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Rear end gear oil sanity check
My sequence of events with a solid 8.8:
Before installing the rear end, I added the specified amount of gear oil with the diff cover off, then added the diff cover.
At some point much later down the road, I had the rear end back off the car with the tires and wheels on it. At some point it was upside down, and some fluid ran out a hole on the top of the diff cover that I wasn't using and didn't notice was an open hole. Maybe a couple oz. spilled out.
I plugged the hole on top, and pulled out the upper fill plug on the back of the diff cover. I had thought that filling to this plug would be the signal that the thing was full. I had to add a bunch more oil, and I might even have added as much as there was supposed to be in there to begin with.
So, too much gear oil is surely a problem, right Shirly? Thinking I need to drain it all, refill it to the specified amount, and not worry about it if it doesn't reach that upper fill plug. Yes? No?
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Senior Member
Fill it until fluid comes out of the upper hole in the cover. Too much gear oil is not good for the system
Last edited by egchewy79; 11-16-2021 at 08:16 PM.
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IMG_1763.jpg
Forgot to attach a picture of the diff cover I have, if that matters.
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Senior Member
Something seems off here. My fill plug is not anywhere near the top of my diff cover. I have IRS, but I don't think the Ford 8.8 cares in that regard. A quick search says the Ford 8.8 takes just less than 2 quarts, which is what I got when I filled mine. (Wasn't watching closely, tried emptying the last of that second quart and ended up putting the last bit on the garage floor).
If it were me I'd double check the diff cover; if I wasn't going to change it I would pull the drain plug to drain the oil, take out the fill plug and fill with a measured 1-7/8 qt. of gear oil since you won't have the fill plug position as a fail safe measure. Don't forget the friction modifier if your diff calls for it.
Good luck.
Last edited by John Ibele; 11-16-2021 at 10:10 PM.
MK4 #7838: IRS 3.55 TrueTrac T5z Dart 347
The drawing is from ~7th grade, mid-1970s
Meandering, leisurely build thread is
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Seasoned Citizen
The fill plug is on the front of the housing on the side of the pinion blister. If you want to top it off, simply fill through the port until oil just starts to drain back out of the port.
If you filled it to the top of that upper plugged port on the cover, you have way too much oil. Open the fill port on the housing and drain oil until it's level with the bottom of the fill port.
https://eastcoastgearsupply.com/file...-fill-plug.pdf
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Senior Member
I stand corrected. The fill plug IS in the housing itself, not the cover. I remember now...
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Be sure to unplug the vent hole, or it will push lube out the axle ends.
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The hole in the top like Railroad said is a vent and should not be plugged it should have a vent part or a nipple with a long tube. The actual vent pipe is best and neater.
I believe Mike Forte sells them.
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Seasoned Citizen
The vent on a solid axle Ford 8.8 is located on the top of the left axle tube near the center housing. It is not located on the top of the center housing or the rear gear cover, at least not from the factory. It will look like this: https://www.americanmuscle.com/ford-...bolt-8614.html
Dart Little M 406" SBC 800 HP N/A & 1,100 HP on nitrous, 2-spd Powerglide with trans brake, 6,000 RPM stall converter, narrowed Moser 88 3.90:1 spool with 35-spline gun-drilled axles & Torino bearings, custom parallel four-link, custom tube chassis & roll cage NHRA certified for 8.5-sec (only two FFR Hot Rods have this cert).
33 Hot Rod Super Pro Drag Racer Build:
33 HR NHRA Cert Roll Cage Build
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AB6479D1-26B8-4C1F-89F7-BC43DC999390.jpeg
N00b here; Is this the right friction modifier?
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Seasoned Citizen
Originally Posted by
PaulRSchmidt
Depends on what differential you have. That appears to be a friction modifier for a clutch type LSD. If you have a clutch type that will probably work but without a part number it's hard to tell what's in the bottle. This is what a Ford clutch type LSD uses but that doesn't mean it's the only friction modifier you can use. Some simply use synthetic gear oil instead of the Ford stuff. https://www.cjponyparts.com/ford-per...021/p/M19546A/
But if you have a gear style LSD or an open differential you don't need friction modifier.
Dart Little M 406" SBC 800 HP N/A & 1,100 HP on nitrous, 2-spd Powerglide with trans brake, 6,000 RPM stall converter, narrowed Moser 88 3.90:1 spool with 35-spline gun-drilled axles & Torino bearings, custom parallel four-link, custom tube chassis & roll cage NHRA certified for 8.5-sec (only two FFR Hot Rods have this cert).
33 Hot Rod Super Pro Drag Racer Build:
33 HR NHRA Cert Roll Cage Build
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Senior Member
I have the common Ford 8.8" rear with the trac-lock.
The Royal Purple fluid has the friction modifier already added. A few years back it was "the" product to use in our cars, according to a number of threads. I used it, and so far, so good. Having rebuilt the rear, as per instructions, I had pre-soaked the discs on the OEM Ford modifier
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