M20 or M21
04-11-2005, 07:50 AM
#1
Drifting
Thread Starter
M20 or M21
I am about to pull the engine and transmission from my 66 Coupe. The engine will be replaced with a 383 crate from GM. The transmission will be rebuilt as well. I have owned the car for over 30 years and I don't really know if this is an original transmission. The car is an air car and had a 3.08 rear end in it until I changed it to a 3.55. Am I likely to find an M20 or a M21 in the car? Can we tell by the case? Trying to get the right rebuild kit.
Last edited by Dennis Beck; 04-11-2005 at 07:51 AM. Reason: Typing
04-11-2005, 08:24 AM
#2
Melting Slicks
Most likely you will have a wide ratio trans. (M20). If it is the original you should find a partial VIN of your car on the rear driver side case. The build code is on the rear passenger side of the case and the speedometer
gear is on the passenger side of the tailshaft.
gear is on the passenger side of the tailshaft.
04-11-2005, 08:24 AM
#3
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Originally Posted by Dennis Beck
I am about to pull the engine and transmission from my 66 Coupe. The engine will be replaced with a 383 crate from GM. The transmission will be rebuilt as well. I have owned the car for over 30 years and I don't really know if this is an original transmission. The car is an air car and had a 3.08 rear end in it until I changed it to a 3.55. Am I likely to find an M20 or a M21 in the car? Can we tell by the case? Trying to get the right rebuild kit.
A sure fire way to tell when you get the transmission out is to put the transmission in low gear and rotate the input shaft one turn. Approximately 2 1/4 turns of the output shaft equals M-21 or close ratio. Approximately 2 1/2 turns of the output shaft equals M20 or wide ratio.
Some will say to look at the number of grooves, or lack of grooves on the input shaft or numbers on the transmission. This is only a guess as OEM parts changed and/or parts have been replaced.
You also need to determine if your transmission has a 7/8's or 1" cluster shaft in the case assuming you have a Muncie.
04-11-2005, 08:32 AM
#4
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If the car came with AC and a 3.08 rear then I would say it is an M20.
Here is web page that will tell you what to look for.
http://www.5speeds.com/muncie2.htm
Here is web page that will tell you what to look for.
http://www.5speeds.com/muncie2.htm
04-11-2005, 12:23 PM
#5
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Originally Posted by MikeM
A sure fire way to tell when you get the transmission out is to put the transmission in low gear and rotate the input shaft one turn. Approximately 2 1/4 turns of the output shaft equals M-21 or close ratio. Approximately 2 1/2 turns of the output shaft equals M20 or wide ratio.
04-11-2005, 02:04 PM
#6
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Originally Posted by JohnZ
If you reverse the above, you'll be in business - count the number of turns of the input shaft to get one full turn on the output shaft; 2-1/4 is an M21, 2-1/2 is an M20.
Gothcha'.
05-18-2005, 11:35 AM
#7
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Newer M20 gear sets....
You may want to consider upgrading to Super street M20 gear sets. These sets have the 2.52 first gear, but a closer ratio spread between 2-3 and 3-4.
Paul
Paul
