NP231 Transfer Case Upgrade to NP231HD w/JB SYE
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By: Berkeley Johnston - 12/2003

NP231 Transfer Case Upgrade to NP231HD w/JB SYE

My new NP231HD (aka: NP231DHD) is a zero-mile unit originally manufactured for full-size Dodge trucks. It came from JB Conversions who bought the cases directly from New Venture Gear. JB then added their short shaft kit (SYE) and converted the shift sector from Dodge to Jeep. The transfer case's Heavy Duty designation is because of the large syncro hub and wider chain, 1.5" compared to the Jeep 1" width. In addition, I had JB Conversions switch from the stock 3-gear input pinion to the stronger 6-gear pinion. This new NP231 transfer case is the strongest it will ever be.



Transfer Case Comparison

Below are four comparison images between the old NP231J (Jeep) and the new NP231DHD (Dodge Heavy Duty). The most important visual differences are the switch from slip yoke tail cone to the new JB SYE and, less obviously, the relocation of the vacuum plug (notice the blue cap on the new t-case). Other differences are the drain/fill plugs... the new t-case uses a hex key design, and my removal of the machining lugs above the front output yoke... the material was interfering with the floor pan. Both cases use the 2.72:1 ratio for the low range.

Original transfer case, NP231J. The vacuum plug is by the speedo gear opening (blue paper rag).
NP231J rear view with stock skip yoke tail cone. Note the vacuum plug location. NP231J top view with stock skip yoke tail cone. The NP231DHD is partially visible. NP231J front view. NP231J bottom view with stock skip yoke tail cone.
NP231DHD rear view with JB Conversions SYE. NP231DHD rear view with JB Conversions SYE. The NP231J is partially visible. NP231DHD front view. Note the vacuum plug location. NP231DHD bottom view with JB Conversions SYE.
New transfer case, NP231DHD. Notice the new vacuum location (blue plastic cap).

About the JB Conversions SYE

The short shaft kit, or Heavy Duty Slip Yoke Eliminator as JB Conversions calls it, gets rid of a number of problems with the standard Jeep NP231 transfer case output. The JB SYE eliminates the leaky tail cone, it replaces the cone with a standard yoke, it shortens the driveline (longer driveshaft), and it replaces the weak output shaft with the strongest shaft available.

The factory XJ slip yoke (left) has a much shallower yoke
    compared to the newer Wrangler style (right). The factory XJ slip yoke (left) compared to the newer Wrangler style (right).
The factory XJ slip yoke (left) has a much shallower yoke compared to the newer Wrangler style (right).

The rear oil seal on the tail cone is prone to leaking because the design incorporates the slip portion of the driveline. Normally, the seal rides on one portion of the driveshaft yoke, but when the Jeep is loaded with gear and taken off-roading, the driveshaft cycles back and forth across the oil seal further than normal. The little-used portion of the driveshaft slip yoke, corroded over the years, can chew up the seal, causing the transfer case to leak. The fixed-shaft design of the SYE maximizes seal life.

Worse than a leaking seal is the very real possibility of a broken rear driveshaft. Driveshaft u-joints are a common trail failure. With no rear-wheel drive, a four wheeler often likes to drive in front-wheel drive for a short (or a long) distance to a place where the problem can be fixed. With the standard slip yoke design (and no rear drive shaft in place) the lubricating oil pours directly out of the transfer case! A broken rear driveshaft and a burned-up transfer case? Not good and no thanks.

With the JB SYE, the slip yoke is replaced with a standard Dana Spicer end yoke. The design of the replacement tail cone along with the conversion to the standard yoke shortens the driveline. A shorter driveline means a decrease in driveshaft u-joint angles... a good thing. Plus, the standard yoke allows easy conversion to a double cardan (sometimes called CV) driveshaft. The double cardan design further decreases the driveshaft u-joint angles. The double benefit of the shorter tail cone and standard yoke allows the jeeper to install higher lifts with fewer problems.

Perhaps the greatest benefit of the SYE is the switch from the known-to-fail stock output shaft to a much stronger design. The JB Conversions output shaft is U.S. manufactured to original New Venture Gear design drawings (including material specs, hardness, heat treating, etc.). Among other features, the JB shaft is 54% stronger in torque capacity than the original design.

Klune-V 4:1 underdrive attached to the NP231DHD transfer case. Klune-V 4:1 underdrive attached to the NP231DHD transfer case.
Here are a couple of shots with the Klune-V 4:1 underdrive attached to the NP231HD/NP231DHD transfer case. Among other benefits, the Klune-V gives you awesome clocking capabilities.

Installation

Swapping the transfer case is a fairly easy operation, as long as you've got the right tools. My installation was greatly complicated because I added a Klune-V underdrive at the same time. (More on the Klune-V next time.) I disconnected the two driveshafts. You can reuse the front because the dimensions don't change. I had to replace mine because of the added length of the underdrive. In either case, the rear driveshaft gets replaced with a double cardan style shaft. (More on the new driveshafts here.)

I used my floor jack and some wooden blocks to support the driveline and removed the crossmember. I dreaded removing the t-case. Some of the bolts are really hidden, but doable with some socket extensions and u-joints. The most difficult part of the operation was physically lifting the cases up and down. For me, that was with the underdrive attached... what a pain. It took a long time to get the floor pan reshaped correctly. I must have lifted that assembly up and down fifty times.

The new t-case's vacuum plug is located further forward compared to the old unit. That worked perfectly for me because of the added length of the underdrive. I was worried about the speedo cable, however. It moved eight inches to the rear. Fortunately, there was enough slack in the cable to reach.

Without the Klune-V underdrive, the exchange would have been pretty easy. Once everything was bolted back in place, I just rehooked the t-case vent, vacuum lines, and speedo cable. The new transfer case functions the same as the old one, but the SYE output and heavy-duty guts give great peace of mind.

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