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peeeot
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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
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FORD DEARBORN
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Your statements are correct so maybe we can sort it out. The fill ports are indeed at different levels with the overdrive port being higher and yes, the two sections (overdrive section and main transmission section) having a common passage internally. So, liquid will seek it's own level meaning in this instance, the O/D port will appear to be low on oil and the trans port will have oil flowing out when the correct quantity of oil is present. That is, when the transmission port REMAINS full, it will be properly filled. Gear oil is very viscous and slow to flow from one section to the other section. Therefore, filling the trans section first gets the balk of the oil in quickly. If you were to to add no more oil to the trans port at this point, much of the oil would drain/flow into the O/D side creating a low oil level in the transmission side requiring the need to add more to the trans port until it's full again with the need to repeat this process until the transmission port REMAINS full. Keep in mind oil will not flow out of the O/D port because it's at a higher level. In theory, you could simply add all the gear oil through the O/D port until oil is running out the trans port but this would take a lot of time due to the viscous nature of gear oil. Internally, this transfer of oil is through the balls and race of the transmission output shaft bearing and that's why it is very slow going. Hopefully I can make sense here because often I can't. Just picture the two ports with the O/D port at a higher level than the trans port and it will be properly filled when oil REMAINS at the bottom of the trans port. So, fill the transmission section first then fine tune by adding oil through the O/D port until oil is running out of the transmission port.
64F100 57FAIRLANE500
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peeeot
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Thanks for the clarification. It makes sense that the reason for the B-W instructions is because of the slow flow between the sections. It sounds like the goal is for the equalized level between the two parts of the transmission to be flush with the bottom of the trans portion fill plug, and thus that the B-W instructions are the way to go.
1954 Crestline Victoria 312 4-bbl, 3-speed overdrive
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FORD DEARBORN
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Yes, that is correct..//
64F100 57FAIRLANE500
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DANIEL TINDER
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[quote] peeeot (9/11/2026)
Thanks for the clarification. It makes sense that the reason for the B-W instructions is because of the slow flow between the sections. It sounds like the goal is for the equalized level between the two parts of the transmission to be flush with the bottom of the trans portion fill plug, and thus that the B-W instructions are the way to go.
Unless, of course, B-W intended both sections should be running at the level of the OD fill port, and the trans. port is merely a way to speed the filling process (?). In that case, you would fill the trans. first, plug it, and then fill the OD unit. Which also means the OD is where you would routinely check for level/top-off. I wonder which is worse: running the trans. overfilled, or the OD underfilled (?). Too much fluid would likely waste energy & encourage leaks, but too little might risk scarce/unobtainable OD component parts.
6 VOLTS/POS. GRD. NW INDIANA
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DANIEL TINDER
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DANIEL TINDER (9/11/2026)
[quote] peeeot (9/11/2026)
Thanks for the clarification. It makes sense that the reason for the B-W instructions is because of the slow flow between the sections. It sounds like the goal is for the equalized level between the two parts of the transmission to be flush with the bottom of the trans portion fill plug, and thus that the B-W instructions are the way to go.
Unless, of course, B-W intended both sections should be running at the level of the OD fill port, and the trans. port is merely a way to speed the filling process (?). In that case, you would fill the trans. first, plug it, and then fill the OD unit. Which also means the OD is where you would routinely check for level/top-off. I wonder which is worse: running the trans. overfilled, or the OD underfilled (?). Too much fluid would likely waste energy & encourage leaks, but too little might risk scarce/unobtainable OD component parts.
6 VOLTS/POS. GRD. NW INDIANA
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FORD DEARBORN
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The O/D section received oil by means of the transmission gears spinning and slinging oil outward and upward where some of it is collected is a cast-in gutter that conducts the oil rearward were it then spills over the planetary gear assembly. The O/D unit doesn't need a level or pool of oil although there may be some level back there. The early 50's 3 speed transmissions didn't have an input shaft seal as was common in the day so as stated above, overfilling would more than likely invite leaks and in this instance, from the input shaft then to the clutch....
64F100 57FAIRLANE500
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DANIEL TINDER
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Indeed. Do to the very viscous nature, timing the fill process can be tricky. The routine I had developed over the years, was to first fill the trans, then the OD, install the OD plug, place a pan under the trans hole, then come back much later to reinsert the trans fill plug. While some fluid does get wasted, it seemed a small price to pay for simplification. You know the OD section wasn’t under-filled, and the trans excess has stopped draining, so no danger of clutch contamination.
6 VOLTS/POS. GRD. NW INDIANA
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