transmission decoding on 68
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transmission decoding on 68
Hello,
How do you determine if the transmission is M20, M21, or M22?
Here is the info I have on the transmission:
date code is P8H26 (Muncie, 1968, June, 26th)
case number on passenger's side is 3925660
case number on driver's side is 3884685
mystery number on bellhousing is 3858403
The engine code is V0708HT.
From my research, GM did not add the prefix for the trans type until 69, so how do you tell the type of transmission for a 1968.
Thanks,
Joe
How do you determine if the transmission is M20, M21, or M22?
Here is the info I have on the transmission:
date code is P8H26 (Muncie, 1968, June, 26th)
case number on passenger's side is 3925660
case number on driver's side is 3884685
mystery number on bellhousing is 3858403
The engine code is V0708HT.
From my research, GM did not add the prefix for the trans type until 69, so how do you tell the type of transmission for a 1968.
Thanks,
Joe
Last edited by 68coupe; Oct 10, 2005 at 08:33 AM.
Team Owner
Joined: Jul 1999
Posts: 26,545
Likes: 46
From: Huntersville NC
Is the trans in a driveable car? Driving test is the easiest way to determine M20 or M21.
M22 (highly unlikely anyway) will have a drain plug. 68 M20 or M21 will only have one fill/drain plug.
M22 (highly unlikely anyway) will have a drain plug. 68 M20 or M21 will only have one fill/drain plug.
Last edited by PRNDL; Oct 10, 2005 at 08:51 AM.
Pro
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From: North Alabama
Thread Starter
Pro
Joined: Sep 2004
Posts: 595
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From: Boston MA
Originally Posted by vettepilot68
This web site should help.
http://www.5speeds.com/muncie2.htm
http://www.5speeds.com/muncie2.htm
If I have the transmission out, I can count the splines and rings.
I guess there is no way to tell just by the numbers.
Thanks,
Joe
Team Owner
Joined: Jul 1999
Posts: 26,545
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From: Huntersville NC
I believe you can't tell from the stampings on a 68 trans. You have to take it apart and count gears. Driving is easy. Using the info on the referenced website, you see:
Year 1966-1974 Type: M20 Rings: Two Ratio: 2.52 / 1.88 / 1.46 / 1.00 / 3.11
Year 1963-1974 Type: M21 Rings: One Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27
Both transmissions (M20 or M21) have a 1.00: 1 4th gear. So you drive along in 4th gear at a fixed rpm, let's say 2000 rpm. Now keeping your speed as constant as possible you downshift to 3rd gear. An M21 has a 1.28:1 3rd gear, so your rpms will increase from 2000 to 2500 rpm (2000 X 1.28). If you have an M20 trans with a 1.46 :1 3rd gear you rpm will jump from 2000 to 2920 (2000 X 1.46). You shift back and forth between 3rd and 4th a few times and it is usually pretty obvious which trans you have.
Year 1966-1974 Type: M20 Rings: Two Ratio: 2.52 / 1.88 / 1.46 / 1.00 / 3.11
Year 1963-1974 Type: M21 Rings: One Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27
Both transmissions (M20 or M21) have a 1.00: 1 4th gear. So you drive along in 4th gear at a fixed rpm, let's say 2000 rpm. Now keeping your speed as constant as possible you downshift to 3rd gear. An M21 has a 1.28:1 3rd gear, so your rpms will increase from 2000 to 2500 rpm (2000 X 1.28). If you have an M20 trans with a 1.46 :1 3rd gear you rpm will jump from 2000 to 2920 (2000 X 1.46). You shift back and forth between 3rd and 4th a few times and it is usually pretty obvious which trans you have.
