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Unread 09-16-2008, 10:58 PM   #1
TPI Monte SS
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ZJ Rear Disc Conversion to 8.25" Write Up - The Whole Truth!!!


I drive my XJ on the streets of Jersey every day, in stop and go commuter traffic, with my kids inside. We deserve better brakes than what the XJ came with from the factory! Whatever I did to upgrade, I had to retain the parking brake feature as well, since NJ State inspection requires it. After some research, I found the ZJ ('94-'98 Grand Cherokee) rear disc setup was almost a bolt-on affair to my Chrysler 8.25" rear. The following writeup will hopefully "clear the mud" with what's been posted on various sites, and give you the real deal about how to swap them in, and do it right the first time. With that said, here's my list of parts and the prices I paid. You might find better deals, but this will give you a rough idea. I still came in way under the price of those aftermarket rear disc conversions!
  • Caliper Brackets/ Backing Plates/ Calipers: 1997 Jeep Grand Cherokee (ZJ) - $110
  • Grade 8 Hardware: 8 bolts, 8 nuts, 16 lockwashers - $10
  • Wheel Studs: Dorman 610-234 - $21 for 10 studs
  • Parking Brake Shoes: Wagner PD666 - $36
  • Parking Brake Hardware Kit: Carlson H7002 - $17
  • Brake Pads: Wagner PAB701 - $43
  • Rotors: Aimco 5360 - $39 for 2 rotors
  • Parking Brake Cable: 2003 Jeep Liberty (KJ) P/N 52128510-AG - $80 for 2 cables
  • Rear Diff Fluid/RTV: 2 quarts fluid/1 tube Red RTV - $15
  • Brake Fluid: 1 quart - $8
Total: $379

So, how do we install them? The first step is to loosen the lug nuts, jack the rear up, place a jackstand at each end of the axle, and take the wheels off. Once that's done, you can start to remove the brake drums. Make sure the parking brake is released, then remove the rubber plug on the bottom of the backing plate to gain access to the adjuster wheel. Using a brake spoon (or wide flathead screwdriver) turn the adjuster wheels inside the slot so the brake shoes will release and make it easier to remove the drums. You may need to beat on the drums from the back side with a BFH to loosen them as well; it's good therapy if you need it.

Once the drums are off, put a drain pan under the rear axle and loosen the top differential cover bolt a few threads. After that one is loose, go ahead and remove the rest of the 1/2" bolts holding the differential cover on. Now give the cover few good whacks with a rubber mallet, and try to use a flathead screwdriver to pry the cover off. It should let go after some "persuasion." Let the fluid drain out, then take off the top bolt and remove the cover. Now, look for any obvious damage to the ring and pinion and spider gears, and check the bottom of the axle for any metal shavings.

The next step is to pull the axles. Put the transmission in Neutral, and turn the driveshaft so you can rotate the carrier around to gain access the crosspin and locking bolt (inside the red circle in the pic below). Loosen the lock bolt with a 5/16" socket on a 1/4" drive ratchet for a little more leverage, then remove it with a box wrench.



As soon as the bolt is out, the crosspin will probably slide out, so be ready to catch it before it falls in the drain pan. Keep an eye on the spider gears and shims too; they may also try to go for a swim. Now, push in on the axles from the outside, and pull the c-clips off the ends of the axle. A magnetic wand is your friend here. Once the c-clips are off, you can slide the axles out. Replace the crosspin and lock bolt for now so the spider gears stay in one place.



Now that the axles are out, you can remove the backing plates and drum hardware. Use a 3/8" line wrench to disconnect the brake line from the wheel cylinder, then bend the line straight up. Next, loosen the four 9/16" nuts that hold the backing plate onto the axle, and slide the whole assembly off and lay it down on the ground. There's no need to disconnect the shoes, springs, or any of the brake hardware; you won't need it again! Leave the parking brake cable connected for now so you don't waste too much time, and attach your new caliper right away so you don't lose too much fluid. Rest the caliper on top of the leaf spring so it's out of the way, and repeat for the other side. Don't forget to inspect the axle bearings and outer seals. The ones on my truck were fine, so I left them alone. If anything looks questionable, fix it now; much easier to do while it's already apart.



Here's where the fun begins; modifying the ZJ backing plates to bolt onto the XJ 8.25 axle flange. I used a Dremel with a small grinding wheel to open up the center hole so the backing plate would fit over the axle tube, then discovered the studs on my axle flange did not line up exactly, and they were too short! I wasn't happy with this, so I enlarged the holes on the caliper bracket, then knocked the studs out of the axle flange with my trusty BFH. I put the ZJ brackets on with 1.5" long 3/8"-16 Grade 8 bolts with lockwashers and nuts. Ah, much safer!



The ZJs are notorious for having rusty/frozen parking brakes, and the reason is the slider cams that push the brake shoe into the drum rusts at the pivot point. I actually thought it was one piece! We were able to free them up, so I wire-wheeled the rust off and coated the pieces liberally with anti-sieze so they will (hopefully) stay rust-free and move when necessary.



Now that the brackets are on, you should install the parking brake hardware. It's easier with the axle out, that's why I removed it again.



Once the parking brake hardware has been attached, you can slide the axles back in, but only if you're going to use steel wheels! Why? Since the mounting face of the rotor is a bit thicker than the original brake drum, there ~may~ not be enough thread showing to safely bolt the wheels back on. I had this problem with my Magnum R/T aluminum wheels. Combine the thick mounting face on the rim with the thicker flange on the rotor, and there's just not enough threads to hold the wheel on; you could have a disaster on the road. After a bit more research, I found the ZJ wheel studs could be made to work; they have the same 1/2"-20 thread, and they're 3/16" longer. The trouble with them is the diameter of the splined area is .003" larger than the original XJ studs. To install them, you'll need to put the axle in a bench vice, coat the splines with some anti-sieze, and pull the studs through with an impact gun. I'm very lucky to have a friend with a killer compressor in his home shop, otherwise I would have been screwed! So, be sure to measure and replace your wheelstuds if you need to. Remove the lock bolt and crosspin again, carefully install the axles and C-clips, then pull the axle shafts out to lock them in place. Re-install the crosspin, and put a bit of Blue Locktite on the threads of the lock bolt and tighten it up.



Now slide your rotors on and rotate the axle to make sure they do not make contact with the backing plates. If the rotors hit anywhere, just straighten the backing plates out with a rubber mallet. Next, put some white grease on the slider pins inside the boots where the caliper mounts to the bracket, and install the pads onto the calipers (the side of the pad with two tabs goes to the top of the caliper). Install the caliper by pushing the top in first, then swinging the lower part in.

There are a few different ways to hook up the parking brake cable. I used cables sourced from an '03 Jeep Liberty (KJ model).



The side up near the adjuster for the hand brake has the same size clips and steel pins on the end as the OEM cables.



The back end has the correct hook to go around the slider cam on the parking brake. It's also spring-loaded like the ZJ cable would have been and the clip is the correct size as well. The only "problem" is the length of the cable. All I did was zip-tie it to the floor above the rear swaybar. Problem solved! Now, bleed the brakes, adjust the parking brake, re-install your differential cover and re-fill the rear, and go for a test drive!


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Unread 09-17-2008, 08:22 AM   #2
xjpro
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ZJ Rear Disc Conversion to 8.25" Write Up - The Whole Truth!!!

Buddy, absolutely great write up, I have this setup sitting on my garage for a weekend project, for sure this write up is going to be printed to help me out during the installation. Very informative and simple to understand!!!! Definetely this should become a sticky!!!!!!!
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Unread 09-17-2008, 01:24 PM   #3
gunselman
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So are the Dorman wheel studs that you have in your parts list similar to the ZJ studs that you used?
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Unread 09-17-2008, 05:04 PM   #4
RckRdyXJ
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Good write up, Deff one of the better ZJ disc swap write ups i have read.


FYI KJ Liberty rear discs are a direct bolt on no mods required, KJs have an 8.25 with discs.

Got any pics with the magnum wheels on your XJ, I love how they look on an xja nd want a set for street wheels.
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Unread 09-17-2008, 05:27 PM   #5
xjpro
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Did you use the ZJ wheel studs or the Dorman 610-234 ones??

Thanks.-
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Unread 09-17-2008, 05:52 PM   #6
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I was gonna do this, and had all of the parts, but I sold it for money for my bumper. I kinda regret it.
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Unread 09-17-2008, 09:04 PM   #7
TPI Monte SS
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Quote:
Originally Posted by gunselman and xjpro
Did you use the ZJ wheel studs or the Dorman 610-234 ones??
I used the Dorman part number; they're exactly the same specs as the ZJ studs, only they're new in a box from the auto parts store.

Quote:
Originally Posted by RckRdyXJ
Good write up, Deff one of the better ZJ disc swap write ups i have read.


FYI KJ Liberty rear discs are a direct bolt on no mods required, KJs have an 8.25 with discs.

Got any pics with the magnum wheels on your XJ, I love how they look on an xja nd want a set for street wheels.
Yep, I figured the KJ brakes would work; my wife would have gotten pissed if I took hers though. They're tough to find in this area, so I pilfered a set from a ZJ being parted out locally. And yep, here's a pic of the Magnum wheels on my XJ -

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Unread 09-17-2008, 09:17 PM   #8
xjpro
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Quote:
Originally Posted by TPI Monte SS View Post
I used the Dorman part number; they're exactly the same specs as the ZJ studs, only they're new in a box from the auto parts store.



Yep, I figured the KJ brakes would work; my wife would have gotten pissed if I took hers though. They're tough to find in this area, so I pilfered a set from a ZJ being parted out locally. And yep, here's a pic of the Magnum wheels on my XJ -

I love that color man, sweet rig!!
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Unread 11-22-2008, 09:52 PM   #9
mtsumedi
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Can you use the axles from a 8.25" ZJ and just swap them in instead of dealing with the shorter studs? I have a pretty inexpensive pull a part locally that may make it worth my while to not have to deal with having to make new/different studs work.
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Unread 11-23-2008, 08:28 AM   #10
RckRdyXJ
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No ZJ came with an 8.25.


BUT Liberties came with an 8.25 29 spline with discs, and the discs are a 100% bolt on nonmodification direct swap on to any xj 8.25.
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Unread 11-23-2008, 08:49 AM   #11
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It would be tough to pull the axles to get the backing plate in a junkyard though, I guess buying the whole axle would work better. Still might be cheaper than the Teraflex kit and you'd have everything you need.

Good writeup dude, I really need to do this. My drums sounded awful on my last wheeling trip.
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Unread 11-23-2008, 09:34 AM   #12
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Great write up!
You didnt feel you needed the Proportion Valve? I really noticed a difference in braking after installing the prop valve.
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Unread 11-23-2008, 02:51 PM   #13
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Quote:
Originally Posted by Steamer98 View Post
Great write up!
You didnt feel you needed the Proportion Valve? I really noticed a difference in braking after installing the prop valve.
Thanks for the compliment! Well, I've driven it a couple thousand miles with the original XJ prop valve and it's been great. I even had to do a panic stop one day in the pouring rain, and the wheels didn't lock up - just stopped MUCH quicker than it would have before. I do have the ZJ prop valve sitting in the shed though, I might install it in the spring time when the weather warms up again.
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Unread 11-23-2008, 03:19 PM   #14
groovyone
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If you use the KJ parts do you still need to change out the studs?

Thanks for the excellent write up!
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Unread 11-23-2008, 08:49 PM   #15
TPI Monte SS
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Quote:
Originally Posted by groovyone View Post
If you use the KJ parts do you still need to change out the studs?
I don't know for sure, but would say, probably yes. Most likely, the hat on the rotor is thicker than the face of the drum, and that's going to take up some of the distance on the studs that you need. Better to be safe than sorry, IMO.
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