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Mountain Dude

Np231J T-Case Upgrade

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The New Process 231 transfer case installed in our Jeeps is a simple, well designed unit. Unforunately, Jeep engineers decided to steal a little of the unit's available strength, when they put it into our trucks. The NP231 is also used in Dodge half ton trucks (231HD) and Chevy / GMC S10 and S15 pickups Blazers and Jimmys (231C). What these other two types of 231's have in common are: 6 pinion planetary reduction and a front drive chain that is 1.5 inches wide instead of 1 inch wide. The good thing is that the internal parts are interchangeable!

 

Since S10s and S15s are a dime a dozen, finding an NP231C from a first generation Chevy is fairly cheap and easy. What would have been even better would be if Chevy decided to use the same 6 bolt mounting flange on the front, instead of the 5 bolt one they have. So you can't do a direct swap. The one I got a hold of was a 1989 S-10 pickup (4wd obviously) that I bought for $200 to get the 4.3 out of, so the t-case was a free bonus.

 

I won't bore you all with the details of the teardown or re-assembly, there are lots of write-ups on the internet already.

 

Here is the original Jeep reduction unit - only three planetary gears

 

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Notice the spaces available for the other three gears.

 

Here is the Chevy unit on the right with 6 gears.

 

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You will have to swap in your input gear from your original T-case, it is not likely that the Chevy one will be the same. Be cautious here. According to Novak Adapters knowledge database, there is a possibility that there are two shapes of gear teeth, make sure they are the same or your upgraded t-case won't last. I don't think it will be a problem since I used a 1989 Chevy t-case to upgrade my 2005 TJ one. 2x the pinions = 2x the strength!

 

Here are the two chains compared side by side:

 

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I was concerned that the older chain would be more stretched than my newer one, but it was not. The Chevy t-case is not driven at road speed all the time like the TJ one due to the front axle disconnect in the Chevy, so the wear and tear is less even though it is older.

 

Here's the old one - 1 inch wide:

 

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And the new one - 1.5 inches wide:

 

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Note that the extra half inch means it is 50 % stronger! When you swap the chains, you also have to swap the drive and driven sprockets from the Chevy unit. The driven one just slides in. For the drive one, you will probably have to drive out the two needle bearing sets from inside the sprocket. I say probably because I am not sure for a stock NP231J since I am running a Terraflex Slip Yoke eliminator kit in mine. I don't remember doing anything with the stock drive sprocket when I installed the SYE kit, but I'm over 50 now so my memory may be faulty ;)

 

As a bonus, the upgraded t-case actually makes less noise and creates less heat than the old one - at least in my Jeep anyway! I have been running it for several years now with zero problems.

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brubakes - not sure about the 242, I have never had one apart. When I did my swap, I spent a lot of time on the internet looking at websites such as Novak Adapters and OMIX - ADA checking on part numbers etc. It may very well be that the back half of a 242 and 231 are the same and the chain part of the swap might be able to be done.

 

Dirteatr717 - if you are talking about something like the Tera-low conversion from Teraflex, then the chain mod can be done on it since it is only the front half that changes for the 4:1 kit. If you are talking about the NP241OR swap, I think it already comes this way.

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i have this done on my 231 and it is a cheap conversion. a few things i ran into and not sure i you have, is the return spring on the rear of the mode fork that presses against the back of the case. because the 241 mode fork is bigger, the spring would compress and bind up and not let me shift into N or 4L. i had to use a spring with a larger I.D. so it slid over the casted part of the fork so when the spring compressed, it did so over the fork end

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