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Semi custom cheap ZJ rear discs on small bolt 9.25 #2524014
07/19/18 11:55 AM
07/19/18 11:55 AM
Joined: Jul 2007
Posts: 1,682
Philadelphia
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radar Offline OP
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radar  Offline OP
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Joined: Jul 2007
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Howdy!

I’ve been posting about all kinds of stuff and learning a lot with my latest project a ground up streetrod 54 dodge truck. Thanks for all the help along the way! This post is about my 1970s 9.25 12-bolt rear disc brake adventure which is not over yet. Most of the 9.25 disc info and kits online are for the bigger bolt pattern late model truck stuff- there is not much online about the 5 on 4.5 bolt pattern 64” wide axles that fit under older mopar trucks and suvs.

So I have this 9.25 rear that I’m rebuilding and I also have a 90s zj 8.25 Jeep Grand Cherokee rear with 5 on 4.5 bolt pattern and fresh disc brakes. They have the tophat style baby drum parking brake too which is a nice feature. I don’t anticipate any C-clip failures but internet lore says the clip is the thing stopping the 9.25 from rivaling the dana 60 in strength. Yeah that and a smaller ring gear and non floating axles! Anyway I think a disc kit would provide a margin of safety in case of a broken axle shaft or c-clip failure by keeping the wheel from sliding out and possibly saving my bodywork and or neck.

I took the backing plates off one side of both axles and tried to put the disc bracket on the 9.25. It didn’t fit- the center hole that slips over the axle tube is too small, but just barely. It turns out the 9.25 tube is about 2.840” OD past the flange and the ID of the jeep disc bracket is 2.80.

A bunch of circles around and around the ID with a sanding roll on a dremel and the disc bracket finally slipped over the 9.25 tube. Now I can see that the four studs don’t line up. Either one side or the other the pair will slip into the holes.

I’ve done a scarebird disc kit on my 72 demon, the rusty hope kit on my ‘54 pickup, and I have made brackets for japanese and italian multi piston calipers to work on harley disc rotors, so I’m not afraid to cobble stuff together if it looks like it will work. The fixed caliper type that squeezes from both sides are harder to mount- if they are not centered over the rotor they can tend to drag and lock up. Floating style one piston calipers are easier since they can move inboard and outboard to take up the wear as one side squeezes.

The jeep brakes are the floating style single pistons. My next move is to install an axle into the 9.25 and compare the distance from the wheel mounting surface to the outside of the backing plate flange. If it’s similar I will consider it a success and install the zj disc bracket on the 9.25. I figure the best way to do that would be to use two of the studs to locate/clock the bracket properly so the bleeder is at 12:00 and replace the other two with a nice clean weld from the flange to the disc bracket.

So today I’m going to do that measuring and maybe tinker a little before going to work. I am putting this post here to help other people that might consider this swap for a strong cheap street axle and also to hopefully catch the eye of anybody that’s done this swap or seen it done- what’s your opinion or experience?

Thanks Moparts
Radar

Re: Semi custom cheap ZJ rear discs on small bolt 9.25 [Re: radar] #2524031
07/19/18 12:25 PM
07/19/18 12:25 PM
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Posts: 1,682
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radar Offline OP
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Replying to myself haha

I went out in the sunshine and had another look. It looks like with a little more grinding on one side of the bracket ID the four studs could slip through at the same time. If the factory bracket is iron not steel then the cold connections (bolts not welds) would be the way to go.

It’s more than just a little bit though- with two studs started the other two are 40-50 thou from hitting their holes...

Re: Semi custom cheap ZJ rear discs on small bolt 9.25 [Re: radar] #2524044
07/19/18 12:43 PM
07/19/18 12:43 PM
Joined: Jul 2007
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Philadelphia
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radar Offline OP
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Yup! It went on!! Looks like the answer is just to grind out the ID until all the factory chamfer is gone on the side where the stud holes are closest together.

Re: Semi custom cheap ZJ rear discs on small bolt 9.25 [Re: radar] #2524082
07/19/18 02:16 PM
07/19/18 02:16 PM
Joined: Jan 2003
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dogdays Offline
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Cool! But then again I like disc brakes. "But drums are enough for the rear, it doesn't do any braking!" True, but by the same thought, two barrel carbs are enough for 360s, because they get around town or on long distance driving just fine. You only need a four barrel for the 10% of the time you are accelerating.

If it was only the reduced pain, blood and effort to renew a disc brake over a drum brake I would vote disc. I also like the feel and progressiveness of the pedal. Also they look better peeking through the wheels.

R.







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