Mitchell overdrive compatibility?

E

EZLIFE

Member
I read an article by Tom Endy that says, “The Mitchell overdrive stub shaft is not compatible with the early pinion gear.” His pictures show the two different gears. Is there any way to tell if my differential has the “early pinion gear” without opening it up? I am pretty sure my differential is not the one that came in the car.
 
I called and talked to Steve Mitchell. He told me you can tell if you have one of the early non-compatible differential. Look at your differential and see if it has a rib on the top and bottom starting where the torque tube bolts to the differential. If you have the reinforcing ribs you have a later design differential and it should work with the Mitchell Overdrive.
 
Well that is true, it doesn't necessarily mean it is early as parts have been switched many times over the years.
 
The early 28's had the 370 ring & pinions that were different and used a different drive shaft. The banjo did not have support gussets. When they made the change to the 378 they continued to use the banjo without the support gussets for a period of time. There is no way to tell for sure which you have until you pull the torque tube off and look at the two front nuts on the pinion sleeve.

Tom Endy

http://www.santaanitaas.org/wp-content/uploads/2014/08/Early-Ring-Pinion.pdf
 
You could pull the fill plug and count the teeth on the ring gear. 37=3.70:1, 34=3.78:1.
 
quickchange said:
Skinny radius rods & skinny torque tube on early ones , if got normal ones very good chance will work ,
Click to expand...
I kniw this is an old thread, but how 'skinny' does the torque tube have to be to be an early rear end?

My coupe has no rib on the centre, but the torque tube is the same diameter as a later tube.
 
There is really no way you can determine for certain the gear ratio of the ring and pinion gears by viewing the outside of the rear axle assembly because all of the components are interchangeable and over the years many of the components have been replaced.

There is a way, however, the early R&P is a 370:1 ratio, not a 378:1 ratio. There is information around the Model A kingdom that describes how you can determine the ratio by marking a rear tire and the engine pully and rolling the car a perscribed distance to see where the mark on the tire ends up.

Tom Endy
 
Take the plugs out. Put the transmission in high. If you have an overdrive put it in direct drive. Jack up one rear wheel. Release the parking brake. Mark the tire that is jacked up with a piece of masking tape or chalk. Use the hand crank and count the number of engine revolutions for 20 revolutions of the wheel. If it is close to 38 you have 3.78. If it is 37 you have 3.70. If 41 you have 4.11.

Why 20? The differential gears will double the revolutions of the wheel.
 
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