To understand the evolution of the Muncie 4-speed you have to look at a series of engineering platforms that led to the final design of this transmission. The Muncie design has roots going back to 1935. I took the time to research the patent number that is cast into most Muncie main cases. It is U.S. Patent Number 3,088,336 (see Appendix). You will see that James W. Fodrea designed the patent; no other engineers are listed. If you look closely, you’ll see that the patent drawings look nothing like the Muncie 4-speed but rather like the BorgWarner T10! Therefore, the Muncie patent is basically a design for the layout of a 4-speed transmission. This “layout” is a 4-speed transmission with four forward gear ratios in the main case, a midplate bearing support, and a reverse gearset in the extension housing.
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BorgWarner was a company founded in 1928 from the merger of Borg and Beck (founded in 1904) and Warner Gear (founded in 1901). They designed a 3-speed transmission, the T85, which was originally used in the 1935 Chrysler Airflow. It was used right up to 1971 in the Ford Pickup F100 with an overdrive. The T10 shares the same case design, gear centers, and 3-4 synchronizer as the T85. The cases have a distinctive similar size, shape, and cover-bolt pattern.
If you look at Fodrea’s 4-speed patent drawings you can clearly see it was based on the T85 platform. His brilliant and well-thought-out idea was to make it into a 4-speed. It’s important to understand that in 1956 General Motors didn’t have the money for the Corvette program. Zora Arkus-Duntov wanted to use the design of General Motors’ employee James Fodrea. BorgWarner was probably the foremost manual transmission manufacturer at the time, and basically wrote the book on manual transmission design. (They are credited with hundreds of manual transmission patents.) It is therefore no surprise that a decision was made for BorgWarner to manufacture this 4-speed transmission based on the GM concept drawings and Fodrea’s patent. It was a very logical decision: Tooling costs would be minimal because hobs, castings, and certain components could be used from the T85 platform. This would be the fastest and most cost-effective way to put a 4-speed into a Corvette with very little risk. Many Corvette restoration books share the common misconception that the 4-speed T10 and Muncie are two separate entities. They are not; the Muncie evolved from the T10.
Happy Accidents Create the Muncie
The 4-speed design of the T10 and Muncie is a very “forward” design. Whether it was a series of lucky choices, or happy accidents, the 1957 design allowed for improvements. Other muscle car transmissions of that era, such as the Ford Toploader and Mopar A833, left no room for improvements because of the layout and initial design of their geartrains. The Super T10 was a later-version design of the T10.
BorgWarner sold the T10 to Doug Nash in the early 1980s, which then sold it to Richmond Gear. In 2012, Motive Gear acquired Richmond Gear and they are still manufacturing the Super T10 today. Several NASCAR transmissions, such as the G-Force T101, are also T10-based. The Muncie saw many improvements during its 10-year production run with General Motors. Auto Gear Equipment (AGE) currently produces Muncie replacement parts as well as new replacement transmissions. Auto Gear sells them directly and also through approved distributors. Auto Gear’s “Syracuse 4-speed” is a Muncie on steroids.
General Motors received royalties for every T10 sold from BorgWarner, so you have to wonder why they would bother making their own 4-speed at the Muncie plant. It appears that BorgWarner had an exclusivity contract with General Motors until 1960. This was the first year that the T10 was used in the Ford Galaxie and Fairlane. Soon after, Chrysler and American Motors began using the T10. With the power levels increasing in GM muscle cars the power capacity of the T10 also needed to increase.
I believe the reasons for bringing the T10 to the Muncie plant were threefold. First was to revamp the T10 to handle more power. Second was the direct benefit of the increased sales volume of the 4-speed GM muscle car market. The third reason was increased T10 royalty benefits.
Muncie Design Changes
Good engineering should allow for improvement to the design. When designing transmissions you have to remember that as vehicles change dynamically (increased horsepower, weight, or gas mileage requirements) the transmission also has to change. Because the Muncie was well engineered, there was room for improvement to the base design. Modern automation gives companies the ability to store incredible amounts of data; it’s much easier to track changes. Today’s VINs (vehicle identification numbers) are even bar coded. A service technician can use a scanner to find a VIN, and any known service issues are easily found.
From the 1960s until the late 1980s General Motors issued Technical Service Bulletins by mail or fax to alert dealerships of potential service issues. I’m not a fan of these but I do understand their importance. I do not like them because, for the most part, they are admitting defects to a design. They fix it if there is a complaint but do not order a recall. Recalls are bad publicity, so it’s easier to fix the problem silently rather than risk sales. The problem is that some cars just aren’t driven very often. The service issues crop up after the warranty period has expired and the owner is left to pay for a repair on something that was defective in the first place.
In the early 1980s I did a great deal of repair work for GM dealers nationwide for the Corvette 4+3 overdrive. It had three major service issues affecting 1984 and 1985 models. I had the bulletins, but most of these service issues happened after the warranty period ended.
I bring this up so you can better understand how service updates and design changes are handled. You also need to remember that all record keeping was done manually and sometimes the changes were left undocumented. It is often very difficult to decipher what part numbers actually match the part you may need because the GM parts books have discrepancies.
The first Muncie has several unique features that were dropped by 1964. It had a small 6207-style front bearing and an aluminum front bearing retainer. This retainer was upgraded to cast iron by the end of the 1963 run. The 3831704 cast main case is unique because the front bore is smaller than it is on later Muncies. The first-speed gear rode directly on the mainshaft. A snap ring retained the first and second synchronizer assembly on the mainshaft. The first-speed gear had a smaller bore diameter as well as a recess in the bore to clear the synchronizer retaining snap ring. It had a thrust washer behind first gear that floated on the rear bearing inner race.
The countershaft diameter of 7/8 inch and front bearing were both carried over from the T10 4-speed design. The shifter shafts had 5/1618 threaded studs.
The GM service manuals are interesting. For some reason, the unique 1963-only items were still used in exploded-view illustrations, which confused many rebuilders into the early 1970s.
The Muncie was designed to shift better than the T10; it used larger-diameter synchro cones. Both the M20 wide-ratio and M21 close-ratio transmissions were offered. These were the only ratios ever offered from General Motors for the Muncie 4-speed:
- M20 first, 2.56:1; second, 1.91:1; third, 1.48:1; fourth, 1.0:1; and reverse, 2.64:1
- M21 first, 2.20:1; second, 1.64:1;third, 1.28:1; fourth, 1.0:1; and reverse, 2.27:1
Two major improvements were issued. The first was the introduction of a larger-diameter front bearing that meant a new case casting and larger-diameter front bearing retainer were necessary. The second was that the first-speed gear now rode on a bushing that was press-fit onto the mainshaft. It stopped against the first and second synchronizer assembly, thereby eliminating the need for the assembly to have a retaining snap ring. Because the bushing was subsequently retained by the rear bearing, the synchronizer could not go anywhere. The first-gear thrust washer was eliminated and the gear was designed to have a thrust surface that ran against the rear bearing’s inner race.
The first-gear design change was done for several reasons. The first was added strength. Whenever you have a snap-ring groove between a flow of power you have a potential stress riser on the shaft. Because the slider engages first gear across the snap-ring groove, a huge stress riser develops that leads to broken mainshafts. First gear also had a tendency to seize to the mainshaft. Cutting grooves and valleys for oil in the shaft only weakened the shaft more. A bushing was used with a “v” notch to promote better oil flow under the gear.
By the end of 1965 the rear extension housing saw some modifications to the casting. Webbing was added to the top and bottom. Small changes in countergear needle bearing spacer tubes surfaced. Some tubes were seamless with four needle spacers while others had a seam with six spacers. There seems to be no specific time when this change took place. By 1965 the shifter-shaft designs changed because they had been snapping. The new thread size of the stud was increased to 3/8-24.
These spacer tubes go inside the countergear. The upper tube has no seam but the lower one does. Because the needle bearings ride against the seam, extra spacer rings are needed to cover the seam. Typically, the seamless spacer had four needle-spacer rings. The seamed spacer tube used six. Most of the later GM overhaul shop manuals show four spacers in the exploded-view diagrams when, in fact, the transmissions used six.
In 1964 and 1965, Muncies in some of the full-size Chevrolet Impalas and Pontiac Catalinas were equipped with longer mainshafts and extension housings to keep driveshafts shorter and reduce harmonic vibration.
In 1963 the Corvette Grand Sport racing program was instituted. The early Grand Sports used a special heavy-duty version of the M21 close-ratio transmission. These special units evolved into what is called the M22 today. According to research by Alan Colvin (author of the Chevrolet by the Numbers books), 57 M22 units were actually built for 1965 production. The engineering change documentation for the M22 is dated December 12, 1964.
The change basically states that a new gearset is to be used with different synchronizer assemblies, the main case is to be modified to accept a drain plug, and the countershaft bore of the case is to be machined to accept a 1-inch-diameter counter-shaft. A letter to Zora Arkus-Duntov dated December 8, 1964, is referenced in this engineering change stating successful use of the M22 in Grand Sport Corvette field testing.
So exactly what is an M22? The RPO M22 stands for Heavy-Duty Close-Ratio. Many people think the gearset had some different alloy compared to the standard sets, but it didn’t. According to original engineering drawings I have of the M22 first gear, it is made of an 8620-alloy steel. The same alloy is used to manufacture the M20 and M21 gears. The difference is the notation on the drawing to add shot peening to the gears.
Shot peening is a process in which the gear is blasted (like sand blasting) with steel shot. Steel shot is spherical and the gear surface develops thousands of microscopic dimples when the shot hits the gear. These dimples reduce stress risers on the area’s gear teeth that can develop cracks because of fatigue.
The tooth counts and gear pressure angles of M20, M21, and M22 gears are the same. The difference is the helix angle. If you reduce the helix angle of the gear you reduce thrust loading on the main case. Reduced thrust loading reduces heat and yields less horsepower loss to the rear wheels, but it increases gear noise. Hence the name “RockCrusher.” Muncie 4-speeds have varying helix angles in the gearsets. Typically, the M20/M21 gearsets have a first-gear helix angle of 26.4 degrees and an input shaft angle of 39 degrees. The reduction in the angle with the M22 is quite substantial. The M22 first gear has an angle of 14.5 degrees and the input shaft is 24.2 degrees.
Several major improvements began in 1966. The diameter of the main case countershaft bore was officially increased to 1 inch. The most common main case casting was 3885010. This larger diameter was necessary because the big-block and small-block engines were producing more power. As a result, all M20, M21, and M22 countergears had to be redesigned to accommodate a larger countershaft. The needle bearings changed from .156 to .125 inch and the diameter of the spacer tubes also changed. New thrust washers for the countergear were also needed because their bore size changed and the location of the holding tang of the thrust washer was also redesigned.
The synchronizer assemblies were also updated to what is commonly called a “shoulder style” synchro ring. The early 1963–1965 ring had a tendency to crack at the strut key slot. Therefore, the ring was redesigned with material added to create a shoulder in front of the synchro teeth. It’s important to know that the later rings used a narrow synchronizer hub to compensate for this increased thickness. If you mix them up you end up with shifting problems.
The M20 gearset’s front end was also redesigned to handle more power. Both the M20 input shaft and front of the countergear were machined with thicker teeth. This was accomplished by reducing the input shaft tooth count to 21 from 24 and the countergear’s maindrive section to 25 from 29 teeth. The 25/21tooth headset ratio is 1.19:1 and the 29/24 ratio is 1.21:1. This yields a slightly different M20 ratio set.
Available Gear Ratios
Prior to 1966 order options really didn’t exist for Muncies. General Motors optioned cars with either the M20 or M21 ratios based on engine types and axle ratios. Many window and tank stickers exist from before 1966 that list cars as having M20s when in fact they had M21s. M22 production was rare in both 1966 and 1967; 29 were produced in 1966 and only 20 in L88 Corvettes in 1967.
Speedometer fitting placement was also changed. Before 1966 the speedometer gear “bullet” fitting on the extension housing was located on the driver’s side in the middle of the shift linkage and below the oil level. The passenger-side speedometer extension housing also has added material between the upper shifter mounting holes. These extension housings always had a tendency to leak as well as get in the way of after-market shifters. For some reason, Pontiac kept the driver-side speedo while Chevrolet and Oldsmobile did not.
A small update was also added to the pivot pin on the sidecover that holds the whole internal interlock and detent system. It was press-fitted into the cover but it had a tendency to fall into the transmission. The new pin design simply added a hat to the end of the pin so it could not fall through.
Beginning in 1967, transmissions had a date designator added to the serial numbers. For prior years only a month and date were added.
1968, 1969 and 1970
The 1968 model was identical to the 1967 model except for a main case casting alloy change. The most common main case casting used during this period was 3925660. The front bearing retainer was also changed to a thicker casting. The height of the casting changed from roughly .325 inch to .450 inch. This made piloting the transmission into the bellhousing an easier operation.
More M22s were produced from 1968 to 1970 than in any other period. Using data from Alan Colvin’s Chevrolet by the Numbers books there were approximately 13,700 M22s made in the era compared to approximately 6,400 made in 1971 and 1972. What that means is that from 1968 to 1970 there were more factory 10-spline-input M22s assembled than the later 26-splines.
The 1969 design changes were subtle, and now all models had drain plugs as a standard issue. It was no longer just an M22 thing.
This was also the first year of “bolt-on” shifter shafts. Shifter shafts damaged because of broken studs or stripped threads were now a problem of the past. The rectangular drive portion of the shifter shaft was also increased in length from .605 to .722 inch. Shifter linkage arms were changed because the drive slot now had to be longer to match the drive portion of the mating shaft. The slot width of .315 inch remained unchanged. Putting a later linkage arm on a pre-1969 shaft can cause the linkage to become loose and out of alignment.
The speedometer drivegear only came as a molded 8-tooth gear. It was held in place with a spring-steel clip. It was obviously a move to save money and consolidate inventory since all transmissions would be assembled internally using all the same parts. Before 1969, different internal steel drivegears were installed to match specific axle ratios and tire sizes. The later plastic drive-gears frequently failed; the transmission had to be disassembled and the equivalent press-on 8-tooth steel gear had to be used anyway.
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.
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.
The 3925661 main case casting was used exclusively and a new tail-housing (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 pre1971 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.
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.
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 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.
Written by Paul Cangialosi and Posted with Permission of CarTechBooks