MTFCA Forum

When using a 39-13 (3 to 1 gear set) and changing just the pinion gear to a 12 tooth gear (to get a 3.25 to 1 ratio), What are the chances of the same shims providing the correct adjustment without adding or subtracting shims? I know ring and pinion gear mesh should be checked, but I am wondering whether I'm going to have to add or subtract shims, and how many, and thickness?

If you have babbit thrust bearings, they should be replaced with bronze. when you replace the thrust bearings, you can place the steel spacers the correct distance to fit the ring gear. The pinion gear is set by using or not using the paper gasket between the sleeve and the axle housing. You also need some good sealer such as permatex or ultra black to keep out grease leaks. Changing either gear, could cause a change in shims, because of wear on gears. It would be good to check a copy of the book the club sells "Front and Rear Axles" or Ruckstell Rear Axle" for step by step directions how to do this.
Norm

Because the pinion gear is of a different diameter, you need to start again to set the correct mesh. Logic says you will need to subtract from the left stack of steel plates/bronze thrust and add to the right. The total stack of both left and right side should be the same, if it was set up correctly, so if you take something from one side, the same amount should be added to the other. How you manage this may be with new bronze thrusts/shims/replacement steel discs.
I used the same combination of gears in my roadster, but it was an initial set-up, rather than a shuffle, so the method differs.

Hope this helps.
Allan from down under.

ModelTWoods wrote: ↑

Wed Jun 02, 2021 12:23 am

When using a 39-13 (3 to 1 gear set) and changing just the pinion gear to a 12 tooth gear (to get a 3.25 to 1 ratio), What are the chances of the same shims providing the correct adjustment without adding or subtracting shims? I know ring and pinion gear mesh should be checked, but I am wondering whether I' going to have to add or subtract shims, and how many, and thickness?

Mesh is way different. I had to "shim" the ring gear 1/8" up. (3-1 ring gear) Made a thick shim from a piece of T6 aluminum. I was told , after the fact, that this is bad news. Will lead to ring gear bolts shearing. We shall see. Might be possible to reduce the bell on a rux to bring the a stock(11-40) ring gear down to fit. I dont see a real good answer. Maybe someone has a better answer.

I have this 3.25 gear set in a T that I dont put a lot of miles on annually. It has, maybe, 2000 miles on it.

The answer may involve "readjusting" axle lengths in combination with the bronze thrust washers to make up for the 1/8" whall keeping the axle ends even on both sides.

Thanks for the responses. The rear end in question is a 26-27 Ruxtell. When I bought it, was freshly rebuilt, according the the seller who is a forum member. If I wanted to use the 3 to 1 ratio, I probably would use it, as is; but since I now live in a hilly area, I am going to take the advice of many on the forum and change to the 3.25 ratio. Having had previous experience with a 13 tooth pinion being so large a diameter that I had to open the axle housing to get it out, I'll check everything, but I suspect all bearings, thrust plates, etc. were changed when it was rebuilt.

Terry, mine was the same application. I hanged the 3:1 gearset in the Ruckstell I took out of my speedster. It may well have worked in my roadster too, but I was not at all enamoured with the compromised brakes that came with the package.

I did not have to resort to a shim behind the ring gear to get the correct rolling fit for the gears. I always set the assembly up first in the left housing, using whatever combination of discs/thrust washer is needed. Then the right side disc/thrust washer combination is selected/machined to suit.

I am very pleased with the 3.25:1 ratio. It is a compromise which suits my car and the driving I do.

Allan from down under.