Build sheet

Discussion in '1978 - 1981 Specific' started by Scott Sievers, Apr 10, 2020.

  1. Scott Sievers

    Scott Sievers Veteran Member

    Messages:
    239
    Likes Received:
    100
    Joined:
    May 22, 2018
    Location:
    SE Iowa
    Anyone got a link to decoding the build sheet? I decided to see if mine was there today.. it was! It says M21 Trans though... I didn't realize they were still using muncies in 1980?? Anyhow, thanks!!!!
    I tried to upload the scan I took of it as a PDF but keep getting the error that the extension is not allowed? .pdf isnt allowed?
     
  2. Gary S

    Gary S Administrator Lifetime Gold Member

    Messages:
    23,087
    Likes Received:
    1,088
    Joined:
    Apr 14, 1999
    Location:
    Bismarck, North Dakota
    M21 on the build sheet is the RPO for a close ratio 4 speed tranny that you could order. It just so happens that many years earlier, GM used M21 as a model number for a close ratio Muncie. These do not mean the same thing.

    In the early 70s, RPO M21 on the build sheet got you a M21 Muncie.
    By 1980, RPO M21 got you a Borg Warner 4 speed tranny.
     
  3. 70lt1z28

    70lt1z28 Veteran Member Gold Member

    Messages:
    3,691
    Likes Received:
    751
    Joined:
    Oct 3, 1999
    Location:
    Beavercreek, Ohio, USA
    And to make it even more confusing, in the early 70's M20 could mean either a Muncie OR a Saginaw.
     
  4. Scott Sievers

    Scott Sievers Veteran Member

    Messages:
    239
    Likes Received:
    100
    Joined:
    May 22, 2018
    Location:
    SE Iowa
    Awesome... and found what I think is the page for the codes on the build sheet..
     
  5. Glava

    Glava Veteran Member Lifetime Gold Member

    Messages:
    409
    Likes Received:
    26
    Joined:
    Mar 23, 1999
    Location:
    Central NY
    Yup, my 79 has RPO M21 on the build sheet -which confused me too when I found the build sheet years ago.
    The Borg Warner is a T-10 aluminum case, which is a decent tranny right?
     
  6. Scott Sievers

    Scott Sievers Veteran Member

    Messages:
    239
    Likes Received:
    100
    Joined:
    May 22, 2018
    Location:
    SE Iowa
    Not entirely sure... Super t10 was upgraded and supposedly pretty strong... My experience from back in the day was to Ditch the t10 and scrounge the salvage yards and find a M21 or better an M22. Although I ran several t10's behind big blocks, as long as you didn't slide your foot off the clutch at 5000 rpm they did okay.. if you did slide your foot off the pedal at 5000 rpm it snapped the output shaft off right where the splines started... Not.... that I ever did that....
     
  7. Gary S

    Gary S Administrator Lifetime Gold Member

    Messages:
    23,087
    Likes Received:
    1,088
    Joined:
    Apr 14, 1999
    Location:
    Bismarck, North Dakota
    The T10 is equal to the Muncie in strength. GM quit making Muncies in 1973 and started buying T10s from Borg Warner. These T10s from 1973-79 were quite close ratio and have aluminum cases. Starting in 1980, EPA standards called for better fuel economy so Chevy dropped the 3.73 rear gears in favor of 3.08 gears to get better mileage. The change required a wider ratio T10. Widening the ratios in a manual tranny weakens it as the wider gears put more stress on the case. I believe you will find that these later wider ratio T10s will have cast iron cases to make them stronger.
     
  8. 1981 Berlinetta

    1981 Berlinetta New Member

    Messages:
    8
    Likes Received:
    0
    Joined:
    Jul 30, 2019
    Location:
    Timmins ON
    not 100% on topic but. I purchased a 1980 Z28 for parts just after Christmas, car is pretty rough. I removed the rear seats yesterday and there was the build sheet. 40 years and rear seat was never removed. Pretty cool
    PS, Saginaw 4 speed, iron case.
     
  9. Gary S

    Gary S Administrator Lifetime Gold Member

    Messages:
    23,087
    Likes Received:
    1,088
    Joined:
    Apr 14, 1999
    Location:
    Bismarck, North Dakota
    Have you checked the casting numbers on that tranny? Some sources indicate that an 80Z28 should have originally had an iron case Borg Warner.
     
  10. 1981 Berlinetta

    1981 Berlinetta New Member

    Messages:
    8
    Likes Received:
    0
    Joined:
    Jul 30, 2019
    Location:
    Timmins ON
    I’ll check casting numbers once I pull it out of the car but using google, checking pictures and charts it’s looking like a Saginaw.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.