Rear Disc Brake Rotor Issue

Kerndone

Kerndone

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Mar 14, 2023
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Los Angeles, CA
Hello all,

Regretfully, my first post here is one for help, rather than an introduction. I recently started bringing my '99 Wrangler back to life. It has been pretty dormant, either in storage or buried in a small garage behind a motorcycle for the past 5+ years.

In the process of a bolt pattern conversion and moving the lugs on my rear Superior axle shafts, I noticed one of my rear brake caliper pistons was cracked. This was a "junkyard conversion" from a '96 Grand Cherokee, and I've been running everything as I pulled it 8 years ago.

I bought new calipers/pads for an '03 Rubicon instead of from a '96 ZJ so that I could use abutment clips and the TJ pads. In turn, I got rotors for an '03 as well.
My question, and I promise I did search, has anyone had issues with the rotors not fitting their axle shafts? These are Superior shafts, just regular Dana 44, not a Super 44 kit.
But after re-drilling the new rotor, I could not fit it onto the axle shaft. Upon comparing the rotors, you can see the new one I re-drilled has a lip on the inside, with a much smaller inner diameter.

I know I converted the bolt pattern and re-drilled the rotors, but I see no interference with the wheel studs, it is the axle shaft flange hitting the inside of the rotor. Do I need to use a ZJ rotor for the rear? Can I grind the inside of the rotor to make room, or is there something I need to look for when purchasing new rotors for a TJ?

Thank you!


OldRotor.jpg


NewRotor.jpg


Hopefully as I get more into my Jeep "refresh," I can start to contribute a little more around here!
 
Last edited:
Kerndone said:
Hello all,

Regretfully, my first post here is one for help, rather than an introduction. I recently started bringing my '99 Wrangler back to life. It has been pretty dormant, either in storage or buried in a small garage behind a motorcycle for the past 5+ years.

In the process of a bolt pattern conversion and moving the lugs on my rear Superior axle shafts, I noticed one of my rear brake caliper pistons was cracked. This was a "junkyard conversion" from a '96 Grand Cherokee, and I've been running everything as I pulled it 8 years ago.

I bought new calipers/pads for an '03 Rubicon instead of from a '96 ZJ so that I could use abutment clips and the TJ pads. In turn, I got rotors for an '03 as well.
My question, and I promise I did search, has anyone had issues with the rotors not fitting their axle shafts? These are Superior shafts, just regular Dana 44, not a Super 44 kit.
But after re-drilling the new rotor, I could not fit it onto the axle shaft. Upon comparing the rotors, you can see the new one I re-drilled has a lip on the inside, with a much smaller inner diameter.

I know I converted the bolt pattern and re-drilled the rotors, but I see no interference with the wheel studs, it is the axle shaft flange hitting the inside of the rotor. Do I need to use a ZJ rotor for the rear? Can I grind the inside of the rotor to make room, or is there something I need to look for when purchasing new rotors for a TJ?

Thank you!


View attachment 415137

View attachment 415139

Hopefully as I get more into my Jeep "refresh," I can start to contribute a little more around here!
Click to expand...

Pretty common issue. The rotor is machined to clear the stock diameter of the flange for the 5 on 4.5" bolt circle and not the larger aftermarket flange that has the extra material for the 5.5" circle. You can have the rotors machined to clear, taper the edge of the flange on the axle or get folks to see if their rotors are machined larger. I'll check ours here in a few and see but I would not count on them being larger there.
 
mrblaine said:
Pretty common issue. The rotor is machined to clear the stock diameter of the flange for the 5 on 4.5" bolt circle and not the larger aftermarket flange that has the extra material for the 5.5" circle. You can have the rotors machined to clear, taper the edge of the flange on the axle or get folks to see if their rotors are machined larger. I'll check ours here in a few and see but I would not count on them being larger there.
Click to expand...

Ok, "pretty common issue" makes me feel better! I was going to order a different brand of rotors for the TJ as well as a set for a ZJ to see if either worked without any machining. I guess I got lucky that the original ones from the donor Jeep fit without any issue. The real bummer is I was not expecting this and I re-drilled everything using my old 5x4.5 to 5x5.5 wheel spacers as a template first. It worked well, but then the disappointment of interference was a real let down!
DrillingJig.jpg
 
Kerndone said:
Ok, "pretty common issue" makes me feel better! I was going to order a different brand of rotors for the TJ as well as a set for a ZJ to see if either worked without any machining. I guess I got lucky that the original ones from the donor Jeep fit without any issue. The real bummer is I was not expecting this and I re-drilled everything using my old 5x4.5 to 5x5.5 wheel spacers as a template first. It worked well, but then the disappointment of interference was a real let down!
View attachment 415157
Click to expand...

If you do the math and apply some basic common mechanical sense, a 5 on 5.5" bolt circle using the smallest stud hole which is 1/2-20 threaded for a 1/2" hole, 1/2 of that stud diameter added to both sides of the circle means the edges of the studs are a 6" diameter circle. The smallest amount I would like past that for good mechanical sense would be 1/8" which puts the minimum flange diameter at 6 1/4" which is also right at what our rotors have as the flat on the inside of the hat.
 
mrblaine said:
If you do the math and apply some basic common mechanical sense, a 5 on 5.5" bolt circle using the smallest stud hole which is 1/2-20 threaded for a 1/2" hole, 1/2 of that stud diameter added to both sides of the circle means the edges of the studs are a 6" diameter circle. The smallest amount I would like past that for good mechanical sense would be 1/8" which puts the minimum flange diameter at 6 1/4" which is also right at what our rotors have as the flat on the inside of the hat.
Click to expand...

I can follow that math! I just measured the rotor I re-drilled and it is indeed right about 6 1/4" as well. My Superior shafts are quite old with press-in studs... and I have my tires back on so I can't measure the exact diameter of the axle flange. I wonder if at some point Superior wasn't worrying about tolerances for fitting inside disc rotors or maybe it is the press-in studs adding that little extra.

I will play around this weekend again and clean up my axle shaft flanges of some rust. I did go ahead and order a ZJ rear rotor to see if those offer a better fit that the TJ ones seeing as how the stock ones fit fine. I'll touch base here just to close the loop.

Thank you for the help and measuring your rotors!
 
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Kerndone said:
I can follow that math! I just measured the rotor I re-drilled and it is indeed right about 6 1/4" as well. My Superior shafts are quite old with press-in studs... and I have my tires back on so I can't measure the exact diameter of the axle flange. I wonder if at some point Superior wasn't worrying about tolerances for fitting inside disc rotors or maybe it is the press-in studs adding that little extra.

I will play around this weekend again and clean up my axle shaft flanges of some rust. I did go ahead and order a ZJ rear rotor to see if those offer a better fit that the TJ ones seeing as how the stock ones fit fine. I'll touch base here just to close the loop.

Thank you for the help and measuring your rotors!
Click to expand...

I'll check the ZJ rotors I have here.
 
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Ok, I received both rotors for a comparison... These are just run of the mill BrakeBest/SureStop whatever brand from your local auto parts store. Don't worry, my front brakes are a 16" big brake kit from you Blaine! In fact, you just helped me get everything fit up to install them with my Yukon hub conversion kit.

Short version:
ZJ rotors: inside flat - 6 5/8", disc thickness 0.438", outer hat bevel
TJ rotors: inside flat - 6 1/4", disc thickness 0.483", outer hat offset
RotorCompare.jpg

InnerRotorCompare.jpg
The ZJ rotors definitely have more clearance, with the flat edge inside the hat measuring about 6 5/8". The only difference is the outside of the center hat has a bevel around the perimeter and the thickness of the disc is about 0.438."
96ZJ.jpg
The TJ rotors (new set) have less clearance, with the flat edge inside the hat measuring about 6 1/4." The outside of the center hat does not have a pronounced bevel around the perimeter, more of an offset face, and the thickness of the disc is about 0.483."
03TJ.jpg
Seeing as how I am swapping from ZJ calipers to TJ calipers, I believe these new TJ rotors will work since the inside is machined down way more than the first one I attempted to use. Once I get the tires off I will measure my axle flanges to be certain, but the lower inner lip combined with the bevel on my axle shaft flange, I don't see interference being a problem.

All that said, it seems like the ZJ rotors are a no doubter to fit without any clearance issues. I just don't know how the thickness difference works with each caliper. I figure it is best to keep the ZJ rotors with ZJ calipers and the TJ rotors with the T calipers!

First TJ rotor to compare. Notice how much that inner lip protrudes!
03TJBB.jpg
 
Kerndone said:
Ok, I received both rotors for a comparison... These are just run of the mill BrakeBest/SureStop whatever brand from your local auto parts store. Don't worry, my front brakes are a 16" big brake kit from you Blaine! In fact, you just helped me get everything fit up to install them with my Yukon hub conversion kit.

Short version:
ZJ rotors: inside flat - 6 5/8", disc thickness 0.438", outer hat bevel
TJ rotors: inside flat - 6 1/4", disc thickness 0.483", outer hat offset
View attachment 415683

View attachment 415693
The ZJ rotors definitely have more clearance, with the flat edge inside the hat measuring about 6 5/8". The only difference is the outside of the center hat has a bevel around the perimeter and the thickness of the disc is about 0.438."
View attachment 415682
The TJ rotors (new set) have less clearance, with the flat edge inside the hat measuring about 6 1/4." The outside of the center hat does not have a pronounced bevel around the perimeter, more of an offset face, and the thickness of the disc is about 0.483."
View attachment 415681
Seeing as how I am swapping from ZJ calipers to TJ calipers, I believe these new TJ rotors will work since the inside is machined down way more than the first one I attempted to use. Once I get the tires off I will measure my axle flanges to be certain, but the lower inner lip combined with the bevel on my axle shaft flange, I don't see interference being a problem.

All that said, it seems like the ZJ rotors are a no doubter to fit without any clearance issues. I just don't know how the thickness difference works with each caliper. I figure it is best to keep the ZJ rotors with ZJ calipers and the TJ rotors with the T calipers!

First TJ rotor to compare. Notice how much that inner lip protrudes!
View attachment 415691
Click to expand...

The only concern that we've run into swapping the discs around to work is the hat diameter sometimes interferes with the inboard edge of the outer brake pads. The ZJ pad is narrower than the stock TJ rear pad.
 
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My reasoning for going with the TJ calipers was based on a comment I saw saying that the TJ calipers allowed the usage of abutment clips. Seeing as how my caliper brackets are older and have some wear marks, I thought it would be better to go with them.
 
Ok, just to finally follow-up in case anyone stumbles upon this. Most folks won't have an issue with this, if they put the rotor on and it doesn't fit, you can just return it. In my case, I had to re-drill the rotor to 5x5.5. So once I did that and then found out the rotor did not fit, I just bought myself an unusable rotor.

The second set of TJ rotors purchased had the same internal dimensions, but one distinct difference being the inside "lip" was machined down about 1/8" - 3/16" lower. This allows the rotor to fit on my Superior shafts without any interference on the outer edge of the axle shaft.

Now I finally have a '03 rotor with a '03 caliper rather than the rotor and caliper from a '96 Zj. And Blaine was correct, while the ZJ rotor had no clearance issues, the TJ pads were very close to hat of the rotor. They weren't rubbing, so it probably would have been fine, but I also like all the parts on the rear axle being for a TJ now.

Here is a pic of the rotor I used on the left and the one on the right with the interference from that raised lip. You can see the casting, where it was not machined down.

20230423_141024.jpg
 
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