67 m-21 transmission oil change procedure......?

[David K.]

Hello to all;

Would like to change out the oil in my 67 4 speed - M21, on my own. Already bought the Castrol 80-90 weight oil, and the syringe pump to execute work.
Could anyone please review the procedure...? I'm concerned about removal of the side oil drain plug...do not want to force it, hence I'll assume it will come off easily. My car was restored at one point, however; I have no idea when the oil was changed. The reason for my desire to change the oil is a noisy hum (like a turbine) when I'm driving normally. There's no vibration or any other issue....just this "turbine sound" coming from the tranny. I know it could be one of many issues, however; I would like to change the oil as a starting point. Hope it's not the bearings etc.....

Thank you in advance.

[Frank D.]

Re: 67 m-21 transmission oil change procedure......?

I generally shoot PB-Blaster or Liquid Wrench on the square bolt's threads and let it soak overnight. I have 8-pt sockets for such work but you can try loosening the bolt with a regular open-end wrench but don't force it and strip things... Sears (and perhaps NAPA) sell the 8-pt sockets in sets of four or individually.

Don't be surprised if a fluid change doesn't clear up your hum....it may well be a mechanical issue but never hurts to try new fluid. I use a 10-quart evacuator pump and suck all the fluid out that will come out and then use a hand-pump (AmSoil makes one) to pump fluid in...

[Duke W.]

Re: 67 m-21 transmission oil change procedure......?

...don't know if this will work on a Muncie or T-10, but the Cosworth Vega guys tell me that they can drain the oil from their Saginaw four-speeds by removing the lowest tail housing bolt.

I bit of gear whine is normal in 1-3, but there should be no noticeable noise in fourth.

Duke

[Jim D.]

Re: 67 m-21 transmission oil change procedure......?

...don't know if this will work on a Muncie or T-10, but the Cosworth Vega guys tell me that they can drain the oil from their Saginaw four-speeds by removing the lowest tail housing bolt.
Duke

That won't work on T-10's or Muncie's.

[Joe L.]

Re: 67 m-21 transmission oil change procedure......?

Hello to all;

Would like to change out the oil in my 67 4 speed - M21, on my own. Already bought the Castrol 80-90 weight oil, and the syringe pump to execute work.
Could anyone please review the procedure...? I'm concerned about removal of the side oil drain plug...do not want to force it, hence I'll assume it will come off easily. My car was restored at one point, however; I have no idea when the oil was changed. The reason for my desire to change the oil is a noisy hum (like a turbine) when I'm driving normally. There's no vibration or any other issue....just this "turbine sound" coming from the tranny. I know it could be one of many issues, however; I would like to change the oil as a starting point. Hope it's not the bearings etc.....

Thank you in advance.

David------


There is no drain plug on the side of a 1967 Muncie M-21. If yours has one, it's been added (or the case has been replaced with a later case).

If you are actually talking about the FILL plug, that's another story. You'll definitely have one of those. These can be a BEAR to get out if they have not been removed in a long while. In fact, in a worse case scenario, you could have to drill or EDM it out. I've "been down this road".

[Richard G.]

Re: 67 m-21 transmission oil change procedure......?

When a GM 4 speed is noisy it is typically a bearing issue.
When the transmission has a drain it is easier to diagnose as one can catch the oil in a clean bucket and drain off the top oil after leaving it settle.
Then look for debris in the bottom of the bucket. However in your case it is unlikely that you can get the hose down far enough to pick up any junk that may be in the bottom of the transmission.
Does the transmission quite down in forth gear? If it makes noise in the first three gears and not in forth it identifies the transmission specifically as the source of the noise.
If this is the case I would save the oil and get the transmission repaired and use in the the transmission after the repair. Incidentally is solves the fill plug issue as it will be out and they are infinity easier to deal with on the bench.
Best of luck

[Patrick T.]

Re: 67 m-21 transmission oil change procedure......?

David------

There is no drain plug on the side of a 1967 Muncie M-21. If yours has one, it's been added (or the case has been replaced with a later case).

If you are actually talking about the FILL plug, that's another story. You'll definitely have one of those. These can be a BEAR to get out if they have not been removed in a long while. In fact, in a worse case scenario, you could have to drill or EDM it out. I've "been down this road".

Joe, I have a side drain plug in my Muncie M-22 FWIW.

[Joe L.]

Re: 67 m-21 transmission oil change procedure......?

Joe, I have a side drain plug in my Muncie M-22 FWIW.

Patrick------


All M-22's have drain plugs. Also, 1970-74 Muncies of all variants have drain plugs.

[Joe L.]

Re: 67 m-21 transmission oil change procedure......?

When a GM 4 speed is noisy it is typically a bearing issue.
When the transmission has a drain it is easier to diagnose as one can catch the oil in a clean bucket and drain off the top oil after leaving it settle.
Then look for debris in the bottom of the bucket. However in your case it is unlikely that you can get the hose down far enough to pick up any junk that may be in the bottom of the transmission.
Does the transmission quite down in forth gear? If it makes noise in the first three gears and not in forth it identifies the transmission specifically as the source of the noise.
If this is the case I would save the oil and get the transmission repaired and use in the the transmission after the repair. Incidentally is solves the fill plug issue as it will be out and they are infinity easier to deal with on the bench.
Best of luck

Richard------


Even on the bench the fill plug can be near-impossible to remove. As I mentioned, I've been down that road, on the car and on the bench. I hope he gets lucky, though.

I think this problem is caused by the dis-similar metal "syndrome"-----plugs are usually cast iron (sometimes steel) and the case is, of course, aluminum.

[David K.]

[David K.]

Re: 67 m-21 transmission oil change procedure......?

Well....I took it out for a 5 mile run, and as someone mentioned, the "whining" noise is non existent in fourth gear, both in acceleration and coasting in 4th without my foot on the gas pedal. The whining only occurs when I'm pushing on the gas in the first three gears. Is this normal ....hence....do I leave things alone ? Whining also occurs from my wife when I spend too much time with this stuff.

[Richard G.]

Re: 67 m-21 transmission oil change procedure......?

Their are a couple possibilities.
First think to know is the power flow is straight through the transmission with no load going to the cluster gear (bottom gears on a single shaft all connected together) when you are in forth gear.

Power flow:
1) input shaft
2) output shaft

In all other gears the power flow goes through the cluster. It goes;
1) input shaft
2) cluster
3) output shaft.

One possibility is the transmission was put together with miscellaneous gears with different wear patterns. Your description sounds like it may be gear related but I caution you as this is a best guess only.
The other possibility is it has a defective bearing and the noise is evident as the shaft is loaded. Interesting it is only under load and not under deceleration. One has to think the fault is not bearings as the noise should be evident in both acceleration and deceleration. Another reason I a guessing at a gear issue. However sometimes the fault doesn't follow logic. Always go by the mechanical inspection as the final say.
If there are no other issues like popping out of gear under deceleration the transmission may well go a long time before causing issues.
If you decide you can live with the noise I would likely change the oil as you mentioned as you don't have any lube history.
If it bothers you have the transmission taken apart and inspect the bearings for defects. If none are found I would suggest replacing the entire gear set. This is likely the only method that will produce the slick shifting low noise transmission you have been looking for.

[David K.]

Re: 67 m-21 transmission oil change procedure......?

Thank you Richard;

I'm going to change out the fluid since I have no history on it. If the whine persists, I'll live with it until it becomes more of an issue. Thank you again for your help.
David