AUSTIN DIFFERENTIALS

Q: Someone has changed out the ring gear. If I remember the Austin has a 5.?? ring gear and mine has a 3.78 stamped on it. I don't know how available the 5.?? gear is, but I think I may bore out the current ring gear to make 3/8 bolts fit and see how the 3.78 gear does.

A Norm Booth: I suggest you count the number of teeth on the ring gear and on the pinion gear. Divide the ring gear number by the pinion gear number will give you the gear ratio. I do not think you can change to a different gear ratio without changing BOTH the matching ring and pinion. I reviewed the factory drawing C-1694 for the American Austin differential assembly, and find the ring gear which is referred to as "spiral bevel drive gear has 42 teeth, 6.46 pitch-rh spiral" and the
"spiral bevel pinion mate" has 8 teeth-6.46 pitch-lh spiral, spiral angle 33 degrees, 26 minutes, and the pressure angle is 17 degrees, 30 minutes. By dividing 42 teeth on the ring gear by 8 teeth on the pinion, we get a rear end ratio of 5.25. I know of no other ratio that was ever available.

The ring and pinion on a American Austin are very tender. It is difficult to
find good used ones, and new old stock ones are non existent.

Q: Do you have any thoughts on this? Will the little 14 -15 hp engine be able to
handle a higher speed rear end?

A (Norm Booth): The American Austin engine only has 10 or 11 horsepower. I suggest you try to come up with the proper rear end. I personally have never set up a American Austin rear end, so my knowledge is limited. I'm copying Lynn James as he has set up an American Austin rear end. He may have some thoughts for you.

A Lynn James: I haven't had to much time to respond to this properly. I'll have to
have a couple of days and review my notes. I can send them to you. I
believe I documented enough info that I could describe how to do this in
some detail. It is a very time consuming project but you have to do it if
you want you differential to survive after disassembling it. You will have
to assemble and disassemble the differential maybe as much as 3-8 times. I
think I did mine 5-6 before I was satisfied with it and was assured that it
was right. After I send the details you will understand. As Norm said they
are very tender, if you just put them together and are off a little it won't
last long. There are no replacements available. I personally have bought 8
rear differentials over the years paying from $100.00 up depending on what
else came with it to get a good set of gears. They were ALL BAD. I was
fortunate to get the last decent pinion that a club member had on his shelf.
I had to match a mismatched set of gears for mine, not a good thing to do
but it was the only thing I could do. So if you can find a good matched set
of gears your in good shape. If you can find a complete differential that
doesn't have a lot of back lash and doesn't make noise you could probably
just use it. If you split the cases and put new carrier bearings in as I
did you will have to set it up. You just can't replace the bearings and put
it back together and expect it to be right .

You can just remove the pinion and inspect it for wear and replace it (the
same pinion) with out having to go through the setup. But you will have to
make sure you don't loose any of the shims and if you replace the gasket
make sure that it is as close to the same thickness as possible.

If the pinion is good the ring will be good, If the pinion is bad the ring
may still be good. If the pinion is really bad i.e. missing, chipped or
excessive where thin the ring will more than likely be bad. Finding a good
pinion is the quest.