Lad's while I was building the the motor I sent me gear box for a service , new seal and bearing the box was in good condition the only other thing I did was fit a new needle roller side cover with cam levers thumbs box went back in then the motor went back in and now the gear box is leaking form around the front main input shaft (not the lay shaft) my pretty sure box is a M20 = 2.52 1.88 1.46 1.00 wide and the gear box place is saying the front shaft has no seal and uses a slinger type arrangement and they can't do anything to stop the leak ? Whats your thoughts ?? I really wanted to stay with Muncie 4 speed...

My Vette's spec's are : 350/350 HP now rebuilt with 425 H.P. & 480 Ft/Lbs Torque engine. Casting #3932386. Date code J28. Engine pad #19S704264 V1004HW.
4-spd M20 transmission with the VIN 3925660 19S704264 stamped on the Aluminum Housing, Casting 30088336, POP 23.

Rear end code, AS81269. 3.70:1 rear end gears.

Jethro in Australia

 

The only external venting of a Muncie 4 speed is through the input shaft. If you have a failing output shaft seal ( probably due to the rear housing bushing starting to fail) you will begin to have leakage at the input shaft. There is a oil slinger plate and the front input shaft nut act as guards against allowing gear lube leakage. There are also the 3 shifter shaft input seals, 2 in your side cover and 1 in the rear housing. The seal failure has to be pretty bad before a Muncie will start leaking out the input shaft. Auto gear makes a new style input shaft and throwout bearing collar with a seal on it but stock Muncie 4 speeds rarely have problems if everything is in great condition. Vintage road racers benefit from the Autogear components. I have Auto crossed a M22 equipped C3 for years with no leakage problems from its GM parts.

My other thoughts are did someone add a vent to the main case? There is a slight potential that the oil return passage is blocked at the throw out bearing collar or the transmission is grossly overfilled, pretty hard to overfill a Muncie but, I have seen it done before.

 

The only external venting of a Muncie 4 speed is through the input shaft. If you have a failing output shaft seal ( probably due to the rear housing bushing starting to fail) you will begin to have leakage at the input shaft. There are also the 3 shifter shaft input seals, 2 in your side cover and 1 in the rear housing. The seal failure has to be pretty bad before a Muncie will start leaking out the input shaft. Auto gear makes a new style input shaft and throwout bearing collar with a seal on it but stock Muncie 4 speeds rarely have problems if everything is in great condition. Vintage road racers benefit from the Autogear components. I have Auto crossed a M22 equipped C3 for years with no leakage problems from its GM parts.

 

Is the front bearing retainer clocked correctly?
I've heard if you put the front input shaft nut on backwards, it will leak.

 

If the countershaft is slightly loose in the case it can leak there too.

 

Ok guys I am having the same problem.

If the rear seal/bushing is replaced along with the side cover seal and shaft seal should that pretty much cure the input shaft leaking?

What is involved in changing the rear bushing and seal?


Thanks,


Bill

 

What fluid are you running in the Muncie? Some synthetics and lighter gears oils are known to creep out the front and leak through the input shaft. A good oil to be running is the Sta-Lube Gear Oil, SAE 85W 90. A lot of the high performance and Muncie rebuilders will recommend that oil.

 

The muncie uses a hydro seal on the input shaft. Just a fancy name for saying there is not a rubber seal. The input shaft nut, by theory, rides very close the nose cone preveting gear lube from leaking past it. Any damage to the nose cone or the nut will cause a leak. If the shop doesn't have the special wrench for installing the nut they may have damaged it. Installing the nose cone upside down or not using a gasket will also cause it to leak. May sure none of the 4 mounting bolts have stripped threads. Make sure there are not lock washers on the four bolts. The bolts must have the lockplates under them. They act as seals to prevent fluid from walking past the threads.
Mike

 

Based on a visual inspection the M20 has never been apart.

I am thinking at 110k miles the countershaft hole on the case is a bit worn.

If that is true I am just going to drive it. I am not in a position to spend a lot on a transmission rebuild.

It was fully functioning and made no noise when removed. It just has lots of leaks.

Bill

 

After several jumpy starts and stalls, I asked my friend to turn back over my 63 w/ M20 muncie. The car began leaking trans fluid from the front of the trans between the bell housing after the abuse. The clutch now slips a little the first two releases of a day out. I decided to read the forums and pulled the motor out with the bell housing. Inside the bell house seemed saturated.

I took a pic and adied to library. Upon inspection, I did not see a retaining nut. I also saw dents on a washer-looking thing (hydro seal?) that also appears to be cross threaded on the input shaft. Do I need to rip-off the hydro seal, replace it and get a new retainer nut? Should I get a new retainer cap too considering the recent abuse? I can find a hydro seal or retaining nut on ebay or common online trans shops. Any recommendations and/or advise?

 

The nut part number is '591150' If you get a thin, cheap Muncie nut wrench; make sure you only use it to TIGHTEN the nut. If the wrench jaws get tweaked by loosening the nut; now you'll have trouble applying enough torque to adequately tighten the nut.
The nut is left hand threaded...so it acts like an Archimedian screw and pushes oil back up the geartrain.

 

I just dealt with this last fall, found the input nut was installed backwards. You'll need a special wrench, very thin, to install this nut correctly.

The hole for the countershaft becomes enlarged over time, heavy use. From what I've heard you have two options. The hole can be sleeved by a competent machine shop. Other option if numbers are not a concern may be to buy a new case from Autogear. The caveat I ran into with the Autogear case was the cluster gear for the M-20 had to be modified. A ring had to be removed for the gear to fit into the case.

I've heard of some people having success covering the end of the countershaft with a gasket or sealer before installing the transmission. Don't know how reliable this fix may be.

 

The ring you speak of is the damper plate thats only found on OEM M20 clusters. It sounds like a good design, but in practice it doesn't work. Grind the heads of the rivets off and it comes right off. No M20 cluster in current production has this "feature"; whether Italian or Asian.

Similarly, Id also remove the little anti-rattle spring that is stuffed in a pocket under the reverse sliding gear on the mainshaft. If that little thing popped out and got in your gear mesh....it'd be very expensive to repair, vs a little gear rattle noise when engaging reverse.
Our mainshafts don't include the pocket for the spring anymore because of difficulty for the end user in installing and retaining the OEM spring properly.