I have a 66 base model coupe which has had the optional 4 speed M20 transmission replaced with a close ratio muncie from a 71. I'm going to have to spend a fair amount to have it rebuilt, and I'm wondering if it makes more sense to rebuild the one I've got, or purchase a rebuilt correct M20, both from a judging perspective and from a performance standpoint. Thanks for any help.
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Re: 66 Transmission
no difference in performance, '71 case is part number ending in 661, '66 ends in 010, not easily visible, not judged. -
Re: 66 Transmission
Rick-----
For 1966, the M-20 was a wide ratio transmission. Personally, I think that the wide ratio transmission is a better trans for street operation since it gives you a lower 1st gear ratio. Combining this with a higer ratio (lower numerically) rear gears provides good 1st gear "off-stoplight" acceleration and lower cruising RPMs for quieter, better mpg highway driving. In any event, the wide ratio always provides improved "off-stoplight" acceleration over an equivalent close ratio trans.
The internal parts and configuration of a 1966 Muncie and a 1971 Muncie are mostly the same, allowing, of course, for the M-20 and M-21 gear differences (input and cluster gear). However, there are 2 other differences between a 1966 and 1971 Muncie: first, the 1971 Muncie output section of the mainshaft is larger in diameter and slightly longer than the 1966, and; second, the 1971 extension housing was redesigned to accomodate the larger and longer output shaft. Consequently, when the 1971 Muncie was installed in your car the driveshaft had to be changed or altered. Since the rear transmission yoke for the 71 is also different than the 66, there are several things that could have been done to install it in your car:
1) install your original driveshaft, shortened about 3/4" and install a conversion u-joint on the forward end. A conversion u-joint would allow the use of the existing driveshaft yoke and "adapt" it to the 71 Muncie rear yoke. It has one pair of opposing fingers of one diameter and the other pair of fingers of another diamter;
2) install your existing driveshaft shortened by 3/4" and replace the front yoke with one of 4-5/16" finger diameter (same size as the 71 Muncie output yoke). This way you end up with a driveshaft with different finger diameter yokes on either end;
3) install the 71 driveshaft and use a conversion u-joint at the REAR position
4) install a 71 driveshaft or made-up driveshaft and install a rear yoke of 3-27/32" finger diameter, the same as the diameter of the 1966 differential's companion (pinion) flange. This also results in a driveshaft with different finger diameter yokes at either end;
5) install the 71 driveshaft and change the differential's companion flange to the 4-5/16" finger diameter size (GM #3879208);
6) install the 71 driveshaft and change the rear differential carrier unit to a 71-79 unit which already uses the above-referenced companion flange.
If your existing transmission is, indeed, a complete 1971 unit and you convert back to a 1966 unit, then you will have to "undo" the above-referenced. Check the casting number on your transmissions extension housing. If it's GM #3978764, then you have a complete 71 transmission and, if you wish to convert back to an original 66 transmission, you will need to "undo" whichever conversion option was used to adapt it to your car.In Appreciation of John HinckleyComment
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Re: 66 Transmission
Guys - The main case number is 3925661, and the tailhousing cast # is 3978764, so it does appear to be a complete 71. I haven't been able to get any identifying numbers off the differential so I don't know whether it is the original 66, or part of the 71 installation. I suspect the latter mainly because the replacement installation was obviously not a good fit when it was done - The flanges on the drive shaft and front differential pinion connection are too large and they actually cut away a small piece of the floor pan in the rear storage compartment so they'd fit. Any way thanks for the great information.Comment
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Re: 66 Transmission
In addition to the items that Joe Lucia pointed out, the '71 transmission has a 26 spline input shaft. If you convert back to a '66 transmission you will have to get a clutch disc to fit the '66's 10 spline input shaft. See www.5speeds.comComment
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Re: 66 Transmission
Joe-----
Yes, that's correct. I should have mentioned that. I got so focused on the rear of the transmission, I forgot about the front!In Appreciation of John HinckleyComment
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Re: 66 Transmission
I would add that the shifter mounting bracket will be located in the wrong place. The 71 tailhousing has only 3 shifter mounting bosses and they are in a different location as compared to the original 5 lug setup. This can cause fitment issues with stock floorpan/console/shifter/linkage. Been there, done that. What type of shifter is in it now? My 2 cents is that the conversion back to stock configuration should be carefully considered at this point. The sale of the old incorrect stuff will help pay for the new correct stuff. I just did this to one of my rides and am very happy I made the switch back. Good luck with what ever you decide.
-Mark.Comment
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Re: 66 Transmission
Mark-----
Most 1966 Corvettes don't use the tailshaft-mounted shifter assembly. Beginning in early 1966, the shifter was moved to a crossmember-mounted bracket, GM #3888263. This bracket was then used throughout the remainder of C2 PRODUCTION. A similar, but different, bracket was used for 68-81. So, after early 1966, Corvettes no longer used any of the threaded bosses on the transmission extension housing.
Personally, I recommend the installation of the GM #3888263 bracket on any 66 originally not so-equipped and, for that matter, on any 64-65, as well.In Appreciation of John HinckleyComment
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