M20 vs M21 - NCRS Discussion Boards

M20 vs M21

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  • Greg Y.
    Expired
    • April 3, 2014
    • 25
    #1

    M20 vs M21

    I'm trying to determine which of these I have in my '66 coupe. According to the Black Book approximately 11 thousand of the M20's were installed and 14 thousand of the M21's in 1966 so it's about a toss up. Is there an accurate way to tell which one I've got? Does each have a specific part number?

    Thanks in advance
    Tags: None
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    • Keith B.
      Very Frequent User
      • March 7, 2008
      • 928
      #2
      Re: M20 vs M21

      If it's the original trans there will be numbers stamped on the housing. Also there is small metal tag with a part number stamped on it.
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        • Joe L.
          Beyond Control Poster
          • January 31, 1988
          • 43191
          #3
          Re: M20 vs M21

          Originally posted by Keith Brodbeck (14640)
          If it's the original trans there will be numbers stamped on the housing. Also there is small metal tag with a part number stamped on it.
          Keith-----

          For the 1966 model year there was no stamping on a Muncie that identified whether it was an M-20 or M-21. However, if the part number tag is still affixed to one of the side cover bolts, that will identify it.
          In Appreciation of John Hinckley
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            • Mike T.
              Very Frequent User
              • December 31, 1991
              • 568
              #4
              Re: M20 vs M21

              This isn't a definitive way to figure out whether you have a wide or close ratio trans but isn't it more likely that if the rear gear is higher numerically, like 3.70 or 4.11, then the transmission would typically be close ratio (2.20 first gear)?
              Mike T. - Prescott AZ.
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                • Jim T.
                  Expired
                  • February 28, 1993
                  • 5351
                  #5
                  Re: M20 vs M21

                  What engine came in your 1966? My 1968 has 327/350HP M21 and 3:70 rear end.
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                    • Jim L.
                      Extremely Frequent Poster
                      • September 30, 1979
                      • 1804
                      #6
                      Re: M20 vs M21

                      How to determine if your transmission is wide or close ratio:

                      Travel in 4th gear with engine turning exactly 1000 RPM. Note your speed.

                      Shift to 1st gear and bring car back up to the previous speed. Note the tachometer.

                      If tachometer indicates 2540 RPM, you have a wide ratio box.

                      If tachometer indicates 2200 RPM, you have a close ratio box.

                      Simple. Easy to remember.

                      Jim
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                        • Greg Y.
                          Expired
                          • April 3, 2014
                          • 25
                          #7
                          Re: M20 vs M21

                          Originally posted by Jim Lockwood (2750)
                          How to determine if your transmission is wide or close ratio:

                          Travel in 4th gear with engine turning exactly 1000 RPM. Note your speed.

                          Shift to 1st gear and bring car back up to the previous speed. Note the tachometer.

                          If tachometer indicates 2540 RPM, you have a wide ratio box.

                          If tachometer indicates 2200 RPM, you have a close ratio box.

                          Simple. Easy to remember.

                          Jim
                          Bingo... I'll give that a try. Thanks to all who responded.
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