Hot Rods - SM318 Muncie information (2025)

I am interested in several discussions about this transmission. It has been misidentified over the years, and I still see them occasionally. The old closed driveshaft Chevrolet vehicles had a similar trans, and it seems it was used in the open drivelines until about 65 or 66. I need to build a 65 C10 and want to be sure to start searching for parts. Seems in the later units, there was some sort of change of the gear angle(maybe). Anyone familiar with how to tell or identify what internal gearing I have? Especially in relation to the gear pitch/angle I am talking about.I saw where 56sedandelivery had an informative article before. Also how do I decode the build date castings on various parts of this unit? Any info would certainly be appreciated. And of course, parts suppliers(especially the 2-3 synchro drum). Thanks!

I think you'll find the "late" 1965-66 gears in transmissions with the fine spline output shaft.

Casting dates are like other Chevy castings. Letter is the month, A=Jan, B=Feb, etc. Then one or two numbers for the day of month. Then one or two numbers for the last digit(s) of the year.

I have Chevy parts books that cover all the years the transmission was made, so if you have specific questions, just ask

I've built hundreds of them. They're a great operating, simple to repair trans. They did exactly what they were designed to do. They won't last long today if used behind anything with a lot of power and driven hard. Consider going to a Saginaw for the many improvements - it bolts in & is going to save you a bunch of money in the long run.

Update- had 2 65 or 66 transmissions- both had 16 spline out put shafts. One housing had 3743368 &L300 the other had 3830210 &11064. I am thinking that 16 splines are not fine spline. I am wondering if the 8 cylinders had the fine spline output shaft. Also is it true that some of these had a different gear pitch? Thank for the responses !

the date codes you listed L 30 0 is from a 1961 model truck, the other I 10 64 is from a 1964 model truck. So neither of them would be the 65-66 transmissions.

Do you have any reference material that describes how you decoded these casting numbers? And how about gear helix being different on some of the internal gears? If the helix angle has been changed , can the different helixes be compared and measure? Thanks Again!

again:

Looking at the parts book, most parts are the same through 68.

I have a vague memory of a close ratio version used in some Pontiacs that had lots of gear differences from the Chevy version. Different angles and tooth counts. Can't find any documentation yet, but still looking.

edit: Seems like gear angles stayed the same,but there was a close ratio version used in some Pontiacs and Chevys, including early Corvettes and later big block cars. all the parts with swap in full sets so no telling what's in there now. But, the close ratio versions Main Drive Gear was larger than the MDG bearing, so pretty easy to spot, with the side cover off.