C8.25 width differences

[Dzimm]

Ok so I'm looking for a 29 spline C8.25 and want disks.  My options are: 

- 97+ XJ axle and get disk swap parts

- 03+ KJ axle

 

They don't seem to be the same width and I cannot find solid answers on how wide they are.  I've read the KJ is wider and narrower than the XJ and I've seen they are about the same so which is it?

 

XJ axle PROS

- $150 for the axle

- less cutting of brackets

- same width as front

CONS

- have to buy disk conversion stuff (maybe less than $100??)

 

KJ axle PROS

- disk swap already done

- better gear ratio (3.73 vs 3.55)

CONS

- $250 for the axle

- Not sure if width differences would have negative impact on the track

- less aftermarket support it seems

- parts are more expensive

 

I was heavily looking at the KJ axle but at this point I'm leaning more towards doing the XJ axle since it seems simpler.  What's the widths and what's your thoughts?

 

 

Thanks for the responses but I should clarify, I want to keep a Jeep axle so no 8.8.   It's for the 92 so 2wd with small tires (24" ish).  

[coolwind57]

Interested in hearing observations as well.  Slowly working on prepping for a rear axle swap in my near future.  I'd like to hear from someone who has done a KJ axle swap.  

[89 MJ]

Why not do an 8.8?

[derf]

KJ axles have a bunch of bracketry to support their triangulated 3 link with coil springs.  You'd have to cut it all off, which isn't difficult with a torch.

 

Having done a disk brake swap, I have to say that the braking performance is a wash at best.  Sure, swapping pads is a ton easier than doing drums and for that, I'm happy I did the swap.  But how often do you really swap pads on the rear axle anyway?

 

Keep in mind also that if you change the gear ratio in the back, you need to also swap gears in the front to match.  So if you go with a 3.73 KJ axle, you need to drop 3.73's in your front.

[75sv1]

How much width difference? The Ford 8.8 is 1.5 inch narrower. I do not feel any tracking difference. And other ways to improve tracking. Do you lose or gain axle width if you swap on disc brakes?  What size tires are you running? And manual or auto?  I am looking at widening a future 8.8 swap on an XJ. I did get some 20mm spacers and ordered some 2.8" lug studs. The spacers are only through holes, and not the bolt on, if that makes sense. The stud I ordered from NAPA. If interested I'll get the PN.

[Dzimm]

2 hours ago, 89 MJ said:

Why not do an 8.8?

Want to keep it all Jeep.  Also trying to avoid width differences.

[Dzimm]

1 hour ago, derf said:

KJ axles have a bunch of bracketry to support their triangulated 3 link with coil springs.  You'd have to cut it all off, which isn't difficult with a torch.

 

Having done a disk brake swap, I have to say that the braking performance is a wash at best.  Sure, swapping pads is a ton easier than doing drums and for that, I'm happy I did the swap.  But how often do you really swap pads on the rear axle anyway?

 

Keep in mind also that if you change the gear ratio in the back, you need to also swap gears in the front to match.  So if you go with a 3.73 KJ axle, you need to drop 3.73's in your front.

You are saying the disks didn't offer much of a difference in your case?  This will be on the race truck so I can use as much braking performance as possible.  No worries on the front, it's 2wd.

[Dzimm]

7 minutes ago, 75sv1 said:

How much width difference? The Ford 8.8 is 1.5 inch narrower. I do not feel any tracking difference. And other ways to improve tracking. Do you lose or gain axle width if you swap on disc brakes?  What size tires are you running? And manual or auto?  I am looking at widening a future 8.8 swap on an XJ. I did get some 20mm spacers and ordered some 2.8" lug studs. The spacers are only through holes, and not the bolt on, if that makes sense. The stud I ordered from NAPA. If interested I'll get the PN.

I don't think swapping on disks will affect width at all.  Drums and disks are about the same thickness so it shouldn't change anything.  This is for the 92 so 24" ish tires, ax15, 2wd and I would like to avoid spacers if at all possible.  Also trying to stick with a Jeep axle.

[derf]

15 minutes ago, Dzimm said:

You are saying the disks didn't offer much of a difference in your case?  This will be on the race truck so I can use as much braking performance as possible.  No worries on the front, it's 2wd.

Disks will help a lot with fade so in a race truck they would be a good upgrade.

[Dzimm]

Just now, derf said:

Disks will help a lot with fade so in a race truck they would be a good upgrade.

You didn't see a difference though in your case?  I've only ever heard of disk always offering better braking.  Did you swap I've the distribution block as well?

[75sv1]

27 minutes ago, Dzimm said:

I don't think swapping on disks will affect width at all.  Drums and disks are about the same thickness so it shouldn't change anything.  This is for the 92 so 24" ish tires, ax15, 2wd and I would like to avoid spacers if at all possible.  Also trying to stick with a Jeep axle.

I had to do spacers so I could keep the rear sway bar. I did run without the swaybar and had no issues. I did run 2" spacers and also no tracking issues. I have since went with WJ 5on5 on the front to eliminate the spacers I had there. I was running 15 inch Canyons by 8. I do have 5 to 4.5 adapters on the rear. I will try and eliminate them latter. 

[derf]

37 minutes ago, Dzimm said:

You didn't see a difference though in your case?  I've only ever heard of disk always offering better braking.  Did you swap I've the distribution block as well?

Not much of a change.  But really, 80-90% of your braking comes from the front brakes, especially on a pickup truck with a light rear end.  When you're braking going into a corner, almost all of the weight transfers to the front and that's where brake upgrades really help.  Going with bigger rotors and calipers make a huge difference in the front.  In the rear, there's only so much you can do before you lock up the rear tires.

[75sv1]

Would 4.10 ratio do you? There are some 8.25s with that gearing for XJs. Then swap on KJ rear brakes.

[Dzimm]

3 hours ago, 75sv1 said:

Would 4.10 ratio do you? There are some 8.25s with that gearing for XJs. Then swap on KJ rear brakes.

I could get away with 4.10 but cruising would put me a little over 3k rpm at 70mph and I would drive this truck a lot during the summer at highway speeds.  The 3.73 would keep me closer to 2800rpm while still giving me a decent bump in acceleration.  3.55 is a little higher gear than I think I want to run which is another big part of why I was leaning KJ axle but I really need to know the width of it compared to the XJ to know if it'll work for me.

[JMO413]

 

I've been looking at both options myself. In the post above Pete says the liberty axle is a couple inches wider. And well Pete said it so it must be true. Lol

[Dzimm]

7 hours ago, JMO413 said:

 

I've been looking at both options myself. In the post above Pete says the liberty axle is a couple inches wider. And well Pete said it so it must be true. Lol

Thanks, didn't come across this one.  Another one for the "couple inches wider" camp.  At this point I've seen that more than anything else so it's probably safe to assume it is slightly wider but the actual numbers would be nice.  "Couple inches" could be a half inch or could be 5, which really makes a difference.  If we had u-pull yards around here I'd just go measure it myself.  

[75sv1]

The 4:10  C8.25 can be found in 4 cyl XJs. Yeah, 3K RPM would be a bit much. 2,800 RPM would be a bit much for me, too. 

[Pete M]

a 4.10 8.25 can be found under 4cly gas KJs too.  car-part.com can help find them.  I just brought home a pair of 4.10 diffs for my Libby.    

 

the 03+ axle is around 2" wider.  don't remember the exact amount but I can measure my new-to-me 8.25 once I get how this weekend.  Or if you can find the thread I did years back about the then newfound info after I measured my 06. 

 

and there's nothing wrong with 3k rpms.  the engine will handle it just fine.  remember that 2.5/auto XJs came with 4.56 gears.

[Dzimm]

1 hour ago, Pete M said:

a 4.10 8.25 can be found under 4cly gas KJs too.  car-part.com can help find them.  I just brought home a pair of 4.10 diffs for my Libby.    

 

the 03+ axle is around 2" wider.  don't remember the exact amount but I can measure my new-to-me 8.25 once I get how this weekend.  Or if you can find the thread I did years back about the then newfound info after I measured my 06. 

 

and there's nothing wrong with 3k rpms.  the engine will handle it just fine.  remember that 2.5/auto XJs came with 4.56 gears.

I can probably make 2" wider work.  If you could get the measurement that would be awesome!  Closest 4.10 axle for either is over 200 miles away so that's probably not gonna happen.  4.56 would be like 3600+rpm lol.

[Pete M]

1 minute ago, Dzimm said:

  4.56 would be like 3600+rpm lol.

 

yup.  what I'm saying is, don't pick numbers that you think are some sort of "limit".   your Jeep has no problem hitting them allllllll day long.  

[Dzimm]

2 minutes ago, Pete M said:

 

yup.  what I'm saying is, don't pick numbers that you think are some sort of "limit".   your Jeep has no problem hitting them allllllll day long.  

I don't question it can do it but I don't really want to drive it screaming all the time.  I'm sure gas mileage would be very poor as well doing that high of rpm.

[coolwind57]

I think my plan is to stick with 3:55 (same as I have up front) when I purchase my KJ axle.  I'm running 31s with no desire to increase that size.

 

Starting to look like no one has actually done a KJ 8.25 swap.  I'm taking my time, as I have nearly zero down time these days, but I'll surely do a write up or at least show progress as I transition out of my noisy D35. 

[Pete M]

1 hour ago, Dzimm said:

I don't question it can do it but I don't really want to drive it screaming all the time.  I'm sure gas mileage would be very poor as well doing that high of rpm.

 

I had 3.92 gears in my Dakota RT and happily traded any potential issues at highway speeds for the insane grunt at low speeds.  torque is fun   and more is better    

[AZJeff]

On 11/19/2019 at 9:47 AM, Dzimm said:

You didn't see a difference though in your case?  I've only ever heard of disk always offering better braking.  Did you swap I've the distribution block as well?

In a race application, it would be best to add in a manually adjustable proportioning valve.  Remove the factory stuff from inside the factory distr. block, and add in the prop. valve.

 

There is NO real diffference is width between disc and drum setups on the same axle.

 

One question, though:   you say you plan to buy the disc conversion stuff for less than $100.   I assume you mean you will acquire the calipers, mounting brackets, and rotors from a ZJ from a junkyard.   If that is the case, you might want to spring for rebuilt calipers if the junkyard ones have been sitting around a while.  Also, do you plan to incorporate the drum parking brake, so just skip that since it is a race truck?

[Dzimm]

1 minute ago, AZJeff said:

In a race application, it would be best to add in a manually adjustable proportioning valve.  Remove the factory stuff from inside the factory distr. block, and add in the prop. valve.

 

There is NO real diffference is width between disc and drum setups on the same axle.

 

One question, though:   you say you plan to buy the disc conversion stuff for less than $100.   I assume you mean you will acquire the calipers, mounting brackets, and rotors from a ZJ from a junkyard.   If that is the case, you might want to spring for rebuilt calipers if the junkyard ones have been sitting around a while.  Also, do you plan to incorporate the drum parking brake, so just skip that since it is a race truck?

That was more a hypothetical question if the yard would sell the disk swap parts for $100 or less because it would then mean the XJ axle with disks is cheaper than the KJ axle with disks, but id be limited to 3.55 gears due to supply near me.  Regardless which I go with, I'll be buying new pads/rotors ect.  I already bought a ZJ dist block and am gonna swap the spring over.  I thought about using a manual adjustable one but I want to do this in stages so I can more easily troubleshoot if I need to and feel out everything as it changes to make sure I like the direction I'm going.

 

The KJ I'm looking at has been in the yard for 3 years so I plan on at least rebuilding the calipers, I'll order new ones if I need them.  I will be using the factory parking brake setup if I can get it adjusted.  Having trouble with my current setup with all new parts, which is a whole different issue...