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

Greetings to all: Your assumption of the overdrive filler port being higher than the transmission port is correct. Adding oil into the overdrive port until oil just runs out the transmission filler port is exactally how I have always filled my overdrive trans.. The overdrive scriptures state: Only check the "transmission" oil level during routine maintenance. As you have stated, clutch contamination may result if the transmission is overfilled. Filling the unit as you have been seems to work just fine. There is a trough spot welded (if memory serves me right) on the plate just aft of the bulk ring/gear that controls the oil quantity in the overdrive unit when driving down the road. I believe the trough funnels excess oil back into the transmission. One would think that oil should be at the bottom of the fill port on the overdrive unit but this is not the case and the overdrive unit doesn't burn up. I will try to include a link to a publication on this subject that you may find interesting. By the way, how did your transmission function when filled with synthetic oil? Hope this helps and good luck, JEFF..

http://www.oldcarmanualproject.com/manuals/trans/Borg-Warner%20Overdrive/index.htm 

64F100 57FAIRLANE500

Jeff,



Transmission functioned fine filled with synthetic lube, except it leaked like a sieve. I wanted to try the stuff hoping it would reduce OD wear, but when I noticed it leaking out of the pressed-in fittings (not to mention the gaskets) after draining the excess, I went back to fossil. I had installed brand new clutch linings when the trans was rebuilt, and thus didn't want to go through the grief of changing them again after they got contaminated from the synthetic overfill. It took several soakings with brake cleaner to finally get rid of the chattering when hot.



P.S. The Borg Warner manual says to first fill the overdrive case to overflow, and then do the same with the main case. Seems rather silly, since there is no way to fill the OD case without overfilling the main, unless they intentionally want the oil level that high initially in the OD unit to pre-lube some parts that must be oiled before a new or freshly rebuilt unit is put into operation? They DO say to do the routine level check from the main case filler port though, so that pretty much answers my question.

6 VOLTS/POS. GRD. NW INDIANA