Unit bearing/disk/knuckle replacement

My 99 XJ has had a grinding noise since I bought it earlier this year. It just sounded like it needed brakes, and was a smokin' deal, so I picked it up. Anyway, I replaced the disks with the cast rotors (like it had on it when I got it), but I still get a grinding sound. Is it possible that the old owner used the wrong discs, and the Jeep was still able to move? (I haven't been able to measure the unit bearing yet... just learned how to tell them apart from the FAQs)

The next question: I'm pretty sure the unit bearings are shot anyway (I can feel some play)... Can I just use the later part number bearing to match things back up with the cast rotors, or is there something else that would need swapping, if this Jeep was set-up for the other bearing? Are the cast disks somehow better than the composites, or vice-versa?

Lastly, During my brake replacement, I noticed that one of the caliper bolts was stripped. I bought a bolt/bushing kit, and re-tapped the threads, but it would still only tighten down so-far. So I'm going to need a knuckle too. At the scrap yard, they didn't have any newer body style XJs (with knuckles), but they did have quite a few <96's. As long as they are the one-piece type (not the old 2-piece brake mount), are 'all knuckles created equal'? It looks like the same knuckle, but I wanted to make sure I wasn't over-looking some minor detail.

Hopefully... there will be an entire front axle that already has new unit bearings/ball joints/ and brakes at the junkyard :laugh:

If there's play in the bearings that might be your grinding sound right there. I think if you had the wrong disks, you would have noticed the incompatibility when you installed them.

There should be no play at all in the hubs, and even when they're tight, they can make a good bit of noise when they wear out.

Remember, many replacements for the composite rotors are cast, so just seeing cast rotors does not actually tell you which hubs you have. You'll probably have to measure to be sure you get the right parts, or make sure you get your parts at a store that will take them back if you guess wrong.

I think all the one piece knuckles are created equal, so that should be no problem. Remember, though, that the caliper bolts really don't do a whole lot of work. If you can get the bolt reasonably tight with a bushing or an insert, it should be fine. The bolts just hold the calipers in place until they engage, but once you're using the brakes the stress is taken up by the ears on the pads and the slides on the knuckle. When examining your knuckle and when shopping for a new one, look out for wear spots and chunks out of those slides. A little won't hurt, and they can be welded if need be, but I'd rather have a sloppy bolt than a chunk out of the slide that hangs up the pad.

Matthew Currie said:

I think all the one piece knuckles are created equal, so that should be no problem. Remember, though, that the caliper bolts really don't do a whole lot of work. If you can get the bolt reasonably tight with a bushing or an insert, it should be fine. The bolts just hold the calipers in place until they engage, but once you're using the brakes the stress is taken up by the ears on the pads and the slides on the knuckle. When examining your knuckle and when shopping for a new one, look out for wear spots and chunks out of those slides. A little won't hurt, and they can be welded if need be, but I'd rather have a sloppy bolt than a chunk out of the slide that hangs up the pad.

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The last time I had the wheel off, the bolt was backed-out a good way. So I'd feel safer just replacing the knuckle.

I had to weld/grind my TJ's knuckles for the brake pads, so I know to avoid the divots if possible.

So... since I already bought the cast-type rotors, and I need new unit bearings anyway, can I just get the part number to match the rotors (no matter which it came with stock)?

The answers to all your questions are in the FAQ for cast vs. composite rotor/hub posting. - > http://www.naxja.org/forum/showthread.php?t=1058413

Instead of replacing the knuckle(unless you just want to) I'd try a helicoil for it. If you're not familiar with this you get a kit which has a drill(or a chart to tell you want size drill to use) a tap and a thread insert. What you do is drill the hole and tap it with the supplied tap. Then thread the supplied thread insert in to the tapped hole and that's it. Your hole is back to it's original size so you can use the correct size bolt and you don't have to fool around replacing the knuckle assembly. Different makers lock the insert in different ways but either way it's good as new. Good luck on your repairs!

Well I figured I'd update this thread:

I managed to find a knuckle that still had the caliper attached, so a quick tightening of the wrench made sure they weren't stripped (glad I tested, because my first choice of knuckle was stripped just like the one I had). I felt a new knuckle was my best option because I was replacing all the ball-joints anyway. This knuckle's divots weren't nearly as bad as mine either, so I didn't have to fix them with welding. After cleaning-up the new knuckle, it was actually repaired with a heli-coil... So someone else fixed it for me...

I bought Timken unit bearings (same part # as my TJ) for the later model XJ with cast rotors. It actually did have the cast bearing/rotor combination. So it was just new parts replacing the old. The rotors had a nice glaze on them, so I had them turned-down. The pads were also new, but had the same glaze. So I hit them with a sanding block until the glaze was gone.

Strangely, the old unit bearings didn't seem bad at all! They were actually a stoutier construction than the Timkens, and still felt tight. Since I already bought the Timkens, I figure I'll just keep these on the shelf as spares... maybe try to re-grease them. The ball-joints were another story, on the other hand. The slopiness I felt before was in the ball-joints, not the unit bearings.
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Which leads me to now:
The whole front end is now new, minus the ring/pinion. The bad part is that I still have a noise! I pulled the plug from the diff cover and did find oil with my finger, but haven't pulled the cover yet. The finger-oil didn't have any sparkles or anything, and didn't appear milky from what I could tell. Time ran out, so changing the diff oil will have to wait for another day. The pinion yoke doesn't seem to have any excess play and is not leaking at all. The driveshaft U-joints and CV are good. I'll check the backlash and gear pattern when I pull the diff cover to change the oil.

What I'm wondering is: Could this be normal drivetrain noise in an XJ? It seams much louder than my TJ ever was, but then again, my TJ has been lifted for many years. Plus, I'm thinking that having a frame may help isolate some of the noise on the TJ. This XJ is bone-stock, and probably a little sagged from stock height.

On a side thought, I never did a thing to the rear... could I be hearing rear-end bearings, or drum brakes dragging? It sure sounds like it's coming from the front end, and is speed sensitive, but I don't know where else to look! I can't keep throwing parts at it, chasing noises... It's getting expensive.

Jack the rear end up and spin the tires to see if its the rear brakes?