Jim McKee On Model 20 Rear Ends (Many posi cars have this ..)

Here is an Answer with a lot of great info on the model 20 rear end ...

Hi Mike,

I think you sent Nick to me with a question about SC/Rambler > rearends. Here is what I told him.> >

Hi Nick,

Roy Woods and Roger Penske Racing had no rearend failures racing AMC Model 20s in Trans Am in a five year period running multiple cars. I ran an IMSA RS car in the early 1970s with the same rearend for four years with no failures.
Unless your car runs faster than 11.0 second quarter mile times there is no reason to run anything other than a Model 20.

The stock keyed axles are only suitable for small street tires, E70 x 14 Polyglass or similar. Current street radials have greatly increased traction and are the limit on stock axles. Using drag radials and certainly slicks are virtually guaranteed to ruin the stock axles very quickly.

It all depends on how you intend to use the car. If your engine makes over 350 horsepower, even on street tires the Moser axles are very good insurance against spinning an axle and smashing the drum into the backing plate.
If you drive reasonably the stock axles have served many SC/Ramblers well, including my original SC/Rambler which now has 57,000+ miles on it. I might mention, I spun an axle sometime back. I have no faith in machining additional keyways or enlarging the single keyway, because then the hub cracks with the same results.

The factory service manual has valuable information for obtaining good service from stock Model 20 rearends. I just finished rebuilding two SC/Rambler rearend assemblies. Both got the following new parts; Moser axles,Timken bearings, positraction clutches, all seals, complete brake redo.

The 3.54 is my recommendation for a street/strip gear, because 70 mph is almost exactly 3500 rpm. Unless you mini tub the car you can't get large enough tires under a SC/Rambler to make use of the additional torque from the 3.73 or 3.91.
Stock SC/Ramblers usually run about 98 to 106 mph in the quarter mile which turns the engine about 5000 rpm. If your engine is capable of more rpm, then a lower rear gear ratio would need to be matched to allow maximum rpm at whatever mph the car will run at the end of the quarter mile.

The "Performance American" booklet shows how to weld the axle tubes to the housing and strengthen the spring perches. Those are the recommended modifications with the Moser axles to go as fast as 11.0 in the quarter mile. Current Moser axles use a Jeep axle bearing that will take all the abuse you can throw at it.

Talk to Rocky or Mike at Moser. Their axle studs fit AMC wheels very well and the whole kit comes with bearings, axles, inner and outer seals, and studs. Remember to remove the spacer in the center of the positraction unit to allow the axles to go all the way into the positraction clutches and plates.
Regards, Jim
PS; The Moser kit is over $200 less than the Mark Williams axle kit and you don't have to modify the emergency brake parts.