[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.