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NP231J Transfer Case Upgrade - 231 Beefing

Posted in How To on May 1, 2010
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The New Process (or New Venture Gear-same guys) 231 transfer case has to be one of the most popular factory-equipped T-cases ever used in a 4x4. It's found under a multitude of Jeeps (Wranglers and Cherokees), Dodge, and Chevy 4x4s. It's a chain-driven transfer case with an aluminum housing and a 2.72:1 gear ratio (though these three traits are common with many transfer cases built today). If you find an NP231 under your vehicle, it will likely have a "J" (Jeep), "C" (Chevy), or "D" (Dodge) after the numbers. All of the versions of the 231 T-case can be improved, but some are stronger than others, and some have better qualities than others. Dodge, for example, used an NP231HD with a bigger chain, and sometimes six planetary gears instead of the normal three found in most 231 T-cases.

We have an NP231J case installed under one of our XJ Cherokees that was working just fine, but we nonetheless felt the need to beef it up before we caused any serious issues with it. By building the transfer case up now instead of waiting until something breaks, we can avoid having any broken parts potentially maiming other parts of our transfer case.

1.Since we had the good fortune of starting with a well-working transfer case, we opted not to order an overhaul kit from JB Conversions. We did however want to strengthen everything we could within the housing. We ended up using an HD chain kit (main shaft gear, front output gear, and chain), a six-pinion planetary gear, and a slip yoke eliminator kit (main shaft, tailhousing, necessary snap rings, and 1310 CV yoke).

When it comes to NP231 upgrades, there's one shop that has it covered better than the rest-JB Conversions. The transfer case specialists and originators of the slip yoke eliminator (SYE) have everything you need to give 1-ton strength to your stock 231 transfer case. For our own personal T-case needs and wants, we were after a heavy-duty chain kit, a fixed yoke on the back of our case, and a stronger planetary gearset. We weren't concerned with any 4:1 low range options and instead wanted to keep the 2.72:1 factory low range since this Jeep was going to be an all-around off-road toy and daily driver. Keeping a 2.72:1 gear ratio allows us to still use low range in the sand to climb dunes with, while 4:1 would have been too low for higher-speed sand use.

PhotosView Slideshow
PhotosView Slideshow

Since we now have a fixed rear yoke on our T-case, we can still drive around in front wheel drive until J.E. Reel is finished with our new CV driveshaft.

Price Breakdown
If you're replacing all these parts in your NP231J transfer case like we just did you might want to think about purchasing an entire ready-to-go transfer case. Our JB Conversions upgrades totaled out to $775, while a new JB Conversions 231J T-case costs $975. If our transfer case was worn out with a chunked out housing, and we had needed the $150 overhaul kit, $175 front housing half and $100 rear housing half, then there would have been no question that buying the complete JB Conversions 231J would have been a better (and less laborious) deal.

SYE kit $200
Wide chain kit $325
Six-pinion planetary gear $250
Total 231J build price $775
JB Conversions complete 231J with all the above parts plus new everything else $975


J.E. Reel Driveline
Pomona, CA 91766
Xtreme Unlimited
Oceano, CA 93445
JB Conversions
Sulphur, LA 70664

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