I have been wrestling with this problem for many years. I always file it away as "solved", and then after enough time passes for memory to fade and the proscribed fluid change mileage interval reoccurs, it again rears it's "ugly head".



The '55 BW 259 OD trans. has two fill apertures. One in the main transmission case, and one in the OD unit. The two cases do communicate internally, but are separated part way up. The manuals say to fill both units, but the fill opening on the OD case is slightly higher than the one in the main trans. If the main case only is filled until the aperture overflows, then the OD unit remains under-filled. But, if filling is attempted through the OD aperture only with the main filler port open (my usual procedure), the OD case cannot be filled to overflowing since fluid will merely run out of the main port as fast as you can pump it into the OD case. If the main case is plugged when it begins to overflow, and then the OD unit is filled completely, the main trans. will thus likely be overfilled (assuming the filler port actually indicates the correct level?) and the oil could thus froth with resulting pressure that could blow it out of the vent valve on top of the main case after compromising the paper gaskets (a situation I experienced when the rebuilder mistakenly installed my transmission completely filled with synthetic lube).

So, you have to wonder why the filler ports are not level with each other. Merely a case of careless design? Filling both totally drained cases from the OD unit only (with the main filler port plugged) takes exactly the manual-specified 4 1/2 pints. But, the main 3 speed trans. when installed in cars without overdrive would then obviously be filled only to it's overflow point under normal circumstances, which would seem to rule out any concept of required extra lube that might be forced into the OD unit under road/operating conditions. Unless of course, the slight overfilling of the main case turns out to be largely innocuous (using standard 50s-era 80 weight lube), but was maybe considered essential by BW/FoMoCo engineers to prolonging OD mechanism life? Running the main unit overfilled could result in leaks (and possible clutch contamination?) and excessive friction (at least temporarily, assuming the excess is designed to eventually blow out the breather valve), but running less than an ideal level in the OD unit might shorten the life of a mechanism who's replacement parts are rapidly becoming unobtainable.

Opinions?


6 VOLTS/POS. GRD. NW INDIANA