69 vs 70 Muncie

[Roberto L.]

Hello friends, while restoring my 70's rear end happened to find some old aluminum Muncies in a gear shop in Argentina. Did a quick check of the numbers and found one to be a 5660 case which, think so, is a 68/69 item. Forgetting the date issues, what's the difference between the 660 case and 70's 661? Is the cover, with TCS switch?

Second, are difference big enough between Chevrolet Muncies and Corvette specific Muncies?

Thanks, Roberto, NCRS #30019, RMC

[Joe L.]

Re: 69 vs 70 Muncie

Roberto-----

The only visible difference, other than the casting number, between the '660' and '661' Muncie main cases is that most, if not all, '661' cases are drilled and tapped for a drain plug. For 68-69 '660' cases, only those used with M-22s were so-equipped.

The side covers for TCS-equipped 70-74 Muncies is different than the one used for non-TCS Muncies, but the side cover is, of course, not part of the main case. The 69 side cover was unique in that it lacked TCS provisions, but did used the improved fastner for the fork levers. This feature was lacking on the several 63-68 Muncie side covers. The 70-74 side covers had the TCS switch provisions as well as the improved, 69 style fastner.

For any given model year, the Corvette Muncie main case was the same as that used on all other Muncies across the whole GM car line. There were several different extension housings, though.

[Roberto L.]

Re: 69 vs 70 Muncie

Thanks Joe, quite clear.
So, my main target in the future is to check the extension of any found Muncie like those mentioned. Eventually I could swap extensions, adapting a non Corvette application to common M20 vette configuration.

Roberto, NCRS #30019, RMC

[Joe L.]

Re: 69 vs 70 Muncie

Roberto-----

Yes, you could. However, the GM #3857584 extension housing casting was used on many other 65-70 Chevrolet applications besides 65-70 Corvette. So, even the extension housings are somewhat interchangeable and relatively common. The '584' extension housing has provisions for shifter mounting which is totally unused on 1966 through 70 Corvette applications.

Beginning in 1971, the GM casting #3978764 extension housing began to be used in Corvettes and many other Chevrolet cars while the '661' main case continued to be used (the '661' was the last Muncie main case produced). Muncies with the '764' extension housing cannot be used on pre-71 Corvettes without changing the driveshaft, transmission yoke, u-joints, and pinion shaft yoke (unless a conversion u-joint or conversion driveshaft rear yoke is used). The 71-74 Muncie mainshaft is designed for use with the '764' extension housing and cannot be used with the '584' extension housing.

The only parts difference between the 66-70 Muncies and the 71-74 Muncies are the mainshaft, extension housing, rear bushing, rear seal, and speedo DRIVE gear. All other parts are interchangeable. As I mentioned previously, 1970 was the first year for the '661' main case but it is functionally interchangeable with the 68-69 '660' and other maincases used for 66-67.