59 ford transmission oil

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Mar 29, 2009

I recently inherited a pristine 59 ford fairlane. It was sitting for many years and I have changed all the fluids among other work to get it roadworthy again. I had several quarts of mobil one synthetic gear oil, so that is what I used in the manual transmission. The rear seal leaked profusely (even before the change) so I replaced the rear seal. The new seal continues to leak a lot. I was careful not to damage it during installation. Could this be caused by the use of synthetic gear oil? If so, will a change to conventional gear oil stop the leak or do I have to change the seal again? Lastly, the yoke coming out of the transmission has some small pits. There were no rough edges, though I did polish it up a bit with some wet/dry sandpaper. Do you think these minor pits might contribute to the seal leaking? Thank you all very much for your responses.
I wouldn't put GL4, 5 or 6 oil in that trans. It calls for GL1 SAE-90 gear oil which is hard to find. I would put SAE40 or 50 engine oil in it. Better choice than the new Gear oils that can eat the bronze parts in side it.
I had a 61 chevy in high school that leaked from the rear shaft even after a seal replacement. Didn't use synthetic then, but it leaked just as bad any way. I ended up taking the tailshaft off and replacing the rear bushing. It was worn enough so that the output shaft would wobble just enough to leak. My shaft had score marks at the bushing from the wobble, so I welded up the shaft and spun it back down to dimension on a lathe in shop class and replaced the bushing and it stopped leaking. I think I got a B- on the repair because of some hydrogen embrittlement in the weld. I would check amsoil's classic car application guide for the right oil, Most of the old iron I worked on back then used gear oil to be compatible with the bushings. Few motor oils have the right additive package for old manual trans units unless the trans has ball bearings throughout.
03cvpi nailed it IMHO. I even had an AW71 in a 245 Volvo do the same thing. Even replaced the bushing once to no avail. I got a new tailshaft (yoke). Problem solved.

It's not the fluid or the seal. You probably took even more material off by smoothing out the pits, so either repair or replace it. Get a new bushing, and seal.

And no modern synthetic gear oil will "eat" bronze alloys.
Go to a junk yard where they have a pile of old driveshafts and take the yolk with you plus a caliper to measure with. You may find one that fits or can be machined down to your tolerances. Does your yoke have a spline or just a keyway? OR like said before, weld it & remachine. Which may be easier.

There is one from a 59 Galaxie on ebay:

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Oh yeah... I somehow forgot about those Speedy Sleeves!
They slip over the male end of the yoke , and use a special seal that is slightly larger to accommodate it.
A nice fresh seal surface.
But any wobble will still make it leak.
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