Drum brake swap from 7 1/4" to Jeep 8 1/4"

[1973dust]

I've done many searches on this and I'm still not 100% sure. I have a 3.55 geared 8 1/4 rear out of, I believe, a 97 Jeep Cherokee. I'm intending to install it in my 73 Duster this winter. I am fairly certain that the 10" drums from my current BBP 7 1/4" axle should be a direct swap to the Jeep rear as it has tiny 9" drums. I'm hoping that someone on here can tell me so I know what I'm getting into before I tear into things.

 

[Charrlie_S]

I don't know for sure, but I would guess it won't work. If the jeep rear has 9 inch drums, then the brake shoes, wheel cylinders, and backing plates would all be different. Now it MIGHT be possible to swap everything, but I have no idea.

 

[1973dust]

Sorry, I re-read my original post and I wasn't very clear. I am looking to swap the complete drum brake setup over, not just the drums.

 

[Charrlie_S]

Sorry, I re-read my original post and I wasn't very clear. I am looking to swap the complete drum brake setup over, not just the drums.

Sorry I can't help at all. I am very familiar with the Mopar 7 1/4 but have absolutely no knowledge of the Jeep rear.

 

[Dave999]

i would suggest if you have disk brakes at the front the 9 inch drums at the rear would be good for brake bias
i.e allow those front disks to do their job before the rear locks up.

This was Aussie A body set up from 67 to 76 . for all 6s and 8s up to 360.
Later they changed the front disks to bigger with supposedly light weight and better calipers and added 10 inch drums on the back 76-82 I never found anyone who said it made much of a difference. most still home in on the mid 70s set up with the 9 inch drums and claw slider 1 pot front disc calipers as the go to set up, for retro fitting to anything else


if you have drums at the front then big ones at the back makes sense you have same all four corners and the bias is provided by cylinder bore and shoe/drum swept surface area.


if you are swapping drum brakes the whole lot needs to go across
so it comes down to whether the back plates bolt up or can be re drilled to bolt up and whether the drums fit the hub on the axle shafts, and wheel flange the PCD.

i think you will have a PCD problem did 7 1/4 stick with the 4 inch PCD
the jeep will be 4.5 with a different hub centering size if that's smaller a press fit ring can be used to center the mopar drum on the end of the jeep axle shaft

presume 7 1/4 has a smaller axle housing end than an 8 1/4 so

1) luck: and the 4 bolts that hold on the back plate just match axle to axle and Mopar used the same dimensions
2) luck: the distance between back plate and axle wheel-mounting face is the same axle to axle i.e mopar persisted with a dimension from housing end to axle shaft mounting face between the 1970s and the 1990s otherwise the drum skirt will rub the back plate or the reverse happens and you have a great big gap.
AND
3) no luck: and they don't match but from smaller axle to big may well give you the option to redrill the new pattern into the back plate provided it doesn't have some wierd contour just where you need a bolt. You can of course off set the back plate to 1 o'clock or 11 o'clock to facilitate re drill if need be, just need slight adjustment of brake pipes to reach.
4) No luck you do all of this and then discover that the drums sit too far in or too far out and rub the back plate as mentioned before... or they don't centre

custom axles could sort that.....

Dave

 

[TrailBeast]

I've done many searches on this and I'm still not 100% sure. I have a 3.55 geared 8 1/4 rear out of, I believe, a 97 Jeep Cherokee. I'm intending to install it in my 73 Duster this winter. I am fairly certain that the 10" drums from my current BBP 7 1/4" axle should be a direct swap to the Jeep rear as it has tiny 9" drums. I'm hoping that someone on here can tell me so I know what I'm getting into before I tear into things.

Why not simply use the Grand Cherokee or Dodge Dakota disc brakes on the rear?
They bolt right on the 8.25.
About the only thing different is you will need longer wheel studs because the discs take up a bit more room than the drums.
I put them on the 8.25 in my Dart.

 

[JDMopar]

Sorry, I re-read my original post and I wasn't very clear. I am looking to swap the complete drum brake setup over, not just the drums.

I have swapped BBP 10" drum brake assemblies from a 7 1/4 to an 8 3/4 several times. If they fit an 8 3/4, they should fit an 8 1/4. It's been a long while since I did that swap, but I can't remember if you have to drill the 5th hole in the bottom of the backing plates or not? If what @TrailBeast suggested above also gives you a parking brake, that would be my choice.

 

[Charrlie_S]

I have swapped BBP 10" drum brake assemblies from a 7 1/4 to an 8 3/4 several times.

What 8 3/4?
An "A" body 8 3/4 will be 4 inch SBP and the small drum center hole. The 7 1/4 BBP 10 inch drums will be 4 1/2 inch BBP and large center hole.
The BBP 7 1/4 backing plates do have 5 holes and I think they are the same pattern as the "A" body 8 3/4 (I don't have a 8 3/4 backing plate that I can measure). I do have small and large bolt patten 9 and 10 inch parts I can measure.

 

[JDMopar]

What 8 3/4?
An "A" body 8 3/4 will be 4 inch SBP and the small drum center hole. The 7 1/4 BBP 10 inch drums will be 4 1/2 inch BBP and large center hole.
The BBP 7 1/4 backing plates do have 5 holes and I think they are the same pattern as the "A" body 8 3/4 (I don't have a 8 3/4 backing plate that I can measure). I do have small and large bolt patten 9 and 10 inch parts I can measure.

I really didn't word that well, now that I re-read it....lol. First one was when I swapped from the factory 7 1/4 BBP rearend in a 71 Challenger to a factory E body 8 3/4 housing. I removed the axles from the 7 1/4, removed the backing plates and lines from the 7 1/4 and suspended the brakes in mid air without cracking the lines. I removed the old 7 1/4 housing and installed the 8 3/4 housing, put the backing plates back onto it with new seals and installed and adjusted the axles. It all fit back perfectly.

The other one was when I swapped BBP 10" drums from a junked 7 1/4 to an 8 3/4 housing cut to A body length and a set of Dr. Diff BBP axles for stock A body length. The backing plate offset is the same on BBP 7 1/4 brakes as it is on the BBP 8 3/4 brakes. I have a few sets laying around from where I junked out a couple of more door 318, 74 model Valiants years ago.

 

[Dave999]

so all good then
the plan laid out by1973 dust works...

Nice

 

[MopaR&D]

Yes it will work, I've done it myself for an M-body ('88 5th Avenue). Swapped from stock 7 1/4" rear end to 8 1/4" out of a Jeep XJ, drum brake assemblies transferred over just fine.

 

[TrailBeast]

I have swapped BBP 10" drum brake assemblies from a 7 1/4 to an 8 3/4 several times. If they fit an 8 3/4, they should fit an 8 1/4. It's been a long while since I did that swap, but I can't remember if you have to drill the 5th hole in the bottom of the backing plates or not? If what @TrailBeast suggested above also gives you a parking brake, that would be my choice.

The parking brake is a drum inside the rotors and is cable operated.

 

[1973dust]

Why not simply use the Grand Cherokee or Dodge Dakota disc brakes on the rear?
They bolt right on the 8.25.
About the only thing different is you will need longer wheel studs because the discs take up a bit more room than the drums.
I put them on the 8.25 in my Dart.

View attachment 1716161324

Thanks, I knew that was one option from the searching I've done. I'm happy, at least for now, with the 10" drums since I've already rebuilt them and they work great.

 

[1973dust]

Yes it will work, I've done it myself for an M-body ('88 5th Avenue). Swapped from stock 7 1/4" rear end to 8 1/4" out of a Jeep XJ, drum brake assemblies transferred over just fine.

Great, exactly what I was hoping to hear. I couldn't believe that someone hadn't already done this before me. This is a really cheap way to get 3.55 gears and the extra axle width will be perfect for my 17" Mustang wheels.

 

[typopup]

Why not simply use the Grand Cherokee or Dodge Dakota disc brakes on the rear?
They bolt right on the 8.25.
About the only thing different is you will need longer wheel studs because the discs take up a bit more room than the drums.
I put them on the 8.25 in my Dart.

View attachment 1716161324

I'm planning on doing this swap over the winter months to my 73 Duster w/ an 8.25" (now if I could only find a hoist that would fit in my igloo). I'm sure somewhere there's an answer to my question in FABO but to make it easier, which years & models of the Grand Cherokees & Dakotas have the proper hardware to do the swap?

I've also read somewhere that you have to enlarge the centre hole in the backing plate to accommodate the larger size of the axle housing??

Thanks in advance!!

 

[TrailBeast]

I'm planning on doing this swap over the winter months to my 73 Duster w/ an 8.25" (now if I could only find a hoist that would fit in my igloo). I'm sure somewhere there's an answer to my question in FABO but to make it easier, which years & models of the Grand Cherokees & Dakotas have the proper hardware to do the swap?

I've also read somewhere that you have to enlarge the centre hole in the backing plate to accommodate the larger size of the axle housing??

Thanks in advance!!

The late 90’s Dakota and Cherokee as far as I know.
My discs, calipers, backing plates came from a 96 Dakota, as did the 3.55 sure grip carrier, ring and pinion gear.
The only mod I had to make was a small casting ring on the back of the rotors the occasionally touched the backing plates.
I used a flap type sanding disc to knock those down.
No holes enlarged or modified.

All this went on and in a 73/74 8.25.
The early 8.25 housing used smaller OD bearings with an ID that still fit the later carrier hubs.
Those bearings are readily available.