XJ C8.25 Rear Disk Brake Conversion - ZJ vs KJ parts - JeepForum.com
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Unread 08-16-2015, 09:18 PM   #1
Charley3
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XJ C8.25 Rear Disk Brake Conversion - ZJ vs KJ parts

ZJ vs KJ parts - Which is easier to do? Which gives better results? Is there noticeable braking performance improvement over properly functioning drums?

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Unread 08-17-2015, 11:22 AM   #2
PimaXJ
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All good questions. These are my notes from researching the same swap. The ZJ has a Dana rear & the central hole has to be enlarged slightly to fit the 8.25. With the KJ, which has the 8.25, everything is a direct swap & is my first choice. As to efficiency of the set-up, my first thought is to determine the swept braking area of the OEM drum (9" or 10") versus the disc set-up. Also, imo a disc set-up is less likely to overheat than a drum set-up, has fewer parts & is easier to maintain. Other concerns are the emergency or hand brake & whether the KJ will interchange. Several people who have made this upgrade have not changed the proportioning valve. When I get closer to doing this swap, I will compare the diameter of the master cylinder bore between the KJ & the XJ as well as take a look at the differences between the two valves if there is any. Although the OEM Trac-Lok is not the greatest in limited slip differentials, the best donor for me would be a KJ with the Trac-Lok. Hope this helps!
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Unread 08-17-2015, 01:25 PM   #3
RedJeepster1
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Regarding the track lock, are you considering swapping in the whole axle? Or just to pull the carrier while you pull the brakes on the KJ? Liberty's used coil springs in the rear so an axle swap isn't as simple as a brake swap.

Otherwise, I agree with everything above. This is something I have been considering since I have a donor ZJ at home. E brake is something I would be interested to learn about for sure. The rest seems mostly self explanatory.
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Unread 08-17-2015, 01:32 PM   #4
WhammyXJ
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Quote:
Originally Posted by PimaXJ View Post
All good questions. These are my notes from researching the same swap. The ZJ has a Dana rear & the central hole has to be enlarged slightly to fit the 8.25. With the KJ, which has the 8.25, everything is a direct swap & is my first choice. As to efficiency of the set-up, my first thought is to determine the swept braking area of the OEM drum (9" or 10") versus the disc set-up. Also, imo a disc set-up is less likely to overheat than a drum set-up, has fewer parts & is easier to maintain. Other concerns are the emergency or hand brake & whether the KJ will interchange. Several people who have made this upgrade have not changed the proportioning valve. When I get closer to doing this swap, I will compare the diameter of the master cylinder bore between the KJ & the XJ as well as take a look at the differences between the two valves if there is any. Although the OEM Trac-Lok is not the greatest in limited slip differentials, the best donor for me would be a KJ with the Trac-Lok. Hope this helps!
This.
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Unread 08-17-2015, 02:17 PM   #5
Charley3
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Sweet. Now I have a general plan. KJ 8.25 brake parts for my XJ C8.25.

Now how about a list/discussion of the benefits compared to drum brakes?
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Unread 08-17-2015, 02:43 PM   #6
WhammyXJ
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Originally Posted by Charley3 View Post
Sweet. Now I have a general plan. KJ 8.25 brake parts for my XJ C8.25.

Now how about a list/discussion of the benefits compared to drum brakes?
http://www.edmunds.com/car-technolog...m-vs-disc.html
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Unread 08-17-2015, 03:23 PM   #7
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I used the disks off of a ZJ, along with the booster/master cylinder and proportioning valve. If memory serves, it is less than 1/8" that has to be taken out of the inside bore of the backing plate for it to fit the 8.25 axle tube. I don't know if my original drums were working properly or not, but I definitely noticed an improvement in braking performance after the swap, especially when towing a trailer. Of course, I also gained some power on the front brakes, as the ZJ master cylinder has a larger bore than the stock one off the XJ. I still haven't taken care of the parking brake. The stock cables won't attach to the mechanism on the disk setup, and the cables off of the ZJ aren't long enough to reach. I just haven't had time to revisit the issue. However, as has already been said, swapping from a KJ is simpler, as the backing plate is a direct fit. I have no idea about the parking brake setup on the KJ vs. the XJ. Best of luck!
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Unread 08-17-2015, 03:33 PM   #8
PimaXJ
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As to the Trac-Lok, the KJ innards will bolt into the XJ housing. Axles are reportedly a different length but are 29 spline as are the '97 & newer XJ. One of my concerns was that the KJ has ABS & a tone ring on the ring gear. Evidently this is bolt on & can be removed for use in a non-ABS application. As to the drum vs. disc efficiency, the key is dissipation of heat & the disc brake system excels in that. The front brakes do most of the stopping & in a way the rear brakes are simply balancing the front brake force. However, imo the benefit for a rear disc conversion besides what I mentioned earlier, is found when towing something behind the XJ. One last thought is if you intend to go off-roading where the trail is wet & muddy, a drum brake system could possibly collect mud & H2O in the drum whereas the disc system will not.
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Unread 08-17-2015, 04:31 PM   #9
Charley3
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I defineatley want the KJ conversion. You convinced me.
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Unread 08-18-2015, 09:18 AM   #10
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I did the KJ swap (here) - A couple notes for you:

1) get new rotors. The OEM ones "moan" and will drive you nuts. There were lots of TSBs around this and DC never fixed it.
2) get a Ford Explorer E-Brake cable for the passenger side, and a ZJ for the driver side.
3) get a complete set of 2001 XJ front lugnut studs and press them in FIRST. The stock rears are too shallow.
4) you can use ZJ softlines to connect the XJ brake lines to the KJ banjos. Get them off the same donor as your ebrake line.
5) Replace the axle to backing plate studs with high quality grade 8 hardware. I used the XJ studs and I will likely be taking my brakes apart to replace them soon. You DO NOT need to use press fit studs, just high grade hardware.

As part of mine I completely disassembled, cleaned, painted everything, and replaced all wear components (except the rotors, doh!). This made install go smoother, but it brought the cost to close to $250. If you use all junkyard parts, you should be able to do the whole thing for sub-$100.
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Unread 08-18-2015, 03:31 PM   #11
Charley3
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The nearest junkyard is 50 miles away and it's a U pull it. I'm handicapped from a car wreck. I can't pull the parts myself.

So I think I'll buy all new parts from Oreilly's or RockAuto. Maybe Moog brand parts, if they make brake parts. I've bought Moog suspension parts before, but never bought brake parts before.
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Unread 08-18-2015, 05:38 PM   #12
Charley3
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Hey XeroOTG, couple questions:

Will those emergency brake cables you suggested be long enough with 3" lift? My lift isn't really that high, but the amount of droop my LCG lift has is like a typical 3" lift's droop.

Also, I currently have Crown aftermarket stainless steel braided soft lines of a suitable length for 3" lift. The have universal fittings on them. Will those work with KJ brakes?
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Unread 08-18-2015, 06:22 PM   #13
KF4SQB
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To my knowledge, Moog doesn't make brake parts. Use Bendix.
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Unread 08-18-2015, 09:41 PM   #14
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KJ brakes are bolt-on bliss.

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Unread 08-18-2015, 10:01 PM   #15
xeroOTG
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Quote:
Originally Posted by Charley3 View Post
Will those emergency brake cables you suggested be long enough with 3" lift? My lift isn't really that high, but the amount of droop my LCG lift has is like a typical 3" lift's droop.
So the explorer cable is long enough for way more lift/flex than what you will need. I have 4" of lift and fully extended there is still tons of room left. IF you are nervous, there are companies that make retrofit cables. The cables in this case are the same for ZJ and KJ backing plates.

Quote:
Originally Posted by Charley3 View Post
Also, I currently have Crown aftermarket stainless steel braided soft lines of a suitable length for 3" lift. The have universal fittings on them. Will those work with KJ brakes?
The soft line conundrum. The aftermarket crown lines are good from the hard lines on the body to the hard lines on the axle. Take a look at the thread I linked to at NAXJA, I put lots of pictures of how I chose to do my brakes. The soft lines I was talking about are about 4 inches long and only go from the axle hard lines to the calipers to make removal easier.

I want to say teraflex makes a set for sale. TF-4355400 looks like what you would need. Do not worry about what they say it fits, I am more interested in the brackets, threads and length.

I used the soft lines from a ZJ because if I tear one back in the woods (I have, and it sucks) I can get a replacement at any parts store. I tend to break my Jeep and I don't like walking, so I meticulously pick parts that are OEM (read: available anywhere) and good quality.

Quote:
Moog doesn't make brake parts
Correct. I prefer Centric Premium. Centric was brought to my attention by the fact that Black Magic Brakes recommend them. Considering they specialize in compounding for Jeeps, I would consider their opinion very in tune with the industry.



ONE LAST THING: If you find KJ backing plates that have missing or rusted dust shields, keep looking. KJ emergency brakes are pinned to these, so it is imperative that they are in good shape.
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My jeep gives a whole new meaning to the phrase, Just Empties Every Pocket!

1975 Jeep CJ5 258 TBI EFI, T-18, D-20, D44 rear, 4.11.
1992 Jeep Cherokee 4.0L, AW4, NP231 October 2009 - August 2012
1998 Jeep Cherokee 4.0L, AX15, NP231 - August 2012
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