Mystery 4_spd

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  • Craig Freeman
    #1

    Mystery 4_spd

    Here is what I have found out so far about my non-original 4-spd. The main housing has the following numbers. On the right side 3925660 US PATENT NO 3088330. On the right side of the tail piece is 3857584 (I have been told stands for a Muncie and possibly an M-22). The side cover number is 3884685 (which tells me it a 68 trans). Here is the real interesting part. Stamped on the edge near where the two parts are joined is 18S404327 P8R31. If I read these numbers correctly the first number says the trans was installed in a 68 vette (8=year S=St.Louis 4237= 4,237th built). The next number/character P=Muncie 8= 68 R= MONTH ??? and 31 the 31st day of the month. And supposedly there should be A,B, or C following this last number. (A=M20, B=M21, and C=M22). I cannot find any letter. What do I have here? Is this a Vette trans?

    Craig
    Tags: None
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    • bob bridge
      #2
      Re: Mystery 4_spd

      I have the same trans in my 77. The numbers are the same as yours except for the serial portion. So far as I can tell it is a 68 M21 corvette trans and it has no letter after the muncie code. I calculated with the speed and revs to come up the 2.20 low. I think they didn't start with the A B C until late 68 or early 69. Bob
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        • bob bridge
          #3
          Re: Mystery 4_spd

          I have the same trans in my 77. The numbers are the same as yours except for the serial portion. So far as I can tell it is a 68 M21 corvette trans and it has no letter after the muncie code. I calculated with the speed and revs to come up the 2.20 low. I think they didn't start with the A B C until late 68 or early 69. Bob
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            • Joe L.
              Beyond Control Poster
              • February 1, 1988
              • 42936
              #4
              Re: Mystery 4_spd

              Craig-----

              Nothing mysterious at all about your transmission. You've correctly identified it. It is a Muncie 4 speed originally installed in a 1968 Corvette. As Bob Bridge mentioned, the "A", "B", or "C" suffix code system did not start until the 1969 model year. Your 3925660 main case was used on all Muncie 4 speed varients during 1968 and 1969. Your 3857584 extension housing was used on all Muncie 4 speed varients from late 1965 through 1970.

              You could identify your first gear ratio by using the methodology that Bob suggests or you could remove the sidecover and count the input shaft gear teeth. A 2.52:1 (wide-ratio) will have 21 teeth; a 2.20:1 (close ratio) will have 26 teeth. M-22 gears will have a lower helix angle than M-20 or M-21 gears, but this is difficult to distinguish unless you're experienced at it or have gears to compare. For original 1968 transmissions, which yours apparently is, M-22 trans will have the lower main case boss drilled and tapped for a drain plug. This is a fairly accurate identification methodology unless someone has drilled and tapped the boss of a non- M-22 trans.
              In Appreciation of John Hinckley
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                • Joe L.
                  Beyond Control Poster
                  • February 1, 1988
                  • 42936
                  #5
                  Re: Mystery 4_spd

                  Craig-----

                  Nothing mysterious at all about your transmission. You've correctly identified it. It is a Muncie 4 speed originally installed in a 1968 Corvette. As Bob Bridge mentioned, the "A", "B", or "C" suffix code system did not start until the 1969 model year. Your 3925660 main case was used on all Muncie 4 speed varients during 1968 and 1969. Your 3857584 extension housing was used on all Muncie 4 speed varients from late 1965 through 1970.

                  You could identify your first gear ratio by using the methodology that Bob suggests or you could remove the sidecover and count the input shaft gear teeth. A 2.52:1 (wide-ratio) will have 21 teeth; a 2.20:1 (close ratio) will have 26 teeth. M-22 gears will have a lower helix angle than M-20 or M-21 gears, but this is difficult to distinguish unless you're experienced at it or have gears to compare. For original 1968 transmissions, which yours apparently is, M-22 trans will have the lower main case boss drilled and tapped for a drain plug. This is a fairly accurate identification methodology unless someone has drilled and tapped the boss of a non- M-22 trans.
                  In Appreciation of John Hinckley
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                    • Craig Freeman
                      #6
                      Re: Mystery 4_spd

                      Joe

                      First of all I checked the trans, there is a drain plug on the main case on the right side almost directly below the US PATENT about 2 or 3 inches and lines up with the space between US and PATENT (roughly). Does this sound correct to you? Also I am still confused on two things. 1)as far as the VIN # shouldn't it be 78S404327 not 18S404327? 2)I am told that P8R31 stands for P= Muncie, 8= 1968, R= October, 31= 31st. If R is tryly October than how can a Oct 31 1968 trans end up with a 1968 VIN #?
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                        • Craig Freeman
                          #7
                          Re: Mystery 4_spd

                          Joe

                          First of all I checked the trans, there is a drain plug on the main case on the right side almost directly below the US PATENT about 2 or 3 inches and lines up with the space between US and PATENT (roughly). Does this sound correct to you? Also I am still confused on two things. 1)as far as the VIN # shouldn't it be 78S404327 not 18S404327? 2)I am told that P8R31 stands for P= Muncie, 8= 1968, R= October, 31= 31st. If R is tryly October than how can a Oct 31 1968 trans end up with a 1968 VIN #?
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                            • Wayne M.
                              Expired
                              • March 1, 1980
                              • 6414
                              #8
                              Re: Mystery 4_spd

                              Craig --- The plug you described below the letters "US" of the casting US PATENT NO XXXXXXX is the fill/level check plug. If it was an M22, there would be a second plug on the same side VERY LOW and forward on the case (almost flush with the bottom) , and directly under the letter "O" in GENERAL MOTORS CORP. You will probably see a "boss" cast for a (potential) drain plug. As for the VIN #'s, I'll let others handle that, but I believe after model year '67, the engines and trannys were stamped with 1 = Chevrolet, 8 = model year, S = St.louis, etc.
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                                • Wayne M.
                                  Expired
                                  • March 1, 1980
                                  • 6414
                                  #9
                                  Re: Mystery 4_spd

                                  Craig --- The plug you described below the letters "US" of the casting US PATENT NO XXXXXXX is the fill/level check plug. If it was an M22, there would be a second plug on the same side VERY LOW and forward on the case (almost flush with the bottom) , and directly under the letter "O" in GENERAL MOTORS CORP. You will probably see a "boss" cast for a (potential) drain plug. As for the VIN #'s, I'll let others handle that, but I believe after model year '67, the engines and trannys were stamped with 1 = Chevrolet, 8 = model year, S = St.louis, etc.
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                                    • Terry M.
                                      Beyond Control Poster
                                      • October 1, 1980
                                      • 15488
                                      #10
                                      Re: Mystery 4_spd

                                      Wayne is right about the VIN for 68 to 72 it is just as you have found: 18SXXXX. 1=Chevroelt 8= 1968 model year.

                                      The P8 and so on indicates the 1968 model year NOT CALANDER YEAR. For a car in the 4000 serial number range October of calander year 1967 is about right.

                                      Be happy.

                                      Terry McManmon


                                      Terry
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                                        • Terry M.
                                          Beyond Control Poster
                                          • October 1, 1980
                                          • 15488
                                          #11
                                          Re: Mystery 4_spd

                                          Wayne is right about the VIN for 68 to 72 it is just as you have found: 18SXXXX. 1=Chevroelt 8= 1968 model year.

                                          The P8 and so on indicates the 1968 model year NOT CALANDER YEAR. For a car in the 4000 serial number range October of calander year 1967 is about right.

                                          Be happy.

                                          Terry McManmon


                                          Terry
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