M21 Has Some Leaks After Rebuild

[Jack O.]

Hi all,

I rebuilt my '69's M21 and I have some leakage at the plug and at the speedo drive (which was a given a new o-ring or gasket - can't remember which it took exactly). When I refilled it I actually used a measured amount of gear oil and had the car/trans as level as possible so I don't think it's a case of overfill which I wouldn't think would cause this. I did have one trans guy tell me he liked to leave a Muncie filled to about 1/2" below the fill plug.
- On the speedo drive, I notice it has a bit of free play in and out - is that normal?
- My plug and threads are in excellent shape so I'm going to add Teflon tape which I wasn't using before.
- Any other suggestions or thoughts?
Thanks,
Jack

[Joe L.]

Re: M21 Has Some Leaks After Rebuild

Hi all,

I rebuilt my '69's M21 and I have some leakage at the plug and at the speedo drive (which was a given a new o-ring or gasket - can't remember which it took exactly). When I refilled it I actually used a measured amount of gear oil and had the car/trans as level as possible so I don't think it's a case of overfill which I wouldn't think would cause this. I did have one trans guy tell me he liked to leave a Muncie filled to about 1/2" below the fill plug.
- On the speedo drive, I notice it has a bit of free play in and out - is that normal?
- My plug and threads are in excellent shape so I'm going to add Teflon tape which I wasn't using before.
- Any other suggestions or thoughts?
Thanks,
Jack

Jack------

You could leave the lubricant level 1/2" below the fill plug but I don't think it's going to have any significant effect on your leakage problem.

You definitely want to use sealer on the fill plug. This is for at least two reasons. The obvious reason is to prevent seepage. However, the bigger reason is to prevent seizure of the cast iron or steel plug in the aluminum case. This is VERY common and if it occurs it can be virtually impossible to remove the plug. I like to use Never-Seize as both a sealer as well as an anti-seize agent. I clean and re-new the Never Seize whenever the plug is removed and re-installed. I didn't always do that, though, and, thus, I learned the hard way on this one.

A little in-and-out movement of the speedometer gear fitting is normal. This is caused by the fact that the retainer does not fit tightly into the key way on the fitting. In other words, the key way slot is wider than the retainer.

Are you sure you used the proper o-ring on the fitting and not just some generic o-ring or one that some vendor SAID was correct? Also, and importantly, note that there is a small lip seal which is pressed into the fitting. This seals the shaft of the driven gear. It's replaceable. However, your best bet might be to purchase a complete new fitting which includes the o-ring and internal seal. It's still available from GM under GM #345215.

Some minor seepage from the speedometer fitting is probably pretty much "normal", though. Back in the old days, automotive sealing systems were nowhere near what they've evolved into in today's automotive world. The modern standard is what we've all become used to but we really can't expect that of 50+ year old technology.