1974 Muncie M21

Discussion in 'VIN, Trim Tag & Drivetrain Decoding' started by RLP, May 18, 2020.

  1. Jonesy

    Jonesy Veteran Member

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    Heres a 74z M21 with the same stamping as mine P4R24B 14N136413 car was built 12B. If you look closely you can see numbers XXXX'd out to the lower left of the P.

    14N136413 trans.jpg
     
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  2. Rocky Rotella

    Rocky Rotella New Member

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    Would you happen to have a copy of that letter?
     
  3. CamaroNmotion

    CamaroNmotion Veteran Member

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    Took a quick look, didn't find it or am I sure that I remembered to save when I saw it online or from a book, magazine...it was many years ago when I saw it. If I come across it I'll post it.
    Meanwhile I did find this attachment and another place to see an explanation about the transmission change would be "70-75 Chevrolet by the numbers" by Colvin, but make sure its a later printing because he made a couple of addendums. File0180.jpg
     
  4. Rocky Rotella

    Rocky Rotella New Member

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    Thanks! I've been trying to find a specific point during the 1973 calendar year that GM ceased its 4-speed for M20 and M21 applications (we know M22 was done after 1972) at Muncie.

    I wrote a book on 70-81 Firebirds for CarTech Books and had spoken with Alan Colvin as well as the person responsible to production scheduling at the Chevrolet-Muncie plant back then while gathering material for the project. From my own research on 1974 Firebirds, it seems like the ST-10 began appearing by February 1974, so your timeline may be accurate. I'd just like to find something concrete.
     
  5. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    Not sure about 74, but you have to be a bit careful about the M20, M21 codes. Those were RPO codes, not necessarily who made the transmission. An example is in 1970, an M20 could be either a Saginaw or a Muncie wide ratio. For 300HP+ applications, it was a Muncie. For less than 300 it was a Saginaw. It may be the same way with the ST10 and the Muncie. Both maybe were called a M21 on the RPO code and this could add confusion.
     
  6. Rocky Rotella

    Rocky Rotella New Member

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    You’re absolutely correct on the RPO code designations.

    In fact, when GM moved to alphanumeric RPO coding in 1963 it assigned M20 for its close-ratio 4-speed applications, which was the outsourced Warner Gear T10.

    When GM began building its own 4-speed during 1963 (at the Chevrolet Gear plant in Muncie, Indiana) RPO M20 was reassigned to the wide-ratio application while the close-ratio 4-speed went to RPO M21.

    During the mid-to-late 1960s, GM began producing light-duty wide-ratio 4-speed manual transmissions for 6-cylinder and low performance passenger car applications produced at its gear plant in Saginaw, Michigan. Those transmissions also fell under RPO M20.

    And when GM stopped producing its Muncie-built 4-speeds in 1974 and began outsourcing again, it appears that it kept RPO M20 for Warner Gear’s Super T-10 for the remainder of the model year but moved it to RPO M21 for 1975. At least in Pontiac’s Firebird option coding.

    So as you note M20 and M21 doesn’t always mean it was a Muncie-built four speed!
     
    Last edited: Dec 2, 2020
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  7. Jonesy

    Jonesy Veteran Member

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    Well you have proof Muncies were still being installed in 74 z28's as late as the 3rd week of March 1974.
     
  8. mallard

    mallard Veteran Member

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    My '74 Type LT Z28 has a production date of April 15, 1974. It's a points car and the Canadian documents show RPO "M21" transmission option however it came with a Borg Warner T10.
     

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