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Tips for rear main, bearings?

Damon Dimick

NAXJA Forum User
Portland, OR
Bought this lovely 88 for $500, and am going through fixing all the crap which had been neglected for 215k. So the rear main is leaking like a sieve, along with all of the other seals on the 4.0. The plan is to do new bearings while I have the pan off, but I've never done them before, or replaced a rear main before. Any tips/tricks/advice that you guys could lend for install? Also, will there be a need for 'oversized bearings'...explain that to me. This is my first time doing any bottom end work, so I'm a sponge for your info. Thanks.
take a good look at the bearings, the rod bearings will definitely look worse than the mains. as far as needing oversize bearings, once you get the crank out, take it to a machine shop and get an expert opinion on the shape of the crank (since you've never done this before) but basically, if there's any ridges in the crank's surface, you will want them ground off, hence the need for oversize bearings.

also, have the machinist put a micrometer on the crank to see if it's already been ground. you can only go so far, and measurement is the best way to know what bearings you need.

also, be sure that you have a good torque wrench. the dial type are the best to use for this. your machinist should be able to get you the torque sequence and specs for everything. (if i remember correctly, for the mains, starting from the center and rotating clockwise out from the center, you torque to 20lb-ft then 40lb-ft then 80lb ft.

don't forget to get "engine assembly lube" and coat the bearings in it.
215k isn't that much. I didn't get the impression the plan here included pulling the crank, and I don't think it's necessary.

"Oversized" (or "undersized" is more correct) bearings are used if the crankshaft has been turned to a smaller diameter than the standard size. If you look on the side of the engine block, passenger side just above the distributor, there is a flat boss machined into the surface with a code number. That code gives the build date for the engine and also tells if the main and/or rod bearings are standard or undersized. If you can get that code number, someone here can help you decipher it.

The biggest problem will be getting the oil pan off without destroying it. Work slowly and pry gently. To remove it once it is loose, you'll need to jack the chassis as high as possible and put it on jack stands, then lower the front axle as far as possible.

The rear main seal is two pieces. Bottom half comes off with the bearing cap -- no problem. To change the upper half, go to a hardware store and buy a short length of 1/8" brass rod. Use this as a pusher to gently push the old seal out of the groove. Brass is recommended because it's softer than steel, so it won't damage the crank surface if it scrapes against it.
the oil pan will come off easily with no damage if the proper steps are taken in loosening the suspension and lifting the engine slightly. i have never damaged a pan or had to do much if any prying. but i have never tried to short cut the job. for that matter i've never done it without the truck being on a lift.so maybe its worse than i think it is. as my perspective is scewed into thinking this is a gravy job.
i remove the track bar at the axle, the shocks at the axle,the anti-sway bar links, the steering link at the pitman arm (if necessary).and i loose the motor while supported with a screw jack and lift the motor. unbolt the pan and anything in the way. and remove. if the motors not knockin' i'd leave the bearings alone.
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Yeah I don't have any noise from the bottom end, but I figured it might be a good time to do bearings as well. Sounds like it's not necessary at this point, thanks for the advice.

Anything else I should consider while I have the pan off? Timing chain? Oil pump? I haven't checked the compression yet, but the motor seems to have pretty solid power and sounds pretty good all around. The rig seemed pretty neglected otherwise though, so who knows what kind of service it ever had?
if it runs good, doesn't burn oil, and compression looks good across the whole thing, I'd just do the rear main and be done with it.

Mine's got almost 315K miles on it, I am NOT nice to it, and I have yet to pry into the engine or tranny (aside from replacing seals) the other stuff... (transfer case, drivelines, axles) was because I broke them.

the things I'd check, if I were you, would be differential and transfer case fluids. they always get forgotten, now would be as good a time as any to change them ;)

other things to look at would be wheel bearings, balljoints, steering linkage and suspension bushings. if they've never been touched, they're all pretty much at the end of their useful service life.

if it has a tilt column, it may start getting loose (it's common) and simple to fix with the right tools.
If the truck has been neglected, while the pan is down I would replace the rod bearings. As my parsimonious colleague already noted, the rod bearings tend to wear before the mains.
I just did this on my 94 XJ with 196,000 miles. I did mine out of vehicle (but that's another story.....)

A couple of the rod bearings showed distinct scores. And the oil pressure had dropped to a little under 20 lbs. at idle.

No stamps on the block to show over/under sized bearings, so I got a new set of Federal Mogul bearings last week, and installed them on Thursday. Plastigaged all of the bearings and they all came out to 0.002" +/- 0.0005". Also changed the timing chain and sprockets (started to slap against the timing cover,) new high volume oil pump and harmonic balancer.

Got it fired up yesterday, and it started on the third crank. Ran it 370 miles today, idle oil pressure is ~37 lbs. at idle, and 55 lbs. at 2500 RPMs.
ChiXJeff said:
new high volume oil pump

Where did you get it and did you have trouble making it fit?

I helped a friend of mine rebuild his 4.0L last summer, and he picked up this high-volume oil pump from god only knows where. I had to heat/beat his oil pan, to reform it to fit over this monstrous oil pump.

Still workin' good... but I'm curious as to the clearance issues :confused:
CheapXJ said:
Where did you get it and did you have trouble making it fit?

I picked up a Melling pump from AZ, complete with a new pickup. It was cheaper than a standard volume pump without a pickup (go figure :dunno: .)

Apparently, it was the same dimensions as the stock pump, and it went in so easy it was boring. No clearance issues at all. I packed it full of assembly grease, and we turned the motor over by hand a few dozen times before I put the plugs back in.

Wonder if your pal didn't get the pickup mounted on the pump right? Personally, I wouldn't have put it in if it didn't fit.
no, it was the physical body of the pump, it stick down from the block about an extra 3/4" or so.

the pump iteslf was physically larger!

It works great, but the fitment was kinda strange :dunno:
True -- Bob from RI had to "massage" the oil pan on his stroker engine to fit around the high-volume pump. I believe his was a Melling, too, but I'm not certain of that.
Hmmmmm........ I'll check the part numbers.

This is annoying...... The AutoZone website lists the pump I bought as a Melling M-167HVS. Which isn't listed on the Melling website. Nor is the M81-A, or the K-81A either. Probably AZ specific part numbers. Unfortunately, I left the box up at Dad's (where I did the work) and it's probably been burned by now.

In any event, I'm happy with the oil system.