Pinion gear to ring gear backlash for 49 series 61

[JoeBetsBlue]

I recently purchased a 49 Cadillac model 61 and it didn't come with any information about history.  Everything is looking okay except for the differential.  The forum seemed to have a flurry of discussions about preload etc in 2014.  I think I understand all that has been said about pinion seals and preload..  I do have a leaky seal and I can replace that. I think I understand enough about the preload etc.  Mine doesn't have any movement forward and back but seems to have more rotational movement than I like.  I can turn the yoke 15 degrees or more in back and forth rotation, sometime even hearing a clunk.  It's quiet when being driven.  I am sure the preload doesn't have much to do with this problem and it must be a very bad pinion gear to ring gear backlash that can't be fixed by giving it more preload.  Would you think my differential is toast?

[Jay Friedman]

It seems there is more backlash than normal, though in my experience, when that is the case, they are noisy when driven.  You may know that except for replacing the pinion seal and bearing, no repairs can be done to these units.  They are held together in such a way that if taken apart they can only be put back together at the factory.  At least that was the fix back in '49.  Today, the best solution is to replace it with a second hand unit in good condition. 

Except for gear ratio differences they are interchangeable from 1941 to 1955 and maybe 1956 and later.  If your '49 is equipped with Hydramatic, as most were, the differential should have a gear ratio of 3.36.  These are the most common available.  Try the following vendors: www.49caddymanemail addresses not permitted; Dennis Akerman Cadillac Parts; Honest John's Caddy Corner; John Gately, who is in the CLC directory in Massachusetts, as well as others.

When replacing it, I think you can get the large diameter circular gasket between the differential and the axle housing from Olsen's Gaskets in Washington State.  Also, be careful not to lose the cone-shaped washers behind each bolt. 

[JoeBetsBlue]

Thanks Jay.  Mine is a hydramatic, 3.36 although I have a hard time seeing the #6.  I mentioned that I have 15 degree or more rotational back and forth movement but once in a while it is a normal, acceptable backlash (little if any rotation).  I put chalk marks on the driveshaft and sure enough every 3.36 turns (or as close as I can guess) it  returns to normal and then returns to excessive lash away for 3.36.  It looks like the ring gear isn't running true, almost seems like a wobble.  I read that the ring gear is riveted to something and maybe some rivets are loose or stretched and is creating a wobble.  Either way something is not right in Dodge city and I won't know until I take it apart which I will do this fall.  Until then I will look for a good replacement (thanks for your list of information) and I will take easy short trips to the malt shop until then.  I bought the gasket from Caddy Daddy and will carefully save the washers.  Thanks again for your help.  Great forum.