Serial numbers from 1969 on were appended with a ratio designator. See Chapter 2 for more information on serial numbers.
The 1970 model was the same as the 1969, and 1970 proved to be a transitional year. The final main case casting of 3925661 was introduced, but there are huge overlaps in date codes. This means a transmission could be assembled in April with a 3925661 case and then in July with a 3925660 case.
The introduction of the transmission controlled spark (TCS) switch on the Muncie sidecover seemed to appear in 1970. This is an emissions device and its function is to disable the ignition system’s vacuum advance until the transmission is shifted into fourth gear. Owners of most of these cars eliminated this system because they felt it hurt performance. It’s quite common to see the switch plugged off or just filled with sealant.
1971–1974
In the final years of Muncie production, the best engineering improvements for strength were incorporated into the gearboxes. The biggest change was the number of splines on both the input and output shafts. The input shaft spline count was increased from 10 to 26 splines along the same 1.125-inch-diameter shaft. The output shaft was increased from 27 splines to 32.
This is a typical transmission controlled spark (TCS) sidecover. The switch is in great shape. Some switches have a bayonet end (shown), and later ones have a pin-type male connector.
The 3925661 main case casting was used exclusively and a new tailhousing (3978764) with only three shifter-mounting holes was added. The transmission was also 3/4 inch longer than older models. Fitment issues might arise when installing one of these transmissions in a pre-1971 car. The shifter bolt pattern does not allow early linkage mounting plates to bolt to it. You have to shorten the driveshaft; of course, you also have to change the clutch disc and driveshaft yoke. Hurst Competition Plus shifters designed for earlier transmissions usually do not fit on these gearboxes. You have to purchase Hurst shifters intended for the make and model car that these gearboxes came in.
A few undocumented synchronizer updates were made. The design of the slider strut key groove was wider and tapered to reduce key wear. These assemblies came with heavier strut key springs. The synchronizer hubs had a very tight press fit to the mainshaft because the hubs apparently were nitrided. (Nitriding is a heat-treating process that adds surface hardness to the hub splines without distortion.) The sidecovers of some 1970 units had the heavier 20-pound detent spring but by 1971 all the covers had these springs.
This photo shows that the overall length of the rectangular drive portion is longer on the bolt-style arm compared to that of the stud type. The rectangular slot of the linkage arm must fit properly. Mixing late long-slot arms on early short-slot shafts causes the linkage to go out of alignment.
All ratios (M20, M21, and M22) came with the 26/32-spline configuration. Even to this day there is a misconception that all “fine spline” Muncies are M22 “RockCrushers,” which is not true. In actuality, more M20 units were produced with this configuration than were M21 and M22 models. The last year that the M22 ratio was available as an option was 1972.
The left shifter shaft has a 3/8-24 threaded stud with machined flats on both sides. The 1969-and-up shafts used a bolt-on style, which eliminated broken studs and stripped threads.
By the end of 1974, Muncies were no longer being installed in GM cars. The weaker BorgWarner ST10 replaced them and by 1975 big-block cars were no longer being produced. At this time, the Corvette had a 165-hp small-block engine, catalytic converter, and “unleaded fuel only” stickers on the gas cap doors. Transmissions had to be geared to work with economy axle ratios such as a 3.08:1 rear. The Muncie was never designed for that. To redesign the Muncie and downgrade it would have cost a lot of money. It was much easier to replace it with the ST10. The ST10 duplicated the current spline configuration, length, and shifter bolt patterns.
The upper mainshaft has 27 splines and the lower has 32. Because larger-spline 1971–1974 Muncies are larger in diameter, a different driveshaft yoke is needed. The extension housings also take different bushings and seals. Make sure you order the proper gasket and seal kits when rebuilding these later units.
The glory days of the 4-speed Muncie were over and BorgWarner once again was back in the saddle. The ST10’s production in GM vehicles was from 1974 to 1988. Because of production quality issues, no Corvettes were produced in 1983. The ST10 was used with an overdrive in 1984 to 1988 Corvettes only and was called the Doug Nash 4+3 (even though the patent for the original Muncie includes the ST10). In a sense, 1988 marked the last year this type of transmission was used in a GM vehicle.
Notice the difference in the drive slots of these arms. The example on the left is for pre-1969 transmissions and the one on the right is for those that accept a 3/8-16 threaded bolt. The slot on later arms always has a circular cutout to make room for the bolt. Adapter clips are available to take up the extra space so you can use later arms on stud-type shifter shafts.
This synchro slider is the earlier design with the smaller strut key pocket. There are no part numbers in the GM system that differentiate these assemblies. Typically, the strut key springs had a lighter tension compared to later styles.
The later slider had a smoother, ramped, strut key pocket. These used a heaver tension spring compared to earlier sliders. Today I use a spring that has a tension between the early and late springs.
The left synchro hub is the standard hub and the right is a factory-nitrided hub. Notice the color difference between them. These hubs fit extremely tightly on mainshafts, and you need to have the proper press clamps or pullers to remove them.
This is the standard Muncie 4-speed case. It has a seven-digit serial number cast into the case on the passenger’s side. These are usually the first numbers used to start the identification process.
It was much easier to stockpile Muncie spare parts and cores in the 1970s than it is today. Reliable sources to validate information regarding serial and casting numbers will get scarcer as time passes. The specialty rebuilders that have