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Tech Reference Page #15

Converting to a 4 speed

Authored by Wes Vann, last modified on November 26, 1998

PLEASE NOTE; Always use jack stands while working under a car. I would also recommend that you have another person present when lifting a transmission into place.

Due to differences between the years on Chevelles, this page will be rather general in terms. I haven't personally done this swap on a Chevelle, but did convert a 69 Camaro from TH350 to Muncie 4 speed several year ago. I was able to pull the auto trans and install the 4 speed in just one day (this doesn't include the pedal and bell-crank assemblies).

I can't stress enough the idea of getting a "factory" manual for your year car and also trying to find a car that you can look over real close that was a 4 speed from the factory!

I'm going to lay this out in a format that goes into the different items that have to be addressed in order to do this conversion. It's not going to be a step-by-step type thing.

Be Careful!

Although I've bench-pressed a 4 speed trans into place, that old automatic just weighs too much. Count on using a good floor jack and also jack stands for the car! We here at Team Chevelle want you to be able to stop by again!

Types of transmissions

1. The M20 Muncie is a wide ratio 4 speed transmission.

2. The M21 was a close ratio transmission (and as far as I know was the most common).

3. M22 transmissions are also close ratio and are the ones called the "rock crusher". They have the gears cut at a different angle than in the M21. This makes them stronger, but also more noisy. I personally feel that you will spin the rear wheels before you break a M21. The M22 may require a different yoke for the driveshaft (this fact is a little "fuzzy").

4. On the Muncie transmissions, it's possible to tell which one you have by the circular cuts on the input shaft t (across the splines). I believe that the M20 had two grooves. The M21 had one, and the M22 had none. I would also wager that somebody has taken the transmission apart in the past and I would NOT trust this test!

5. There is a difference in the side covers on the newer Muncie's in relationship to the older ones. On the side cover, there is a "pin" mounted just about in the center. This pin is a pivot shaft inside the transmission that is part of the linkage that prevents you from putting it in two gears at once. On the newer transmissions, this pin has a shoulder on it. In other words, when viewed from outside, it looks like a very large nail head. On a older side cover, all you will see is the end of the shaft (pin). If you get an older side cover, replace the pin with the newer style. (I've had this pin fall into the trans. I figured it out before the pin ended up between the gears!)

6. There was also the Saginaw and Borg Warner transmissions.


You will need a bellhousing for a Chevy. You can't use a Buick, Pontiac, or Oldsmobile! The bolt pattern for the Chevy engine is totally different.

There are two sizes of bellhousings. This is to accommodate the different diameter flywheels. The small diameter bellhousing is part number 3858403. The large diameter bellhousing is part number 3899621.

Although a shatter proof bellhousing may seem like overkill, the cost of a stock one may be close enough to make it worth while. If you are going to do any form of drag racing, just do it (it may save your legs!).


Well, you need that extra pedal! That's the whole idea.

What you need is both the brake and clutch pedals. The way that they work is that when you have only the brake pedal, there is a pivot "pin" for it. When you have both pedals, the pivot pin is part of the clutch pedal it's self.

The pedals should have nylon bushings for them. I recommend that you replace them at this time.

Through the firewall

It would seem that Chevy didn't do you any favors here. From what I understand, you will have to drill the hole for the clutch pedal and then add the "factory" grommet (also called a bellows). Get the grommet before doing any work.

Transfer interrupted!

>Bell-crank assembly (Z-bar)

The bell-crank is the linkage that pivots on the ball joint mounted on the frame and the other ball joint on the side of the engine.

In order to mount this, you have to add the bracket that is on the side of the frame (close to the inside surface of the front tire). You can buy reproductions of this bracket from various suppliers. On the older Chevelles, the bracket gets welded to the frame. On the newer Chevelles it bolts on (the holes may be there, but will need to tap them for the bolts.

The pivot ball that goes on the engine just screws in. The location is near where the oil filter is. Every small block that I have seen has had this location already taped for the pivot ball. On some of the big blocks, there isn't a provision for this pivot ball.

The big block cars require a different bell-crank than a small block due to the bar having to clear the oil pressure sending unit. (thanks go out to Al Rodenwald, aka bigdog1)

Flywheel, Clutch, and stuff

You will need a flywheel (an automatic transmission uses a "flex plate"). If you get one that has the same ring gear size, you will not have to replace the starter. They come in 168 and 153 tooth versions.

Be aware of the fact that some G.M. engines are externally balanced (400 small block and 454 big block), and as a result, you have to have the correct flywheel. Also keep in mind that newer small blocks have a different crank (rear seal area), and as such, their flywheel is also specific to that engine.

I used a standard diaphragm type clutch and it was always adequate for the mild 350 engine that I had. There are all sorts of different types of clutches and you will just have to figure out what you need. Just don't sell your self on that mega dollar multiple disk set-up because your friends will be impressed!

Put in a new throw-out bearing any time that you have the transmission out! They don't cost that much and the dollars are worth the pain in pulling the trans again!

You will have to add the bushing that goes in the end of the crankshaft. This bushing is for the input shaft for the transmission. There are roller-bearing bushings that can be used, but I really don't see the need.

Don't forget the throw-out bearing arm & it's pivot ball (that mounts inside the bellhousing).

Another item that tends to get lost is the spring that holds pressure on the push shaft between the cross-shaft and the throw-out bearing arm. If you don't have this spring, when you release the clutch real fast, the push arm can fall out. This means that you have to drive home without a working clutch.

Shifters (and humps)

When I did the conversion, I used a Hurst Comp plus shifter and it always worked well. The Comp plus (at the time, I'm not sure about now) had several different levers that come through the floor with different curves. That way, you could change the position of the shift knob.

If you do the conversion on a Chevelle, there is a metal "hump" that has to be added to the transmission tunnel. They can be purchased and welded in place for that stock look.

If you plan on using a "stock" center console, you will have to use the stock (GM) shifter or the Hurst Comp plus in order for it to come out in the correct area of the transmission tunnel.

Additional notes and comments

1. I was able to use the drive shaft and transmission yoke from the TH350. I believe that if you are converting from a Powerglide or (short tailshaft) TH350, you will not have to modify your driveshaft or yoke. If the car had a TH400, then you would need a longer driveshaft.

2. A car with an automatic trans will have a neutral safety switch. On some (if not all) cars with a manual trans, there is a switch that makes you depress the clutch in order to crank over the engine. Although I'm not really recommending this, what I did was just bypass the switch. It's normally (don't bet the house on this) a heavy purple wire.

3. If your car had a column shifter originally for the automatic, you may want to change it. Your options vary depending on what year the car is. There are too may variables to list here!

4. I was able to use the original transmission support. I just got new rubber trans mounts.

5. The speedometer cable just hooked up! Don't count on it being that simple, you may need a longer or shorter cable.

6. When you bolt the clutch assembly to the flywheel, you should use an alignment tool. These are nothing more than a plastic molded piece that is the same as the transmission input shaft.

7. I found the easiest way to bolt the transmission up, and line up the clutch disk, is to first bolt up the bellhousing to the engine, and then bolt up the transmission to the bellhousing. When you are trying to slide in the transmission, if it doesn't line up, you can have a friend depress the clutch pedal. This lets the clutch disk float free.

8. Use only top quality bolt for the flywheel to crank connection and also for the clutch pressure plate! You can get grade 8 bolt "kits" from places like PAW (mail order). I'd recommend "loctite" "stud and bearing mount" for the bolts that go into the crank. Use standard "loctite" for the clutch pressure plate.

Good luck and be careful (a transmission falling off the jack onto your hand isn't a pretty site!)