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Old 03-06-2021, 08:02 PM   #1
Alaska Jim
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Default Early '51 Merc. overdrive Transmission

I am reinstalling the 3sp w/ o/d transmission in my '51 Merc. after rebuilding it. My question is how much gear oil does this transmission hold. My factory manual only shows capacities for a 3 sp . manual transmission, not the overdrive. It calls for 3 1/2 Pints. I also have a book by Randy Rundle that states it holds almost a gallon, and that "after filling both the front, and rear half's of the transmission, you will have about a pint left over." Since there are 8 pints in a gallon, that would mean that the transmission with the overdrive holds about 7 pints. this is a 3 1/2 pint difference. Do any of you know for sure what the correct amount is? when I drained out the transmission for removal, I got les than half a gallon in the drain pan. I know that you are supposed to fill each part of the transmission until the fluid comes out of the fill plug, starting with the overdrive section first, then the main transmission section. I want to make sure that I do not have to wait for a specific amount of time for the 2 levels to equal out. I want to make sure the Trans is filled correctly. I do not want to have to pull this trans out again. After 50 years of working on cars, maybe I am second guessing my self, It is not like I have never filled a manual transmission before. I am just trying to find out the correct amount of fluid. I know the recomended type is API GL1 80-90 wt. Thanks -- Jim
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Old 03-06-2021, 08:43 PM   #2
40cpe
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Default Re: Early '51 Merc. overdrive Transmission

This was posted on the Early V8 site a few days ago.
"According to the 1949-1950-1951 Ford Passenger Car Shop Manual
Transmission standard: 3 1/4 pints with overdrive 4 1/2 pints
Note: with overdrive drain at two plugs, refill through transmission only."

The note about refilling refers to topping off. You are correct about filling from both compartments per the BW manual. I think because it takes so long to migrate.
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Old 03-07-2021, 06:42 AM   #3
Mac VP
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Default Re: Early '51 Merc. overdrive Transmission

Prefill the OD section with about a pint. Fill the rest through the filler plug on the passenger side of the main gearbox. Then let it sit overnight in a warm room. Recheck the level at the main filler plug on the gearbox and top off as needed. The oil will level out between the overdrive section and the gearbox section faster than you think. Keep the transmission level during the filling process. Any checking and topping off should be done at the main filler plug in the gearbox.

The GL-1 gear oil will work but it lacks the good additives of the later grades. We recommend using GL-4 oil in a straight 90 wt. if you are considering using a GL5 oil, check the packaging specs to make sure it is okay with yellow metals.
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Old 03-07-2021, 10:34 AM   #4
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Default Re: Early '51 Merc. overdrive Transmission

The original Lincoln Mercury Overhaul Manual lists the oil capacity for the different transmissions used from 1949 thru 1951 but it is listed by fluid weight and not by liquid volume measure. The 1949 thru early 51 Mercury overdrive transmission capacity is listed as 3 1/2 lbs. I used one of the calculators for weight to volume (pounds to cups) for lubricating oil and it came out to 3.8 pints. Many times this can be problematic so I have always filled very close to the lower edge of the filler plug hole. I use AeroShell 100W motor oil with no Lycoming additive for my transmissions. It's pretty warm here in Texas so I use this to keep the viscosity near straight grade SAE-90 gear lube. The Sta-Lube GL-4 is fine as mentioned previously by Mac. The GL-4 is the replacement for the original GL-3 mild EP specification that was called out in the manufacturer's recommendations. There are enough carry through openings between the transmission and overdrive sections to allow fluid level equalization but, as Mac stated, it's not a bad idea to put some in the OD section if you are planning on driving the car as soon as the oil change is finished.
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Old 03-08-2021, 03:32 AM   #5
Alaska Jim
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Default Re: Early '51 Merc. overdrive Transmission

Thank you Mac Van Pelt, and rotorwrench, for you information. I have some GL1 and was going to use it, but I may go buy some GL4, although I have read that it has ep additives, and that it is not recommended for the early transmissions with the overdrive. I appreciate your input. You both have more experience than I with these older vehicles.
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Old 03-08-2021, 07:59 PM   #6
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Default Re: Early '51 Merc. overdrive Transmission

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Searched all the local parts stores today, and did not find any GL4, only GL5, and also some full synthetic gear oil. I may have to use the GL1 80-90 mineral oil that I already have. need to hook up the clutch linkage, and change the yoke on the driveshaft tonight, then maybe tomorrow check in Anchorage ( 50 miles away ) for some GL4. After that, whether I have GL4 or only have the GL1, I will fill the transmission, starting with the overdrive section. Thanks to all that have replied to my questions.----- Jim
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