Filling Rear Differential - Should I Add More Fluid?

Messages
643
Location
Florida
Limited Slip 373 9.75 Ford Differential. I put in 3 quarts M1 75W-140 LS friction modified gear oil with (1) 4 oz bottle of Ford's friction modifier. I probably wasted 8-10 ounces with pre lubing gears, axle bearings, axle shafts and splines before and during reassembly, with probably 2-6 ounces wasted from overflow, spillage, etc. while lubing parts. When filling, with the vehicle level, the fluid did not come out of the filler hole. My understanding was that this is the stopping point when the system is full. I cannot see how far below the filler hole the fluid is, and cannot tell by feel with my finger. My options: 1) Leave as is, knowing there may be a few ounces less than 3 quarts of fluid, plus the friction modifyer 2) Buy another bottle of Gear Oil and fill until it comes out of the filler hole as originally targeted 3) Add another 4 ounce bottle of Ford Friction Modifier (which I have), keeping in mind I have already added 1 bottle to the already fortified gear oil I have used. I want to do it right and price is not an issue. What would you do?
 
Messages
5,223
Location
NJ
I'd buy another bottle of gear oil and fill it until it drip out. You came this far. Might as well finish the job properly. Of course, you'll probably put in a tiny amount and it will start to overflow. But at least you'll know it's done right. Using a pump is much easier if you're not already using one.
 
Messages
35,288
Location
NY
Fill it to the proper level. You want it level with the lowest point of the fill plug, [ starting to running out] or a hair below which you would easily see or feel.
 

gathermewool

Site Donor
Messages
8,590
Location
New England
OP, why were you pre lubing?
Originally Posted by john_pifer
Does it specifically call for filling until it starts overflowing? I know my Tacoma calls for the level to be a little bit below the fill hole.
That sounds crazy to me. Why didn't they just design the full hole a little lower?!
 
Messages
5,543
Location
Suburban Washington DC
Originally Posted by gathermewool
OP, why were you pre lubing?
Originally Posted by john_pifer
Does it specifically call for filling until it starts overflowing? I know my Tacoma calls for the level to be a little bit below the fill hole.
That sounds crazy to me. Why didn't they just design the full hole a little lower?!
So you don't make a mess on the shop floor.
 
Messages
4,947
Location
VA
Option #2 One thing that I learned is to give the fluid time to settle down before checking. I noticed when this when I filled my diffs until they dripped out and immediately reinstalled the plugs. I went back to check them the next day and none dripped out. Apparently, it takes a little while for the fluid to level out. YMMV
 

gathermewool

Site Donor
Messages
8,590
Location
New England
Originally Posted by atikovi
Originally Posted by gathermewool
OP, why were you pre lubing?
Originally Posted by john_pifer
Does it specifically call for filling until it starts overflowing? I know my Tacoma calls for the level to be a little bit below the fill hole.
That sounds crazy to me. Why didn't they just design the full hole a little lower?!
So you don't make a mess on the shop floor.
No, it's a serious question? You don't just drain and refill the diff? Besides, it sounds like the OP spilled some anyway.
 
Messages
8,214
Location
Champlain/Hudson Valley
My vehicle calls for the "1/4inch to 1/2inch" below the fill hole. It seems to me (and everyone else) that filling to the base of the fill hole couldn't make a difference. Perhaps the gear oil churns too much at that level? After seeing those "clear diff cover" videos it's hard to imagine it could make a dime's worth of difference.
 
Messages
365
Location
Northeast Georgia
This is where I use a small Allen wrench as a level gauge. Insert the short end into the inspection hole and point it straight down...while keeping the long end somewhat level on the bottom edge of the fill/inspection hole. Pull the Allen wrench out to see where the level is.
 
Messages
2,413
Location
Lakeside CA
How can you do this: "pre lubing gears, axle bearings, axle shafts and splines before and during reassembly" and not know how to check the oil level? Just wondering. Add more oil if you cannot touch the oil level with your finger through the fill hole.
 

Fitz98

Thread starter
Messages
643
Location
Florida
Regarding prelube: The axle shafts were pulled to replace seals and bearings. Since the bearings are lubed with differential oil, I wanted to prelubricate them so they don't run dry until the fluid gets to flow to them. Also prelubed axle splines before installing.
 
Messages
4,177
Location
Texas
I agree . Purchase another bottle of gear oil / lube ( it is not that expensive ) & fill until it runs out . Then replace the the " plug " & call it good .
 

Astro14

$100 Site Donor
Staff member
Messages
12,338
Location
Virginia Beach
On my 2000 Expedition, the proper axle fill was something like 1/4" to 1/2" below the fill plug level. So, it wasn't just a "fill it until it runs out" kinda' deal. I actually used a very small hex key wrench to check the level after adding the specified quantity. Check your factory service manual, or subscribe to an on-line servicing information site, to get the correct level for your Ford.
 
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