Bench Build NP231J

[Cameron23]

I have another thread on here trying to identify the cause of a noise in my 99 TJ. I isolated the noise to the Tcase, and only when the output shaft connected to the front driveshaft is spinning (with or without the driveshaft).

I’m still new to Jeeps and the way my brain works I have to understand how everything goes together. Best way to do that for me is to take something apart and rebuild. So I decided rather than pulling my Tcase and potentially messing something up, I would find a 231J in the junk yard and rebuild that, to swap in.

Just pulled this NP231J out of an older model (square headlights). It was mated to a 4 cyl and AX-5 so I assume I’ll need to potentially swap over some components from my transfer case with mine being a 4.0 and AX15. Gonna need to research.

For right now I’m interested in the best quality most complete rebuild kit for an NP231J, and any advice from people who have done it.

I’ll document the process in this thread as I go.

 

[macleanflood]

A SYE kit will convert that case to the right spline count.

Here's my thread...

Thread 'Chasing 231J leaks'

Jun 27, 2025

On my way home from Moab my 231J started hemmoraging fluid.

I've since replaced the front output seal and the shifter o-ring and retention ring.

It's still leaking.

Thinking it might be this...which I believe is a shift detent spring and ball.

Anyone have any idea what sized o-ring? I have an assortment...might have something.

Thoughts on this being the source? I think I need to check my vent too...might be over pressurizing.

-Mac

• macleanflood
• Replies: 150
• Forum: TJ General Discussion

• 

-Mac

 

[Cameron23]

A SYE kit will convert that case to the right spline count.

Here's my thread...

Thread 'Chasing 231J leaks'

Jun 27, 2025

On my way home from Moab my 231J started hemmoraging fluid.

I've since replaced the front output seal and the shifter o-ring and retention ring.

It's still leaking.

Thinking it might be this...which I believe is a shift detent spring and ball.

Anyone have any idea what sized o-ring? I have an assortment...might have something.

Thoughts on this being the source? I think I need to check my vent too...might be over pressurizing.

-Mac

• macleanflood
• Replies: 150
• Forum: TJ General Discussion

• 

-Mac

Wasn’t planning on an SYE, since I’m back to stock ride height. Full transparency, wasn’t aware that the spline count would be different. Is there benefit for an SYE on stock ride height? Is there another option to get correct spline count?

 

[macleanflood]

Other option is fully dismantle your existing case and harvest the main shaft...

And if you're doing that well there's no point in touching the one you bought...might as well just pull yours and see what's wrong with it.

No need for a SYE in your application...but...the SYE kit replaced the main shaft and would enable you to convert the 231J you bought.

Alternatively there are plenty of stock 231J shafts around. I've got at least two. Put a shout out to your local Jeep group on FB and you'll probably find a few.

-Mac

 

[Cameron23]

Other option is fully dismantle your existing case and harvest the main shaft...

And if you're doing that well there's no point in touching the one you bought...might as well just pull yours and see what's wrong with it.

No need for a SYE in your application...but...the SYE kit replaced the main shaft and would enable you to convert the 231J you bought.

Alternatively there are plenty of stock 231J shafts around. I've got at least two. Put a shout out to your local Jeep group on FB and you'll probably find a few.

-Mac

Sorry, I’m new to working on Jeeps and transfer cases. What’s the reasoning behind why the splines wouldn’t be the same? Both are NP231J, with the same ratio.

 

[Cameron23]

Sorry, I’m new to working on Jeeps and transfer cases. What’s the reasoning behind why the splines wouldn’t be the same? Both are NP231J, with the same ratio.

Nevermind I figured it out. So because I pulled it from a 4cyl and I have a 6cyl, I need a new input shaft that has 23 splines instead of 21?

 

[macleanflood]

I had a 231J from a 4 cylinder and sold it and bought two more 23 spline ones for what I sold it for.

Regional markets varry. And time of year.

-Mac

 

[Cameron23]

I had a 231J from a 4 cylinder and sold it and bought two more 23 spline ones for what I sold it for.

Regional markets varry. And time of year.

-Mac

This is probably the route I’ll go. The search continues for a 23 Spline 231J. When I find one and build it…. Or if I give up and yank mine (still runs fine in 2WD with no front DS) the rebuild will go in this thread.

 

[Cameron23]

Alright everyone I’ve decided to bite the bullet and just pull mine out and rebuild it and not f*** it up. I’m doing the NOVAK rebuild kit as it seemed to be the most complete.

Gonna be replacing the rear axle U joint as well because when I pulled it the caps on either side were nearly falling off from gravity.

I’ve almost torn it all the way down following some YouTube guides but I haven’t located anything that I feel would be responsible for that noise that led to all this.

Some pictures from the tear down so far

 

[Cameron23]

As far as the rear drive shaft u joint this is what I ordered but Amazon has got me sweating that I ordered the wrong thing.

Can someone smarter than me confirm that this is fine for the rear axle U joint?

 

[hear]

I believe some of the YJ input shafts are different lengths as well.

 

[Cameron23]

As far as the rear drive shaft u joint this is what I ordered but Amazon has got me sweating that I ordered the wrong thing.

Can someone smarter than me confirm that this is fine for the rear axle U joint?View attachment 706615View attachment 706616

Unrelated to the above question but I didn’t want it to get lost in the thread, but when I disassembled I was able to get everything out without removing the shift selector. Am I good to leave it alone or will it interfere with reassembly?

 

[macleanflood]

Shouldn't cause any issues but it does have a seal.

-Mac

 

[GregBelleville]

As far as the rear drive shaft u joint this is what I ordered but Amazon has got me sweating that I ordered the wrong thing.

Can someone smarter than me confirm that this is fine for the rear axle U joint?View attachment 706615View attachment 706616

I believe that u joint is for the front axle not the driveshaft at the rear axle which should be the Spicer 5-1310-x for the stronger non greasable version.

 

[Cameron23]

I’m sure you experienced guys saw this question coming. What’s the best way to get this bastard out?

I have a slide hammer with one set of jaws. I tried pointing them out and tightening the best I can but it comes out with enough force. The teeth are too fat to get under the bearing.

I have the big 4 auto parts stores near me. And a HF down the street. I just want to make sure I get a tool that’s going to get it done without damaging the case.

 

[macleanflood]

Slide hammer.

Or if you have a welder fat tacks around the perimeter will shrink it.

You probably can heat the case up enough to get it to move too.

-Mac

 

[hear]

Is that bearing part of the rebuild kit? I would make sure you have a replacement before you gnarl that one up in removal.

 

[Cameron23]

Is that bearing part of the rebuild kit? I would make sure you have a replacement before you gnarl that one up in removal.

Yes, it is included in the NOVAK rebuild kit.

 

[lumberjacklloyd]

You need a pretty good slide hammer kit to get that puppy out. Or the bread trick if you can find a scrap piece of metal the same size. Definitely replace it if you have started pulling on it and scarred the rollers.

I have the SKF kit and it worked well.
https://www.skf.com/us/products/mai...-dismounting/bearing-pullers/internal-pullers

 

[Cameron23]

Update.

I used the OMT Blind Hole Collet Bearing Race and Seal Puller Kit to get it to move up slightly then I used my OEM slide hammer (much longer slide for more power) and two jaws, plug some heat to get this bastard out.