[NYGTO]

Hi, can anyone tell me what is the correct gear tooth count for the M20 transmission, I'm referring to the gear INSIDE the trans and cable. Thanks

[Formulajones]

Are you talking speedo gears?

They had various versions. Problem with the M-20's and some M-21's is that they had 2 different diameter drive gears depending on the output shaft, and the smaller one has been out of production for quite some time and unavailable. So if you have to change the drive gear you're backed into a corner. If lucky you might find some on ebay or possibly someone started repopping them by now but that was a no go last time I checked.

Since the drive gears can be large or small, that means the driven gears also come in different diameters. Those are all still available at the moment. It'll either be .840" diameter or .870" diameter. Put the wrong diameter driven gear in there and it either gets chewed up in short order, or just won't fit at all.

The larger diameter driven gears for the small drive gear are only available now in 22-23-24 tooth counts, that's it. So if you need more correction than that you are forced to use an external gear reduction box that is a case by case custom made unit for your application. They run a little over $100 last time I had one made.

If you're lucky enough to have the larger drive gear, you have many more choices in drive gears and driven gears, they take the typical 350 turbo stuff that is still readily available most everywhere.

Sorry for the long explanation but that's pretty much what you run into with those things.

[NYGTO]

Thank you for your help but I should have been more clear, I didn't realize all those variations.
I'm talking about a 1964/5 M20 that would be turning 3:23 gears.
Would the drive gear be 6 or 7 tooth?

['ol Pinion head]

7 tooth is most common. Don't believe you'll find anything else in a stock early M20
6 tooth was used with real low rear gearing, tough to find used drive gear even 30 years ago.

[Formulajones]

Originally Posted by NYGTO

Thank you for your help but I should have been more clear, I didn't realize all those variations.
I'm talking about a 1964/5 M20 that would be turning 3:23 gears.
Would the drive gear be 6 or 7 tooth?

With 3.23's it would likely be a 7 tooth drive gear. No way to know for sure without physically putting eyes on it but I don't typically see M-20's with a 6 tooth drive gear, which is used with more rearend gear, typically seen more with M-21's.

With that said, running a typical 26-27" tire it's going to need something like an 18 tooth driven gear.

Unfortunately, since you have a 64-65 Muncie I'm going to bet it's the smaller diameter drive gear, and they don't make the larger .870 diameter driven gears for it that you need. Only 22-24 driven gears are currently available for that setup. Again hard to say for sure on the drive gear diameter but one way to confirm is to pull your current driven gear and mic the diameter. .810 is for the large drive gear, and .870 is for the small drive gear.

If you find you have the small drive gear, you'll most likely need to contact Transmission center and have an adapter made to speed up your speedometer. That's who I use when I need a speedo adapter.

[NYGTO]

Thank you both for your help, I'm having an early M20 rebuilt as a spare.