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Short Summary
Main recommendation: Change rear differential fluid by draining the old oil, cleaning the mating surfaces, sealing the cover with RTV silicone (if no gasket is available), and refilling with the correct gear oil until it reaches the fill hole. The video demonstrates this on a Mazda B3000 / Ford Ranger-style open differential, with the procedure applicable to most rear differentials.
Key Points
- Confirm the fill plug can be unscrewed before draining, since a stuck plug would leave you unable to refill.
- Check whether the differential is limited slip, as the tag on the unit indicates; limited-slip units need limited slip additive, while open differentials do not.
- Keep the top cover bolt partly threaded during draining so the cover does not drop and create a larger mess.
- Use a plastic scraper rather than metal, because any scratch on the mating surface will cause a leak.
- Use RTV only if no gasket is available for the cover, and choose a silicone that is rated for differential fluid, since some gear oils break down ordinary silicone.
Details Worth Noting
- Concrete instructions:
- Remove the fill plug first and check the fluid level by reaching in with a finger; fluid should reach the bottom edge of the fill hole.
- Loosen the cover bolts from the top down, leaving the middle bolt for last, fully removing all but the top one, which is loosened only about halfway so the bottom opens and drains while the cover stays attached.
- Pry the cover off with a flathead screwdriver placed between the case and cover, then let the remaining oil drain.
- Clean the cover and differential mating surfaces with brake cleaner sprayed onto a paper towel, scrape stubborn old gasket material with a plastic scraper, and check the sealing surface is smooth with no cuts or grooves.
- Check the gears for cracks, chips, or visible wear while the cover is off, and clean any magnet in the housing.
- To make an RTV gasket, cut the nozzle to about 1/4 inch, lay one continuous bead around the inside of the bolt holes starting at the top of the differential, and let it sit about 15 minutes until tacky.
- Set the cover on within 15 minutes to an hour, hand-tighten both bolts, then tighten in a star pattern until a little RTV squeezes out, and leave it for about an hour.
- Torque the cover bolts in a star pattern: first to 10 ft-lb, then to the spec of 25 to 35 ft-lb (the video uses 25 ft-lb); earlier, during initial hand-tightening with RTV, ChrisFix mentions an alternative of about 5 ft-lb each before letting it sit.
- Fill with gear oil (the video uses 80W-90 GL5) using a pump until fluid starts leaking from the fill hole, then reinstall the fill plug and torque to 15 lb-ft.
- The video notes the procedure works for almost all rear differentials, but the fill plug location varies by vehicle; the transcript does not say to check your manual.
- Using a breaker bar may be needed if the fill plug is tight, and the drain/fill plug is magnetic, so clean off metal shards.
- The creator states that a manufacturer gasket is preferable when one is available; RTV is a fallback.
Bottom Line
A rear differential fluid change is a manageable DIY job if you take care with the mating surface, use the correct sealant or gasket, and torque everything to spec.
