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How to Identify a Muncie Four Speed

Technically Speaking: Get it in the right gear

James Berry Jun 22, 2016
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Q: Hello, my name is Zack. My dad and I are in the process of restoring a 1964 Corvette. My hope is that we can get the car running and driving for my senior prom next year.

The Corvette was originally equipped with a Muncie four-speed but the transmission was missing when we bought the car. I know we will probably never find the original transmission but I would like to get the most correct transmission possible for the car.

We recently went to a local swap meet but our problem is that all of the four-speed transmissions look the same. In fact, we bought a BorgWarner transmission by mistake. Is there an easy way to tell which transmission we should be looking for? I guess I am looking for a Muncie transmission guide for dummies.

Thanks,
Zack

A: Zack, I am excited that you and your father are working on a project car together. This is what makes the car hobby so great.

The Muncie four-speed transmission was used with many different models and engine combinations from 1963 through 1974. And you’re right, Zack, not too many people actually know how to distinguish a Muncie four-speed transmission from a Saginaw or BorgWarner, or even what type of Muncie four-speed they have.

If you don’t know what to look for it can be tricky to tell exactly which transmission you are looking at so here are a couple of methods to help you quickly recognize a Muncie.

At a quick glance the Muncie can be easily distinguished from the Saginaw since the reverse lever on the Muncie is mounted on the extension housing of the transmission, whereas on the Saginaw transmission the reverse lever is mounted in the side cover of the transmission.

The main difference between the Muncie and the BorgWarner is that the Muncie has a seven-bolt side cover and the BorgWarner has a nine-bolt side cover.

The basic appearance, design and operation of the Muncie four-speed did not change much during the period when the transmission was used in GM production cars.

However, there were several design upgrades and modifications made during the total production run. The Muncie was used with most performance General Motors engines.

Identifying The Muncie

Muncie four-speeds were produced in two different ratios: the wide ratio (M20) and the close-ratio (M21). There also was a second, heavy-duty close-ratio version: the “Rock Crusher” (M22) that was offered on some of the high-performance big-block models, starting with the 1965 Corvette.

The choice of transmission usually was determined by the engine size and rear axle ratio. An axle ratio of 3.73 and higher usually came with a close-ratio transmission. While vehicles with an axle ratio of 3.55 and lower usually used a wide-ratio transmission.

Several things are needed to identify a Muncie four-speed transmission correctly. One of the best ways to identify a Muncie is the casting numbers on the external components of the transmission.

2 Main Case Numbers 2/4

Main Case Numbers
Casting Number Year Notes
3831704 1963 Patent Pending, 7/8" Bore, Small Bearing
3839606 1963 - 1964 Patent Pending, 7/8" Bore
3851325 1964 - 1965 Patent Pending, 7/8" Bore Mostly Used In 1964
3851325 1964 - 1965 Patent Number, 7/8" Bore
3864848 1965 Patent Number, 7/8" Bore
3864___ 1965 Patent Number, 7/8" Bore, last 3 Digits Milled Off Of Case
3885010 1965 - 1967 Patent Number, 1" Bore
3925660 1968 - 1970 Patent Number, 1" Bore
3925661 1970 - 1974 Patent Number, 1" Bore

3 Tail Shaft Housing Casting Numbers 3/4

Tailshaft Housing Casting Numbers
Casting Number Year Notes
3831731 1963 Thin Fin Tailshaft, Driver-Side-Mounted Speedometer, 27 Spline
3846429 1963 Thin Fin Tailshaft, Driver-Side-Mounted Speedometer, 27 Spline
3846429 1963 - 1965 Thick Web Tailshaft, Driver-Side-Mounted Speedometer, 27 Spline
9779246 1964 - 1965 Pontiac Catalina Long Tailshaft, Side-Mounted-Speedometer, 27 Spline
3857584 1966 - 1970 Passenger-Side-Mounted Speedometer, 27 Spline
3978764 1970 - 1974 Passenger-Side-Mounted Speedometer, 32 Spline

4 Early Side Cover Casting Numbers 4/4

Side Cover Casting Numbers

It is easy to determine an early Muncie four-speed transmission side cover from the later models by the way the transmission shifter arms are attached. The 1964 through 1968 Muncie side covers had a stud coming out of the side cover that the shift arms attached to using a nut.

The 1969 and later model side covers had a hole in the side cover that the shift arms attached to using a bolt.

Side Cover Casting Numbers
Casting Number Notes
3831707 Early Side Cover 1963 - 1965
3884685 Early Side Cover Used With 3857584 Tailshaft Housing
3950306 Late Side Cover - No Switches
3952642 Late Side Cover - Long Boss - With A Transmission Controlled Spark Switch
3952648 Late Side Cover - Short Boss - With A Transmission Controlled Spark Switch
335308 Late Side Cover - Long Boss – With A Neutral Safety Switch

We have identified the Muncie four-speed at a glance and have reviewed the transmission case numbers. This should get you started. Next time we will go over the input shaft and output shaft splines, rings or grooves that may exist around the input shaft splines. We will also go over how to read the date codes.

Zack, thanks for the letter and I am sure you and your father will have the car ready for your prom. Vette

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