'65 M-21 4-Speed
 

...and, its from a Fremont Chevelle.
Milled 848 case, March 10th assembly date. My research shows this to be a big block only trans with this case #. Appears to be all original external components and assembly bolts. Vin in pics.. Alan Colvins book suggests it to be either a Z16 or a COPO chevelle trans. Can someone please help clarify? The seasoned racer I purchased it from this morning claimed it has been buried in his shop for at least 35 years. The ramrod was a welcomed bonus. Thanks for any and all help. Dave

After further research, I found my answers. Oh well, it certainly was exciting for a few hours anyway.
Dave.

And that would be ?? I was excited to see what it was, also...:biggthumpup:

Mitch, I honestly don't know. I'm getting conflicting info. I know it's not from a Z. Probably have to figure out a complete vin, something like 138375z128230 and go from there?? That's above my pay grade lol. I still welcome any help or comments.
Dave.

Strange indeed, unless I'm missing something.....highly unlikely. The Z plant only made chevelles, yet all Z16 chevelles were built in KC, so that leaves a M21 slated small block.
Didn't happen, so it's most likely not a M21, but rather a M20. Never seen a 848 case either, so need help there. Never seen that patten no. on a main case either??

All Z16's came with M20's with the regular 1965 Muncie casting # of 3851325. No M21's. I presume that was due to the mandatory 3.31 axle in all Z16's. And as already noted, no Z16's were built at Fremont.

Fremont built other GM A-body cars too. Maybe it came in a GTO? Fremont Chevelle VIN ending 5Z128987 was the last Chevrolet to leave Fremont at the end of February 65. Which is higher than your VIN stamp of 5Z128230. But your trans was assembled March 10, well after the Chevelle with VIN ending 5Z128230 had already left the plant back in February. So unless the VIN stamp has a factory mistake in it, it could not have come in a Fremont Chevelle. (But VIN stamp mistakes were very common at Fremont, so...)

RPO production totals for 65 Chevelles have no M21's listed. There is not even an ECL for an M21 in a 65 Chevelle. So if it had come in a 65 Chevelle it would have to have been some sort of COPO. But as to the one known 65 COPO 396/425 Chevelle, it was built at KC like the production Z16's, so not that car.

Regarding the "848" case, per https://www.chevydiy.com/muncie-4-sp...ication-guide/ :

"Corvette production determined how Muncie design evolved. By late 1965, the 396 Turbo Jet engine, rated at 425 hp, was introduced. A very small number of 396 engines also found their way into the 1965 Impala. I learned from speaking to many builders that the rare 3864848 case seems to be in a few Corvettes and full-size GM cars with big-block engines. The 3864848 case with the last 848 numbers milled off is in some full-size cars, such as the Impala. This usually indicates that the gears used in the milled cases might be of a different steel alloy. The case appears to be identical to a 3851325 case so perhaps it is made of a different aluminum alloy. It appears that more milled cases are in circulation compared to unmilled cases. They all seem to come from big-block cars."

I have no idea how accurate any of that is, though. The second and third sentences are certainly not entirely accurate (396/425 was more of a "mid" model-year release than "late", and there were a lot more than a "very small number" of 396's installed in 65 Impalas), so I take it all with a grain of salt ...

Having the Fremont assembly location, it seems it was in a full size with the 396/425 engine. Still rare and with Jeff's comments about possible different alloys in gears and cast material, still a mystery.


Someone should be able to piece together a vin for a Fremont full size car from what you have now.

As always, thank you Jeff for taking the time to share your knowledge.
I guess the only other clue might be the lack of a division number stamp before the year number. It looks like they ran all 4 divisions through Fremont in '65. Buick big blocks were introduced in '67. Pontiac had the uniblock, so possible..but I'm leaning towards the Olds F-85, 442 through the process of elimination, assuming that what is stamped is correct.
Dave.

I don't think they built full size Chevrolets at Fremont in 65. Only A-bodies and trucks. Can anyone confirm?

Correct
 

Impalas.org Warren’s site