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Front Diff Oil Change - Not an Ideal Situation

February 21, 2018.

Not all that long ago, the truck got a gear change - from the 4.10 factory gears to 4.88's - meaning that it's now easier on the engine to turn the bigger tires as it climbs up hills, over rocks, and generally towards adventure.

Within a couple weeks of the gear change, I'd driven far enough to get 500 miles of break-in complete on the rear diff and changed the oil - and everything there seemed A-OK, which was nice. And then on our trip to Death Valley, we ran as much as we could in 4WD to give the front diff a workout and complete its break-in. That turned out to be great - since most of the roads were easy - so the diff could break-in without getting overworked.

And now, it was time to change the oil. I was hoping it'd go as well as the back.

But of course, it wouldn't. In fact, it wasn't an ideal situation at all.

I got started by removing the skids. They had to come off anyway to change out the speedo gear, and were easy enough to remove, though heavy.

Unlike the rear diff, the front diff fill and drain plugs aren't a standard bolt head, but are instead Allen head bolts - where the hex key/bit goes into the bolt. I have no idea why this is, but with the necessary 10mm Allen key in hand, I wasn't worried.

(Note: Both the fill and drain plugs on 98-04 are 10mm, but on pre-98 models the drain is 12mm.)

The plan was to remove the fill bolt first (you always want to be sure you'll be able to refill anything you take out, plus it allows air in when you remove the drain plug) and then the drain.

So I inserted the wrench and...that's weird, the bolt wasn't tight at all. As in, even though it was screwed what looked like "all the way," it turned freely.

Definitely not ideal. I wondered if a bunch of gear oil had leaked out. Were my gears toast? I'd put over 4000 miles on the truck with these gears, and several hundred in 4WD. It would suck for them to be wrecked already.

I paused to evaluate. Slower is always faster in these situations.

There was no indication of oil spilling down the diff from the fill hole. There was no oil "puddle" on the skid plate. I'd tested the air locker on the front diff a couple times and it'd held pressure for the short time it'd been on, so I decided that everything was probably OK. Not ideal, but probably OK.

So out came the drain plug. And then the oil.

There wasn't a ton of oil that came out. I expected a bit over a quart, and got a bit less than a quart out - OK, I decided, given the locker and the fact that there was no way to get all the old oil out.

Then I checked the drain plug for debris. There was definitely some sludge on the magnet, but after spreading the sludge out on an aluminum plate there were no chunks. Seemed normal.

So it was time to re-assemble and fill. I put some new crush washers on the bolts and started with the drain, getting it tightened so I could refill the oil.

Once again, I chose to go with Lucas 80W-90 non-synthetic gear oil at the recommendation of JT's Parts and Accessories, but this time I couldn't just squeeze it out of the bottle and into the diff (like I could on the rear). The front is much tighter, and while I might have been able to do something in a pinch, it would have made quite the mess, I'm sure.

Luckily, I'd planned ahead and had a pump in-hand. This guy was only a few bucks and worth every penny. One tube into the oil and one into the diff and I pumped away.

I ended up putting in about the same amount of oil as came out - a good sign that I hadn't lost a bunch of oil - and I was sure to overfill the diff a bit (plugging the fill hole quickly as oil was seeping out), given the diff drop I have installed, which changes the tilt of the diff slightly, thus the "fullness" level of the fill hole.

And then, I buttoned everything back up.

I was not happy to have found the fill bolt loose, but I was happy that everything seemed OK, that I was done with the break-in and oil change, and that the truck was ready for more adventure.

So that's 3-1 in favor of happy; a definite win in my book!


  1. Jim
    Jim March 6, 2018

    Another comprehensive under-truck adventure. Good job on the explanation. However, you young people don't know how to get greasy. Between those gloves and the fact that all your gaskets seemed to be tight, there was little chance to get good and greasy. Back in the day, when we were finished with an oil change (no matter where), we had a nice layer of 90 weight on our clothes, hands and on the garage floor. The grease on the floor allowed us to enjoy the oil change for months by not only stepping in it and spreading it around, but by the sweet aroma and memories of the oil change.

    • turbodb
      turbodb March 6, 2018

      Hahahaha, did spill some oil as I overfilled the diff. And that gear oil is the worst of the oil smells! Luckily, outside the shop, so some kitty litter and it was fine.

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