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M20 Oil Replacement (1/11)
 10/21/18 2:49am
NJAus
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, - Australia

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Currently own a white 1969 427 Big Block Coupe C3 Corvette


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Hi All,

When it comes to changing the transmission oil on the 69BB, can I get away with just removing the fill plug and using a hand syringe pump to remove and refill?

Does the fill plug require a special socket tod remove it? I don't want to damage it.

What type specifications of gear oil should I use? What is the oil capacity of the M20.

I have also noticed what I think is a oil leak from the M20. A front leak will also drip from the bell housing cover plate?

On idle there is no leak, but after the car is shutdown 10 mins later there is a drop or two. There also is a small streak from between the bell housing and transmission mating surface.

At the moment I would like to change the oil and monitor it. If the oil is overfilled will the leak be more pronounced? Will high rpm also make the leak more pronounced?

Thanks







|UPDATED|10/20/2018 11:49:37 PM (AZT)|/UPDATED|
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Re: M20 Oil Replacement (2/11)
 10/21/18 3:59pm
73shark
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Overland Park, KS - USA

Vette(s):
1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day.


Joined: 7/9/2003
Posts: 567

My M20 4 speed has both a drain and a fill plug. As to the front leaks, it could be the rear main seal in the engine is leaking. As to the type of lube for the transmission, I believe you can find it in the back of the owner's manual. I'm currently out of town and can't check mine.

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1973 L-82 4 spd

Re: M20 Oil Replacement (3/11)
 10/21/18 5:00pm
NJAus
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73shark said: My M20 4 speed has both a drain and a fill plug. As to the front leaks, it could be the rear main seal in the engine is leaking. As to the type of lube for the transmission, I believe you can find it in the back of the owner's manual. I'm currently out of town and can't check mine.


From a bit of research, 69 M20 had no drain plug, the boss is there to allow the drain plug to be drilled and threaded. This is why I think I will have to use to hand pump to drain and refill through the fill plug.

I have replaced the rear main and oil pan gasket, there is a bit of oil on a couple of the oil pan bolts but nothing significant, might just need a re-torque. The rear main area doesn't seem like it leaking when I swabbed it.
 
The bell housing cover is dry also inside and out.
 
I did try to remove the fill plug with a spanner but it wouldn't move.....I will have to order a 8 point socket and give it a go.


|UPDATED|10/21/2018 2:00:43 PM (AZT)|/UPDATED|
Re: M20 Oil Replacement (4/11)
 10/21/18 7:45pm
F4Gary
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Grapevine, TX - USA

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I've been told NOT to use synthetic gear oil.

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Re: M20 Oil Replacement (5/11)
 10/21/18 8:20pm
manchestersharkLifetime Member
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Equinunk, PA - USA

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From experience, a newly rebuilt M-20 with valvoline 80-90 gear oil will shift sluggish. Get Penzoil Gold Synchromesh 65-80 gear oil. It is best used with the brass synchronizers. Shifts a lot smoother and will not stick the synchronizers to the shaft.
Re: M20 Oil Replacement (6/11)
 10/21/18 9:27pm
Adams' AppleLifetime Member
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5/8" wrench for the fill plug. Best way to get it loosened(if too tight or corroded) is tapping on it square on the face with a hammer. Then try to TIGHTEN it a smidge first.
Some Muncies could be drained by removing the lowest bolt on the tailhousing. Some bolt holes were drilled all the way thu, some were not...always worth a shot.
75w gear lube is fine in most applications..
The correct lube measurement for Muncies and BW T10s is about 1 inch down from the fill plug...NOT running out of the fill plug.
Leak running down the front between the trans and bellhousing will be the countershaft leaking. There is no seal of any kind where the countershaft fits into the front of the case. It depends on a super tight fit to not leak...and even then, they always leak eventually. The only way to guarantee it to not leak is to clean it really well, and lay a thin bead of sillycone on the shaft where it fits thru the front of the case.
There should never be a leak from the front bearing retainer(where the throw-out bearing rides) unless the unit is waaaay overfull, or someone forgot to put a gasket on it.



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Re: M20 Oil Replacement (7/11)
 10/21/18 9:40pm
73shark
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Overland Park, KS - USA

Vette(s):
1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day.


Joined: 7/9/2003
Posts: 567

F4Gary said: I've been told NOT to use synthetic gear oil.



Why, what are the consequences?

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1973 L-82 4 spd

Re: M20 Oil Replacement (8/11)
 10/22/18 9:54pm
Adams' AppleLifetime Member
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Much more prone to leak than regular dino oils. The smaller molecules of synthetics will find(and leak from) places that regular lubes will not.   Thumbs Up

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Re: M20 Oil Replacement (9/11)
 10/24/18 6:42am
NJAus
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Adams' Apple said: 5/8" wrench for the fill plug. Best way to get it loosened(if too tight or corroded) is tapping on it square on the face with a hammer. Then try to TIGHTEN it a smidge first.
Some Muncies could be drained by removing the lowest bolt on the tailhousing. Some bolt holes were drilled all the way thu, some were not...always worth a shot.
75w gear lube is fine in most applications..
The correct lube measurement for Muncies and BW T10s is about 1 inch down from the fill plug...NOT running out of the fill plug.
Leak running down the front between the trans and bellhousing will be the countershaft leaking. There is no seal of any kind where the countershaft fits into the front of the case. It depends on a super tight fit to not leak...and even then, they always leak eventually. The only way to guarantee it to not leak is to clean it really well, and lay a thin bead of sillycone on the shaft where it fits thru the front of the case.
There should never be a leak from the front bearing retainer(where the throw-out bearing rides) unless the unit is waaaay overfull, or someone forgot to put a gasket on it.


I am finding it impossible to get a hammer in there to tap the plug square on the face.

Furthermore, I got a 9/16 8 point socket and when I attached it to the socket wrench I can't rotate it because the socket wrench hits the exhaust pipe...surely it can't be this difficult....
Re: M20 Oil Replacement (10/11)
 10/24/18 11:36am
F4Gary
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Grapevine, TX - USA

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73shark said:
F4Gary said: I've been told NOT to use synthetic gear oil.



Why, what are the consequences?


Something about the synchronizers don't like it.  I'm not the expert, I just read that on this forum or another Corvette forum.


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