NP231 Tech Thread

[Krod]

Since so many of us are using the NP-231 transfer case in our TJ's/YJ's/XJ's/ZJ's etc.... I figured I'd start a thread with some tech on this case.

Its mainly a starting ground for ideas for my own build, but feel free to add your own writeup or tip to the list, I'll edit this post to add the topics as they get posted.



TOPICS COVERED SO FAR:
1. Widechain conversion
2. Adding more planetary gears for strength
3. Doubling your 231 with another 231
4. Swapping a 231 or 231HD into a ZJ
5. Typical failures/weaknesses

[Krod]

The NP231 is easy to widechain from the junkyard.
Find a 231C (Chevy’s 231) and strip the front yoke, front output shaft, chain, and sprocket on the mainshaft. Heres the Chevy chain (right) next to a jeep chain






Just replace the jeep main shaft sprocket with the chevy piece, replace the jeep front input shaft with the chevy shaft, and the chain will fall in.







Heres a chevy mainshaft right next to a stock jeep mainshaft




And a chevy mainshaft next to my AA SYE mainshaft

[OverkillZJ]

231hd uses the chevy sized parts, doesn't it? I know it has more planeteries too, or something like that... It's been awhile since I've farted around with 231's though.

[Krod]

231HD is 6 planetary
231J/C is 3


The chain in a regular NP231 is an inch wide. The chain in the NP231HD is 1.25" wide. Pretty sure the 231HD's are really the 241.

Stock 231 on the left, Stock 241 on the right:



Good article on the widechain and 6 gear conversion, using 241 parts.
http://www.madxj.com/MADXJ/technical...rade.htm#parts

[jeepxj3]

Good tech Matt! You planning on doing this for your next build?

[Krod]

Good tech Matt! You planning on doing this for your next build?

Why the hell not.


Still have a trans for me? Come summer, i've gotta get moving on this stuff.

[jeepxj3]

Yup, I got a spare 231 case too, so when I blow up the one I got ill think about doing this... But I think I would rather build and flip a 300, or go Stak.

[psychobilly]

I built my first 231 doubler about a decade ago now. I strongly believe the 231c is the best wheelin t-case to ever come from the factory. Its tough, light, well geared and anyone with wheelbase can double it for dirt cheap. Any 231 output shaft is the same diameter and spline as the chevy/gmc 231 input. Same as a dana 30 axle shaft too my the way which helps make it real simple to double.

231-231 requires some aluminum welding but the rest any hack mechanic can handle







Other combos will require shaft fab. I built one to double into a nissan using a slip yoke and a chunk of tranny tailshaft (could also use a busted dana 30 shaft).

[DMG]

That is some very cool tech right there but there is no way a chain driven case with a slip yoke and a weak case is the best.

I built my first 231 doubler about a decade ago now. I strongly believe the 231c is the best wheelin t-case to ever come from the factory. Its tough, light, well geared and anyone with wheelbase can double it for dirt cheap. Any 231 output shaft is the same diameter and spline as the chevy/gmc 231 input. Same as a dana 30 axle shaft too my the way which helps make it real simple to double.

231-231 requires some aluminum welding but the rest any hack mechanic can handle







Other combos will require shaft fab. I built one to double into a nissan using a slip yoke and a chunk of tranny tailshaft (could also use a busted dana 30 shaft).

[BigZees]

Bear in mind that some newer dodge pickups also use the venerable 231. I got one for my YJ that was a direct bolt-in from a 3/4 ton dodge pickup. already had wide chain and heavier planetaries. Just a little fyi.

[psychobilly]

there is no way a chain driven case with a slip yoke and a weak case is the best.

Maybe your right, as every application has different needs and its not ideal for everything. But what makes a good wheelin case? I say when the dust settles the lighter, stronger, cheaper, better geared case will be the victor. No case is ideal in stock form, all need gearing options to really perform.

So now compare the 231 to the dana 300. The 231 is stronger, lighter, better geared and dirt cheap to double. As shown above any hack mechanic could do most of it in his basement. Aluminum welding is the only area that may require outsourcing.

Chain drive is stronger than gear and the aluminum case by design is stronger too. Theres a lot of like 1970's redneck idealogy in wheelin. You know, from a time when a real wheeler had to be a heavy f350 with a 180 hp big block, open headers and cast iron has to be stronger than aluminum. But in the real world use the gear drive dana 300 while in 4wd being gear to gear will try to spread apart. They are well known for exploading under torque. The chain drive 231 will pull together while in 4wd and can only be broken by bashing it off a rock (skid plate?) or bottoming a driveshaft. The 231 case will never bust under torque, never ever. Compression vs tension. Compression will win. And you will never find a broken 231c chain. It is simply way..way stronger than any u-joint you would/could/should ever put in front of it.

So I consider the 231c the best case to ever come from the factory. And its not that I'm too biased seeing how I just built myself a dana 300 doubler for the vitara. I needed passanger output so a 231 wasnt an option. The 300 doubler is heavy, expensive, I think I can break it and even though I bought the kit to do it it was more difficult to do than any of the 231 doublers I've ever built from scratch. Still bad ass though.

[OverkillZJ]

Chain drive is stronger than gear and the aluminum case by design is stronger too.

[psychobilly]

Yep. An aluminum chain drive case can bust if it is bashed off a rock but simply wont (or shouldnt) fail under torque. Thats part of the reason a np231 has a torque rqting of around 2000 ft lbs while the grand daddy 205 has a rating of about 1100 ft lbs. Both strong but the 231 is just pretty much unbreakable under torque. Any I've ever seen fail have been as a result of a rock, bottomed drive shaft or a busted poorly designed doubler crossmember (oops).

[OverkillZJ]

No rock involved, it was on double whammy in Moab (two ledges, hard to climb as both tires hit both ledges at the same time) - but he didn't hit it off of anything, this is one of two times I've seen them crack in half under torque.

Edit: I'm still not sure how exactly this happened to be honest, you'd expect the chain to break before the outpust shaft!

[SirFuego]

For what it's worth, I swapped a 231 into my 93ZJ 5.2L. The transmission required the long input shaft.

My donor vehicle for the 231 was also a 93ZJ, but it had the 4.0L. It came with a short input shaft, so they needed to be swapped.

There is a tab that connects the linkage to the housing and the tab sizes are different (at least between the 249 and 231). I simply reused the tab from the 249 to keep the same linkage geometry. I'd imagine you would want to do something similar for other applications.

Swapping in a 231 into a ZJ also nets you some extra ground clearance allowing you go with a higher clearance tcase skid.

[psychobilly]

Edit: I'm still not sure how exactly this happened to be honest, you'd expect the chain to break before the outpust shaft!

The chain would be stronger than the output. Those chains are unreal strong. Wear is the only way they will break. That failure above would be hard to explain. Was it a SYE? If so there in may be the weakness (possible stress riser or?). The factory torque output ratings of the 231 exceed a rockwell axle so there is an explaination for that failure but I dont know it. Its not that the case is weak though.


How strong is a 231?
NVG rates the 231hd at 1940 ft lbs Note that the 242 is only 1400-ish
http://www.newventuregear.com/tcases.html

1940 ft lbs going into a rockwell axle will create 13036 ft lbs at the axle shaft.

Whats that mean?

"The 2 1/2 ton U joint style, max. static load is 3,600 foot lbs. of torque. The convel model "E" used in the 2 1/2 ton is 4,310 foot lbs. of torque. The rezepa is estimated 10% to 15% weaker than the U joint style. Just for grins the dana 60 - 1480"---steve differential eng.

Differential engineering build some of the baddest axles out there and that guy knows his stuff. I trust his numbers. So as mentioned, there is a reason for that failure (joint bind creating stress, aftermarket shaft, who knows????) but a weak case wasnt the reason. I'd bargain just about any other case could of failed under the same circumstances, if not sooner.

[andrew2516]

Matt this seems like a good easy upgrade...

What are your plans for the jeep? Full widths? bigger tires?

I hear a lot of talk about it but never hear exactly what you plan on.

[Krod]

^^ More than likely due to that output shaft breaking. That allowed the back-torque from the driveshaft to flog the output housing around, shearing it off from the rest of the case.

I'll agree with billy on this one...the only 231 case failures that I've ever seen have been due to factors OUTSIDE of the case itself (too short driveshaft + bent leaf spring = TC failure). The failure in Matt's pic above was probably due to the Ujoints NOT failing, and all that torque from bouncing on rocks coming back to snap the output.




There is a reason why the ATLAS, STAK etc... are THICK aluminum. They have to be to survive the loads placed on the housing by the gearsets. Same reason the 231 etc... can be of such a "lighter" material....no gears!

And I've ALWAYS been of the mentality that stouter is better, but there are some cases where engineering wins and shear beef is just unnecessary weight.

Matt this seems like a good easy upgrade...
What are your plans for the jeep? Full widths? bigger tires?
I hear a lot of talk about it but never hear exactly what you plan on.

Thats because I'm still planning...lol
I don't like to do things twice, so rather than plan as I build, design is getting done on computers and paper first. Nothing exciting to post about, so I'm not yet. I'll update my thread when it comes time for real stuff.

[psychobilly]

Transfer case chains are "leaf" type chains. leaf chains are very strong. A 1" leaf chain used for fork lifts has a min tensinle strength of 18,000 lbs. Them chains are tough. I wanted at one time to build a portal axle using t-case gears and a short leaf chain. I searched and found suppliers that would make any length chain but I lost interest in the project.

Output shafts of the 231c are of very good quality material too. I tried to cut one on my bandsaw once. It just ate the teeth right off and didnt even scratch the surface of the shaft. I've cut axle shafts with it no problemo.

[OverkillZJ]

I understand the Aluminum argument, I just haven't seen folks have the greatest of luck with 231's. I ran one hard for years (JB conversions 231HD / SYE and Tera 2low in it) and it was great, but I've seen two break in person, one of which it snapped completely in half, but again I didn't exactly do an analysis of why, could've been too long of a front shaft hitting into it or what not. The chain thing is what I don't get, seems there's a lot of chains stretching or breaking, but overall I wouldn't argue using one, they're great cases, but I can't call them indestructible

[Krod]

nothing is indestructible.....See my sig.


I think the failures due to "chain stretch" have been on higher mileage cases (lets face it...most of the 231's that are being wheeled hard enough to break the chain are not spring chickens). That is a maintenance issue...not a weakness in the TC design.

The whole "chain stretching" argument is pretty divided too. I'd prefer not to take us there LOL. Lets stick to tech!!!




Nice work so far guys! Who else has some 231 info !?

[gonecheenin]

nothing is indestructible.....See my sig.

And that is the truth right there, no doubt

Nice work so far guys! Who else has some 231 info !?

Whelp, you can put a 231 behind a SM465 chevy with nothing more then a drill bit & the 32 spline input from a NP241

The low range shift collar came in various designs, some with wider teeth then others

some very early 231's used a steel low range shift fork which is easily bent

The planetary gear cut differences are very easy to see once you have the gears in front of you (blunt teeth or sharp teeth)

Dodge truck 231D's came with factory wide chains, larger sprockets, and SYE'd (unverified)
And those parts will all swap into a Jeep t-case (ask me how I know all about breaking a 231 case due to a driveshaft being collapsed from a bent leaf spring )

[OverkillZJ]

I forgot about your YJ destruction incident...

[DMG]

Bear in mind that some newer dodge pickups also use the venerable 231. I got one for my YJ that was a direct bolt-in from a 3/4 ton dodge pickup. already had wide chain and heavier planetaries. Just a little fyi.

A 3/4 ton dodge would have a 241, not a 231. There was a hd version of the 231 in some 1/2 ton dodges. I replaced a few.

Chain drive is *not* stronger than gear. The 231/241 series cases have high torque ratings but do not take shock loads or being bashed on rocks.

[mudforblood]

I think (opinion) that a chain drive itself is also stronger but the aluminum case is the weak point. A chain drive cast iron, steel or whatever it is cased transfer case would be sweet. Does anyone know if one exists.

Anywayyyyysss...

Back on topic with NP231 tech, kinda.

I had an auto in my 95 ZJ and swapped in an AX15 from the a junkyard XJ. Not knowing there was a difference in transmission output shafts and 231 inputs, I just took whatever AX15 I could find.

Of course it was different so I just rigged up a drill to the clutch disc and put in 5th. Then I "machined" the bearing spot on the output shaft. I figured that was stronger that just cutting it off.

Old auto output:


New "machined" AX output video: