Rotor Question

I was going to NAPA today to pick up some pads and rotors to do my front brakes this weekend when the guy at NAPA asked me which type of rotor do I have: composite or cast? I didnt know, so he told me to bring in the old rotor and they could check. I guess one of them sits about 1/8" lower? My question is does it really matter which type is put on, is there an advantage in one vs another?

TIA
Dave

The incorrect rotor will bind up in the caliper and bracket. It will be 1/8 in too far in or out. Easiest way is take the old one in and match it up.

Three different hub/bearing units were used over the years on XJ's. The hub/bearing units are the same part number for both sides of the vehicle. The model years for hub/bearing assemblies were:

1) 1984 through 1989 (early style rotors)

2) 1990 through mid-1999 (composite rotors)

3) late-1999 through 2001 (cast rotors).

** You need to match brake rotors to the unit bearing/hubs on your XJ D30 axle. **

Lay the rotor on your bench and measure from the bench to the top of the rotor.

Composite rotors will have an overall height of 3 3/16". Composite rotors will have a thin sheet metal like appearance at the lug nut holes where the studs come through and measure about 1/8 "(.125) thick in this area. Composite rotors have a rounded off corner at the wheel mounting surface and look like two pieces joined together.

Cast rotors will have an overall height of 3 1/16". Cast rotors are thicker at the lug nut holes and measure between 1/4" (.250) to 5/16 (.320) in this area. Cast rotors have a square corner at the wheel mounting surface and look like they are one solid piece of metal.

Parts store replacement rotors may have a different appearance but should measure about the same.

On late 1999 - 2001 unit bearing/hubs the ring around the 36MM shaft retaining nut is almost 3/4 inch tall and the hub has longer lug nut studs than older hubs ( see picture).

On early 1990 - 1999 unit bearing/hubs the ring around 36MM shaft retaining nut is about 1/2 inch tall (see picture).

Two steering knuckle designs were used: 1984 to 1989 and 1990 to 2001. Left and right side knuckles have different part numbers.

Two types of brake caliper styles were used: 1984 to 1989 and 1990 to 2001. Left and right sides are different part numbers.


1999.5 to 2001 unit bearing/hub on the left, 1990 to 1999.5 unit bearing/hub on the right
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COMPOSITE rotor below -

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CAST rotor below -

Wow ! Excellent post.

Any idea why they called them composite?

Hmmm, I put solid steel ones on my 98 after the composites that were replaced under a recall were shot last year, wagners. Same with both TJ's, got rid of the composites and replaced with steel, no issues, they went right on. I don't turn my rotors, I generally do two sets of pads then replace them. When I buy them I the counter guy says 'ok, 98, do you want cast or composite' the ones they sell are both the same size.
Turning them cost $15 per rotor, replacing them is $25 or so though I guess I could throw them on my buddys/customers machine but that would entail an extra trip from the house/driveway to the shop and back.

Ecomike said:

Any idea why they called them composite?

Click to expand...

I think it is becaues they are (or look like) 2 parts joined together, a sheet metal like center and a cast metal brake surface.
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So it turns out they were cast rotors. Replaced them and only encountered one problem. The top bolt of the two bolts that hold the caliper to the axle (cant think of the name of the piece that it attaches to) will keep turning once its screwed in. The bottom bolt does come to a stop it will just keep pushing on the caliper that is attached to the piston, what I think happened is when I was putting the bolts back in the top one may have gone in at an angle and kept going when it should have stopped. The question being, is this going to be ok, the bolt does not come out and it is pretty tight in the hole, or is the caliper going to fall off one day and completley f things up?
TIA
Dave

It was a good thing that I decided to fix the problem with the caliper today. Turns out the plate that the top bolt goes into was stripped, if you pulled hard enough it would have come out. Luckily I bought a Heliocoil set and that solved the problem, better than new.

So due to the different knuckles and calipers, you can not make a newer 99 rotor replace a 89?