The Ford shop manuals I have access to do not provide specific guidance regarding the o/d fill procedure.  I have seen an old Warner Gear overdrive instruction manual that says to fill the o/d to bottom of fill plug, then the main transmission to bottom of plug, and for normal filling/checking to just use the main transmission plug.  Elsewhere, I have seen people recommend filling both parts to the bottom of the plug.

What I am confused about is that the two fill plugs are at different heights and the two parts of the transmission are interconnected, thus the unit cannot be filled to the bottom of both plugs.  Either the transmission is full and the overdrive unit oil level is below the plug (underfilled), or the transmission is overfull and the o/d is full to plug bottom.

There are lots of different configurations of R10 and R11 overdrives, so it's unclear whether the proper fill procedure is one-size-fits-all or per-application.  Looking at photos it appears that in some examples the o/d plug is lower than the trans plug, and vice versa.

Does anyone know which plug I should fill to?

If it matters, I'm currently working with a T85/R11 unit from a 1960 Ford.






1954 Crestline Victoria 312 4-bbl, 3-speed overdrive