1970 grand prix 4 speed help
 

What is the correct number muncie 4 speed for a 1970 Grand Prix SJ 455 i have either 478218 or 478173??

I don't have access to my GM parts books, so all I know is that for 1970 the transmission should be an M-21. More help, perhaps, via this link:

https://www.motortrend.com/how-to/gu...-transmissions

where did you glean either of those numbers from?

I'd like to help, but am confused by the question.

The numbers the OP gave are out of an early to mid 70's dated Pontiac Master Parts book. That might have worked at the dealer 4 plus decades ago. Such an over the counter complete new Muncie 4sod for such a '70 model would have CTC- (a single digit number) stamped in very small digits into two spots on a "661" case.

Looking for a correct '70 GP 455 application Muncie 4spd, it would indeed be the close ratio/ M21 with correct speedo drive gear for 3.31 & 3.55 rear gear. Same two letter coded M21 was avail behind a '70 455 GTO. Originally, a very early '70 model application would have a "660" casting Muncie case. Later in the Fall of '69, Muncie 4speeds were built with the first "661" casting cases. Both casting cases were combined with the same small output tailshaft housing for thus applucation. Not extremely hard to come up with either '70 casting number M21, it's coming up with a correct dated one that will be a challenge if one is very serous about restoration.

Thanks!

That was my guess, but my knee-jerk reaction was that the logical numbers to use for determining the correct transmission might be the main case casting numbers, or maybe tail housing casting numbers - not a part number that has gone obsolete with time.

Would either of those 478--- numbers be observed on a Muncie ID tag?
(which would have also had the transmission code)

I might not know much about GP's, but that doesn't mean I'm not interested;
I would have thought that finding an four speed in the GP would have meant it was the M20.

https://forums.maxperformanceinc.com...1&d=1626839546

I take it both of those part 478--- numbers were used for the M21.

Sorry for disrupting the conversation.

Short, and to the point!

"part #"...."pick code"..... USAGE

478173.......(DJ)...........'70 233-235-237 & GTO 400 M20
478174.......(DL)...........'70 GTO 400 usage M21 w 3.90 & 4.33 gears (6 tooth speedo drive gear)
478218...... (DP).......... '70 GTO/GP 455 usage M21 w 3.31 & 3.55 gears (believe... 8 tooth drive gear, will have to confirm in '70 DP coded M21)

thanks :)