PS
There is a difference between the M40 and the M45 trans.
the M40 that was stock fitted to the B20 has a Different Input shaft than the M45 that was stock fitted to the B21.
The M40 input shaft has fewer and thicker splines than th3 M45
IF you indeed have a M45 and NOT a M40 mounted to your B20, then the Input shaft on that M45 has been changed. The M40 input shaft has fewer and thicker splines.
Same for the versions of these trannies with Overdrives attached --- M41 and M46
I know this because I have owned both a 75 and and 80 with 4spd gear boxes as well as an 87 with an M47 5spd
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