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Mj vs. Xj brake calipers.

J

jdahnk3

NAXJA Forum User
Location
Apache Junction
Ok searched and just couldnt find the answer i need.
I picked up a 88 mj 4x4/4.0L/AX15/D30/D35. Well i was thinking about changing brakes and was looking at the caliper differences and the mj caliper is much bigger compared to the xj caliper. In the hot rod world bigger is better in most cases.
So my question is are the mj calipers better than xj calipers?
 
Excellent question. I would like to know as well. Sounds like a cheap upgrade to the XJ braking system if so.


Edit- Advanced shows the front calipers for the 92 MJ/XJ to be the same part #, but the pads are different.
 
Last edited:
I thought the braked were the same. Are you comparing the same year calipers? It actually wouldn't matter about overall physical size, it would be piston size and brake pad size. If you have both laying around, could you measure the pistons?
 
I know both the older MJ and the Older cherokee use a different part number than the newer ones for brake pads. I think they were the same for both models, and in 1990ish they both switched over to the newer style.

With a properly working brake system there is really no advantage. I can lock up the tires on my 1991 with 33's after going down a mountain. If you are going to upgrade, go with the dual diaphram brake booster and new MC swap from a 1996 cherokee. Worlds of difference there. It is the only upgrade the 91 has. If you want more, grab a set of WJ knuckles and calipers. And do a high steer upgrade while you are at it.
 
wolfpackjeeper said:
I know both the older MJ and the Older cherokee use a different part number than the newer ones for brake pads. I think they were the same for both models, and in 1990ish they both switched over to the newer style.

With a properly working brake system there is really no advantage. I can lock up the tires on my 1991 with 33's after going down a mountain. If you are going to upgrade, go with the dual diaphram brake booster and new MC swap from a 1996 cherokee. Worlds of difference there. It is the only upgrade the 91 has. If you want more, grab a set of WJ knuckles and calipers. And do a high steer upgrade while you are at it.
Click to expand...
I have a set of pads for an '87 XJ. Would I be able to install them on my '96 XJ?
 
jdahnk3 said:
Ok searched and just couldnt find the answer i need.
I picked up a 88 mj 4x4/4.0L/AX15/D30/D35. Well i was thinking about changing brakes and was looking at the caliper differences and the mj caliper is much bigger compared to the xj caliper. In the hot rod world bigger is better in most cases.
So my question is are the mj calipers better than xj calipers?
Click to expand...

The AX15 went into XJ and MJ AFTER 3/9/89. Unless your MJ was converted, it is a Peugeot BA10/5. I hope for you it was.
 
chainsawhutch said:
I have a set of pads for an '87 XJ. Would I be able to install them on my '96 XJ?
Click to expand...

ok, so let me see if I can remember all of this correctly, it's been a long time since I dealt with my D30. I wrote all of this down a while back, you can probably find those posts by searching my username.

Anyway.

There's not really any difference in the brakes until 97(?) I think, and you can even mix and match amongst those parts.

For instance, my D30 has
97 housing
97 shafts
87 knuckles
97 bearings
97 rotors
87 calipers.

As long as the bearings match the rotor the caliper does not matter.

As long as the pads match the caliper (early vs late) The year does not matter.

As long as the caliper matches the knuckle it will work.

I pulled the original CAD axle and gave it to lawncher, it was geared 3.07 so it was likely factory original. The brake calipers were identical to the XJ axle.
To my knowledge the only difference in the braking system between MJ and XJ were the rear brakes and the proportioning valve.
The MJ could be had with wider drums in the rear when ordered with the Metric Tonne package, or ordered with the tow package, or special ordered with the D44. Otherwise it got the same D35 brakes the XJ got.
The rear proportioning valve is also different, there was a weight sensing valve in the rear that limited how much fluid the rears got based on a linkage attached rear axle to bed. The proportioning valve up front is different to accomodate a third emergency circuit. So that if the fronts failed, it would bypass the weight sensing valve and sned full flow to the rear. It also takes to a disc brake swap pretty well.
Again, it's been a long time since I researched any of this, but that's what I remember. Search my posts regarding brakes from about 3 years ago and you'll dig up my thread with part numbers and cross references and the like.
 
I have swapped an XJ front axle under my MJ. Twice even. The first time was to get a 4.10 geared axle and the second was when I bent that one. The entire front axle, including the brakes are identical and a straight swap. Things do change on the newer models as the rotor height and calipers changed.
 
chainsawhutch said:
I have a set of pads for an '87 XJ. Would I be able to install them on my '96 XJ?
Click to expand...
I believe at some point in the 80s or early 90s the bolt pattern that holds the calipers to the knuckles changed. Hopefully someone with more RENIX/early HO experience can chime in on this one, I know 90XJay mentioned it to me at one point while we were discussing his ongoing game of musical axles.

Also remember that 2wd vs 4wd, the brake and knuckle hardware is different apparently. I will know more when I throw the new front axle in my 2wd MJ hopefully this weekend - I bought all new brake hardware for the axle that's going in it to avoid any problems since it's going to be my DD in the future.

mid 99 the unit bearing / hub, rotor, and caliper design changed iirc. I know for a fact that the unit bearing did, you can tell which you have by comparing the height of the hub flange in the center right around the axle shaft nut. Make sure you get ones with the same height flange (new ones are taller iirc) and you will be OK.
 
wolfpackjeeper said:
so it looks like the new ones are the same size and shape, but have a bracket on the back to hold it to the caliper and piston.
Click to expand...

newer are actually a little more "banana" shaped and definitely don't fit in place of the older ones-- I had a set given to me that were "supposed" to fit when I bought the Heep and didn't realize the difference until I had pulled it apart.
 
kastein said:
I believe at some point in the 80s or early 90s the bolt pattern that holds the calipers to the knuckles changed. Hopefully someone with more RENIX/early HO experience can chime in on this one, I know 90XJay mentioned it to me at one point while we were discussing his ongoing game of musical axles.

Also remember that 2wd vs 4wd, the brake and knuckle hardware is different apparently. I will know more when I throw the new front axle in my 2wd MJ hopefully this weekend - I bought all new brake hardware for the axle that's going in it to avoid any problems since it's going to be my DD in the future.

mid 99 the unit bearing / hub, rotor, and caliper design changed iirc. I know for a fact that the unit bearing did, you can tell which you have by comparing the height of the hub flange in the center right around the axle shaft nut. Make sure you get ones with the same height flange (new ones are taller iirc) and you will be OK.
Click to expand...

This is correct. They redesigned the caliper sometime in the late 90's. You can tell wihch one you have by what bolts are holding it to the knuckle.
7MM allen head screws are the early style.
Hex headed bolts (10MM?) are the new style.
All of the 7mm allen haeded calipers will interchange, and so should the brake pads.
Brake pads intended for the 7mm allen calipers are not interchangeable with the hex headed calipers, as those have a different mount that holds them to the knuckle.
 
Is that supposed to read late 80s?

My 96 and every other 90s XJ I've seen have 13mm head bolts - they are an 8x1.25 thread, according to xjtrailrider and a few others who have had the pleasure of working with those threads.
 
my '88 and '89 each had the allen bolts-- my '91 and '94 were hex head bolts. I think the change was roughly around the same time as the Renix/H.O. split.
 
kastein said:
Is that supposed to read late 80s?

My 96 and every other 90s XJ I've seen have 13mm head bolts - they are an 8x1.25 thread, according to xjtrailrider and a few others who have had the pleasure of working with those threads.
Click to expand...
yes it was. I fat fingered it.
the changeover happened sometime shortly after chryco bought the Jeep line.
FWIW, I much prefer the allen bolt guide pins. Which is why my 97 axle has renix era calipers on it.
 
Hate to bring up an old thread but this confuses me a bit.

"much prefer the allen bolt guide pins. Which is why my 97 axle has renix era calipers on it. "

Does this mean that his 97 axle has Renix era steering knuckles to bolt the calipers up??

My research indicates.

You have to match Renix era or Chrysler era caliper to steering knuckle or unit bearing to disk rotor. In theory you could put a Renix era
wheel bearing and rotor on a Chrysler era steering knuckle/caliper combination or vice versa.

In my case I want to put chrysler era knuckles that are reamed out for one ton steering on my renix axle and my understanding is I can keep my matched unit bearings and rotors but need to swap out the calipers. This comment seems to indicate that you can actually bolt up renix era calipers to the later knuckles. They do look pretty close to me. I will buy new calipers anyway but it makes me curious. Ideas anyone?? I will try to match them up when I do the tear down and I will report back.
 
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