Differential replacement on '41, maybe

[Guest unclefogey]

Received a call from my buddy telling me that it looks like I finally will be able to get my hands on the rear end out of his '55 Special. He has finally had it with the repair bills on the Dynaflow and is going to go with a big block Chevy and drive line replacement.

First question, what would the possible ratios be in a '55 Special with an automatic transmission?

Even though I lucked out and have a 3.9 to 1 in the '41 Special, I would like to be able to cruise at 60 at a little less than the 2700+ rpm that she turns now. I probably will lose a little pulling power on the low end, but it shouldn't be too extreme.

That was the good news. The bad news is that last year, the friend heard what he thought was a strange noise coming from the rear end, jacked it up, pulled the filler plug and was hit with a gusher of a combination of gear oil and automatic transmission fluid.

As backround to the question I am about to ask, on the trip out to Seattle in 2007 in the '41, I, and two guys on Honda Goldwings, stopped at what we thought was an operating gas station on Hyway 12, it wasn't, somewhere on the high plains of Montana. In our conversation about where to get gas, one of the guys said he had a '51 Buick with a problem, and that problem was that in the previous year, the seal had gone out on the Dynaflow putting ATF into the rear end, so that his only course of action to get it running again would be an engine and driveline replacement.

Second question is, is the migration of ATF into the rear end fatal to the rear end? I would assume that there are degrees of damage caused by this situation. Could repair be a possibility or if the damage might be to the ring and pinion this would be a "Live with the 3.9 and get over with it", situation?

[Leif Holmberg]

It seems that 1955 D.F ser.40 has 3.6-1 and 1955 C.T has 3.9-1

Leif in Sweden.