I have acquired a GM 4 Speed trans. The Casting Numbers on it are, 388501 and 3884685 and on the tailshaft it has 3846429. If anyone call help to ID what this is and may be out of, I would greatly appreciate it. What style transmission is it? Muncie 21 or 22? Is it worth much? Thank You

Welcome to TC! You can find a wealth of info here but it helps if you post in the best forum. If you post this in the tag team section, someone could probably help you there. Kevin

You may have left a 0 out of the casting number.

Casting: 3885010 Years: 1965 - 1967
Ratios: M20 2.52 1st , M21 2.20 1st, M22 2.20 1st

Kev

Ratios of the M20 , M21 , M22


Year 1963-1965 Type: M20 Rings: None Ratio: 2.56 / 1.91 / 1.48 / 1.00 / 3.16
Year 1966-1974 Type: M20 Rings: Two Ratio: 2.52 / 1.88 / 1.46 / 1.00 / 3.11
Year 1963-1974 Type: M21 Rings: One Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27
Year 1967-1974 Type: M22 Rings: None Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27
Input Shaft Tooth and Spline Count Related to Year


Year 1963-1965 Type: M20 Rings: None Spline: 10 Tooth Count: 24
Year 1966-1970 Type: M20 Rings: Two Spline: 10 Tooth Count: 21
Year 1970-1974 Type: M20 Rings: Two Spline: 26 Tooth Count: 21
Year 1963-1970 Type: M21 Rings: One Spline: 10 Tooth Count: 26
Year 1970-1974 Type: M21 Rings: One Spline: 26 Tooth Count: 26
Year 1967-1970 Type: M22 Rings: None Spline: 10 Tooth Count: 26
Year 1969-1974 Type: M22 Rings: None Spline: 26 Tooth Count: 26

Kev

Put it in first gear, mark the input and output shafts, and see how many turns of the input shaft it takes to make EXACTLY one turn of the output shaft. If it's 2-1/4, it's an M21; if it's 2-1/2, it's an M20. :thumbsup: