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Rear Diff (Fluid Change)

15823 Views 46 Replies 11 Participants Last post by  elfhearse
Well, since I am at the 30K mark (actually 32K) I decided it was time to change some fluids.

I have ordered the "gaskets" that are needed for the front diff and TC, so I have not done them yet. But the rear just needs some RTV to seal the drain and fill plug.

First I have to say...if you have the M226 rear diff (OR model and Manual tranny) buy diff armor ASAP. As much as I don't like it, I think I'll be buying the AC armor for the short term. The abuse to the bottom of the diff, the drain plug, the cover and the bolts made this process more painful than it should have been.

I started out by purchasing 2 qts of Royal Purple. It's cheap ($9 a qt) and the people that use it seem to have good experiences. I figure I'll be changing out the oil every 30K so I don't have to go with the expensive Amsoil. I also picked up some Permatex Ultra Grey 82194 and this really cool funnel with a clear hose (it's got a handle, measure marks, a lid, and a shut off).

Once home I crawl under and since I had some seeping around the lower part of the cover I figure I'll check the level. I undo the fill plug and oil starts to run out (I had expected this as the pinion angle is pointed up). Due to the amount of fluid that came out the fill plug I think it was full.

So I look under and the drain plug is filled with dirt. I take a small screwdriver and clean it out. I go to put a 3/8� drive extension to remove the plug and it doesn’t fit. I look again and it’s so ground down that the hole is about half the size. So I break out the dremel and open the hole up. So now the 3/8� extension fits in, I remove the plug and nothing comes out. I look at the plug expecting to see a grey goo on it (fine metal shavings mixed with gear oil)…nothing. I stick my finger up in the drain hole and feel a “cap� of sorts. I push it up and oil starts to drain.

With the oil drained out I start to remove the cover so that I can pound it back into shape and reseal it. I loosen all the top bolts but when I reach the two bottom bolts they are so ripped up that the socket or wrench don’t have enough sides to grab on to. I break out the vice grips and they still do not budge. I start looking close and realize that the bottom of the diff and the cover are ground to the point that they are fused together and fused to the bolt. I break out the dremel and grind down the bolt enough to get a 12mm wrench on instead of the 13mm. Still no go. So a break out the torch and head them up. After about 30 mins I finally get the two bolts out.

So I go to remove the cover and find out that there is not enough clearance with the brake line. So I remove the brake line from the axle end (now I am going to have to bleed my brakes). I get the cover off and clean it up.

I go back and look at the inside of the diff, it looks good. The teeth are meshing well and no sign of any problems. I clean off the edge where the cover sits and notice a lip on the bottom where it was ground down. Figuring the cover is not going to sit properly with this lip, I break out the dremel again. Get everything cleaned up and pull the little plug of grey goo from the bottom of the diff. It’s about the size of a quarter and about ¼� thick. This is typically normal, but I can’t say I have ever seen that much metal in a diff.

I apply the RTV to the cover and stick it back on. Finger tighten all the bolts (moving the crewed up ones to the top since I don’t currently have new ones. Torque them all down to 31ft-lbs in a star pattern (just like you would a car tire). Put a little RTV on the drain plug and torqued it down to 25ft-lbs.

Now, since the pinion angle is off, I can’t just start filling or else I am not going to be able to put the correct amount in (someone needs to come out with a new cover will a high fill hole). So I jack up the back end and place jack stands under the rear axle. Even with it up this high, I still was only able to get 3.75pt instead of the 4.25pt’s that the SM calls for. That leaves me 1/2 pint short. The only way I can think of getting that extra ½ in is to park it on a very steep hill (maybe my father’s driveway). For now I am running it a bit low.

I put the brake line back on, but because I do not have a self bleeding kit, my brakes are not bled (I’ll be doing that tonight).

So what should have taken less than an hour to do took closer to three from prep to clean up.
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Wow. Great write up.

any picts?

Edit: Guess what I just ordered?
I have some (both batteries died while taking them). I'll try and get them up over the weekend.
Great write up! I will try to get some pictures when I do mine. Was there a little check valve by the drain plug? It sounds like you couldn't drain the fluid until you pushed that in.

I am figuring I will park on a steep drive too, to get all the fluid in.
No, it wasn't a check valve, it was the build up of fine metal particles and oil that caused the plug. The drain plug has a magnet and usually when you pull it out the grey goo is on it. The problem was the build up of metal was so large it covered the entire hold and the plug couldn't pull it out.

I think what this means for most is that for at least your first change, you should pull the cover and clean out the inside of the diff.
That's about all bad news. Dawg I was counting on you to use severe gear! Shame shame shame. But heck, I've always used some gear oil off the shelf and never had a problem, so I don't think any gear oil comes into play unless something happens. Like you run it dry and don't discover it in time. I had Lucas save me once like that.

Anyways, get ya a set of these at Sears, They are quite handy to remove rounded off nuts and bolts, come in all sizes. You gave that dremel tool a work out eh?

And, I'm not getting why you can't fill it. Some of the oil probably stayed in hidden way back up in there somewhere? I think it's more important to put in what you took out. (Same amount) I had the same experience when I was checking mine, It was leaking but still poured out the fill hole. Jim is about ready to ship his cover, but it's a bolt on no removing the cover that is on type of deal.

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A bolt on is nice, but the only issue I see is if your drain plug gets fouled up like Jason's did.
I think we have to wait and see how it looks. I hope Jim thought of that already. mc
I ahve a gazillion tools, but I do not have a set of bolt extractors. I have never needed them. :)

Like I said, Since I will be changing the gear oil every 30K, just about anything will do as long as it meets the mfg specs.

The reason for the not filling is since the fill hole is a fixed height in the cover and the AAL (with the ~3º shims) tip the pinion angle up (which tips the cover back, which lowers the fill hole). With the diff perfectly level full should be just below the hole, with the diff tipped back, the full line would be above the hole.

It's the reason why most aftermarket covers coem with a high fill hole. They assume you are lifted and there for changing your pinion angle.
I knew I missed something. Yea, They make a funnel that fits into that hole with threads, It has a clear tube. Maybe you can do that, Fill until it settles at the point you want it in the tube and then do a quick switch with minimum loss and put the plug in. But, that won't help when checking the fluid huh? New cover time! You just invented a need, break out the Fab equipment and get into business! MC
The jack trick usually works, but I just couldn't get it high enough. The slopped driveway trick also works, but my driveway is just about flat.
A flat driveway in Pittsburgh? That's the rich part of town! :) Man on the way to our Terminal there is a driveway that has got to be like a 30+ degree slope. (Rt 28 S Highland park bridge exit then south again.) Ok when I do mine, will a set of ramps work? Mild Daystar lift. MC
Quick question - for those of us that have the locking rear, we shouldn't need to add any friction modifiers to the gear lube right? I should be able to buy gear lube and pour it in. On my old Chevy I had to buy some special additive.
It's off the shelf stuff, a C200 is different stuff than the Locked one. But you can buy right stuff off the shelf and use it bare naked. MC
Lockers don't need anything special since it's flat out locking the diff. With LSD (ie. Posi-traction) you need the friction modifier because of the clutch plates.
Thats what I thought!

BTW Amsoil Severe Gear 75W-140 made my diff quieter and less 'shock' is noticed when decelerating/accelerating. I love the stuff!
I`m approaching 30,000 miles in another 1000 or so, so I guess I better get on the band wagon and service the difs and auto tranny...

is there a filter in the trans? and will the filter change and whatever amount of fluid comes out replaced with new sufice as a service?...Thanks...PR...
Palerider where in NJ are u from?
kokopop said:
Palerider where in NJ are u from?
I`m near LBI...PR...
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