainer bolt pattern is "asymmetric" so that the retainer can only be installed in one orientation and that orientation has the oil return "trough" and the oil return hole in the case aligning. If for any reason the "oil trough" or return hole gets plugged, then you will definitely get a continuing "leak" from the front of the transmission.

There are two other things that can cause leaks from the front of Muncie transmissions:

1) wear in the threads in the main case for the front retainer bolts;

2) distortion of the countergear shaft hole in the front of the main case.

Muncie transmissions in long service often develop one or more of the above maladies.

[Domenic T.]

Re: M20 leaks

Bill,
If you suspect a overfill then check the oil level. The book says 1/2 " below the filler hole (cold), but it was found that was to high as the warm oil will thrust forward during braking and out the retainer. The drain hole can only drain so much.
It is said that if your pinkie can touch the oil that it is full enough.

DOM

[Bill L.]

Re: M20 leaks

Bill,
If you suspect a overfill then check the oil level. The book says 1/2 " below the filler hole (cold), but it was found that was to high as the warm oil will thrust forward during braking and out the retainer. The drain hole can only drain so much.
It is said that if your pinkie can touch the oil that it is full enough.

DOM

I will make sure when I refill that it is not over-filled. It is out to replace side cover gasket and shift lever seals. Shifted fine so if it still weeps a little I will live with it.

Thanks, Bill

[Bill L.]

Re: M20 leaks

When I replace the side cover gasket should I use sealer of any type or just torque to spec?

Should I use thread sealer on the bolts?

Do I use any sealer on the TCS switch threads?

[Joe L.]

Re: M20 leaks

When I replace the side cover gasket should I use sealer of any type or just torque to spec?

Should I use thread sealer on the bolts?

Do I use any sealer on the TCS switch threads?

Bill------


I use a very thin film of RTV on both sides of the side cover gasket. If the sealing surfaces on the cover and main case are perfect, you might not need it but few of these are perfect. The sealer, if applied as described, will not cause any problem.

Definitely apply sealer to the bolts. Many of these go through to the lube-filled area of the main case.

I would not apply sealer to the TCS switch as it might cause a grounding problem. However, definitely use a new gasket.

[Bill L.]

Re: M20 leaks

Thank you Joe.

Bill

[Joe L.]

Re: M20 leaks

Thank you Joe.

Bill

Bill------


By the way, the TCS switch gasket is GM #3929530 and it's still available from GM.

[Bill L.]

Re: M20 leaks

Bill------


By the way, the TCS switch gasket is GM #3929530 and it's still available from GM.

HI Joe,

GMDirect says it is a back up lamp gasket?

Thanks, Bill

[Joe L.]

Re: M20 leaks

HI Joe,

GMDirect says it is a back up lamp gasket?

Thanks, Bill

Bill------


Yes, the back-up lamp application is its "group of prime use" (although it was not used on a Corvette for this application). However, it's also used for the TCS switch application.

[Bill L.]

Re: M20 leaks

Hi Joe,

One more question. What is the GM part number for the shift lever seals? Google was not my friend today.

Thanks, Bill

[Joe L.]

Re: M20 leaks

Hi Joe,

One more question. What is the GM part number for the shift lever seals? Google was not my friend today.

Thanks, Bill

Bill-----


Side cover shaft seals are GM #3831716. This is a part that's been continuously available from GM for 50 years!

[Bill L.]

Re: M20 leaks

Bill-----


Side cover shaft seals are GM #3831716. This is a part that's been continuously available from GM for 50 years!

Wow! Do not see that ofter.

Thanks, Bill