Differential Cover Upgrade / Gear Oil Change on a Ford 9.75″ Differential (Transit / F150)

Having a Yukon Gear locker installed, I wanted to be able to change the gear oil in my differential regularly.. and the factory cover makes that a nightmare. Sealed with only black RTV from the factory, draining the oil involves removing twelve bolts, breaking that RTV seal, cleaning both surfaces, reapplying RTV, reinstalling twelve bolts, allowing time to cure, and filling with oil. Well overdue for my break-in change, I opted to ditch the factory cover for something with a drain plug, and ditch the RTV sealant for an actual gasket.

Before buying an aftermarket differential cover, I recommend watching the series by Banks Power (link) where they demonstrate gear oil flow using clear covers, which shows why many aftermarket covers may be more detrimental than helpful.

Parts and Tools Required:

Gear Oil: The Transit manual calls for a 75w85 gear oil meeting the “WSS-M2C942” specification. I only found one readily-available choice outside of the Motorcraft brand. Motorcraft is full synthetic and includes no additives for limited slip differentials. The Red Line alternative does have some LS additives, though this is not detrimental if you don’t have a limited slip differential.

The factory oil capacity is 2.75 quarts. After installing the G2 differential cover, I was able to sneak in just about 3 quarts before reaching the fill hole.. so either way, you’ll need to order quantity 3 of either oil choice.

OR

The Motorcraft oil requires a bottle of friction modifier/additive if you have a limited slip differential installed. The Red Line oil says that it includes friction modifiers and you should only add more if you notice chattering / noise while in motion.

The 9.75″ differential has been in use by Ford in various truck applications for over a decade now, and there are numerous instances of people using different weights and classifications of oil in it without issue. My backup choice of oil, especially if you need something available locally, is Mobil 1 75w-90. I found this locally for about half the cost of the two options meeting Ford’s called out specs above.

Differential Cover: There are a lot of choices out there, but your best bet is to choose a cover which mimics the contours of the factory cover. Anything completely squared off should be out. I highly recommend visiting the Banks Power video I shared in the beginning of this post to see how the oil flows in motion, this will make total sense afterward.

I chose the aluminum G2 Axle and Gear “Hammer Series” 40-2012ALB. The interior of the cover is rounded rather than boxy, resulting in smoother oil movement. The price is very reasonable. The style is nice without being over-the-top. The top link is for powdercoated black finish and the second is plain aluminum.

Differential Gasket: Short and sweet. LubeLocker seems to be highly recommended everywhere. Alternatively, the cover I’m installing included black RTV, so you could save the $20.

Tools:

  • Socket Set (specifically, you’ll need a 1/2″ deep well socket for the factory bolts)
  • Hex / Allen Socket Set (specifically, you’ll need a 6mm and an 8mm)
  • Torque Wrench capable of 30 ft. lbs. (Harbor Freight cheapy will work, I use a Husky from Home Depot
  • Small Razor Blade Scraper
  • Silicone Gasket Remover (The CRC product I used in the pictures doesn’t appear to be available online, it works great for vinyl decals but wasn’t super effective on this RTV. Reviews for this Motorcraft option seem good. GooGone would be another alternative)
  • Fluid Pump (I used a funnel with tube, it took forever but worked eventually)
  • Flush Cutters (for cutting several zip ties)
  • Shop Towels
  • Oil Catch Pan
  • Zip Ties
  • Paint Scraper and Hammer (or something to use to break the RTV seal)

Installation Steps

  1. Park wisely.

    Your van should be on a level surface, and you’ll need to remove your spare tire to make room to work.

  2. Relocate the ABS wire attached to the diff housing bolt studs.

    Unclip the ABS sensor wire from the three top bolts. Should pull straight off the studs by hand. Go ahead and cut the connectors and zip tie this to the rigid line above the differential circled below.the abs line we are relocating

  3. Position your catch pan, then using a deep well 1/2″ socket, loosen all 12 bolts. Remove all but the two on the top corners (or wherever) to hold the cover up once we break it loose. Very unlikely that any oil will drain until the next step.bolts removed, catch pan in place

  4. Breaking the RTV seal.

    Wedge a flat paint scraper or other thin tool between the differential cover and housing. Using a hammer or mallet, drive it in until oil starts to drain. Let the bulk come out and then use this tool to break the RTV seal all around the cover. I actually used a knife on this step as my RTV is very fresh and I wasn’t successful with a paint scraper. Just be cautious not to scrape the housing mating surface. Sorry, I forgot to take a photo of my knife sticking out of the side of my diff! 😀

  5. Clean the valley.

    Allow the oil to drain, then remove the remaining two bolts and differential cover. There’s a valley at the bottom of the differential housing, clear it out with shop towels. This is where any potential metal shavings or debris is going to be, so clean thoroughly.cleaning remaining oil from the valley

  6. Prepare the housing mating surface.

    Cover everything but the mating surface with shop rags or towels. Wipe off any oil and spray mating surface liberally with RTV remover. Scrape at a low angle with a razor blade and repeat process until surface is smooth and clean. It doesn’t have to be absolutely perfect, but it’s worth doing right.cleaning the old rtv off the mating surface

  7. Get the new cover and gasket ready.

    A little awkward: Take the new differential cover and push two bolts through from the front, then place the LubeLocker gasket over the bolts. Holding the diff by these bolts, put into place and start threading the two bolts into the differential housing. Install the remaining bolts until the threads just run out, but don’t tighten them. You’ll probably have to shift the cover around to get all bolts started. Don’t force anything. The gasket installs dry – don’t add any kind of sealant to it or the mating surfaces.

  8. Torque in two steps.

    Using a torque wrench set to 15 foot pounds and a 6mm Hex / Allen socket, tighten each bolt per the diagram on the gasket packaging until the wrench clicks. Repeat the pattern, but set your torque wrench to 30 foot pounds this time. I read online various specs, but the fact is, the Ford spec is for RTV and has no bearing in this situation. I saw a few accounts ranging from 28 to 33, all reporting no leaks. I chose 30 foot pounds and it’s worked out great.

    Note: The 6mm socket fits the aftermarket bolts sort of loosely. I was able to torque without issue, but be careful as I could see these stripping easily. I tried a 1/4″ Hex / Allen socket, but it didn’t seem to fit. Your mileage may vary!

  9. Check out the magnetic drain plug! Snug it up and remove the top two bolts.

    Ensure the drain plug is snug, then remove the other two fill plugs. Use a funnel with tubing or a simple transfer pump (recommended above) to transfer oil into the differential. The third quart brought the level high enough for the oil to start flowing out of the center fill hole. This is your indicator that you’ve filled the differential. Do not fill to the upper bolt hole.

  10. Finishing up.

    Insert the middle and upper fill bolts to snug. Dispose of your oil at a local auto parts store. Drive a couple of miles to bring the diff oil up to temp and check for any leaks. Admire your work!

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