Well, this whole crazy thing started when I started researching the similarities between the Jeep AW4 and the Mitsubishi V4AW3 transmissions. I kept my eyes open and found an NP231 from a 2001 Wrangler with a Slip Yoke Eliminator (SYE) already installed. I also found an AW4 tailhousing from an unknown year Cherokee.
My first steps were back in December when I tried mounting the AW4 tailhousing and np231 to the V4AW3. The tailhousing bolted up just fine and the splines ended up being the same size and count. This initial "test fit" did let me know that if I wanted this to work I would need a "short" input shaft for the NP231. Over the years NP231's have come with different input shafts varying in spline count and lenth. There are three lengths and two spline counts: 21 and 23 splines. The V4AW3 is a 23 spline tailshaft. I also used this opportunity to check the clearances with the floor, exhaust, crossmembers, and t-bars. I felt confident that the NP231 was a good fit and at the most would need minor clocking. After this wrenching session, I reinstalled the Active-trac t-case so I could continue driving the truck while I shopped around for more parts. I would recommend messing with the Active-trac t-case as little as possible! It is a heavy beast to bench press into place.
Since I first started dreaming of this mod, I knew that I wanted to make a dual transfer case setup. I felt that the dual case provided the following benefits:
1)Allowed for selectable ranges, ie:1:1, 2.72:1, & 7.4:1. With these options, even the middle speed is lower than the stock 1.90:1!
2)It was something I could piece together as time and money permitted.
I had three options to choose from in trying to create a dual NP231:
1) Build the components myself. I didn't like this option simply because I knew it meant welding a mid-shaft together. I wasn't crazy out a welded shaft being in the heart of my drivetrain.
2) A Mad Rooster Offroad crawlerbox
3) Or a D.D.Machine "Box4rocks"
I decided on the D.D.Machine kit. I chose this kit because the guy that makes them seemed to really stand behind his product and know what he was doing. Also this kit had provisions for oil filling and draining. I was also less than thrilled with how the Mad Rooster guy handled himself when answering an email of mine.
I was lucky enough to find a Box4rocks 23 spline kit second-hand that had not yet been installed and came with all the parts that would be needed from a donor NP231 to make the doubler. I got even luckier when I found out that this doubler donor was a 23 spline short input shaft!
Here are some pics of the Box4rocks and doubler parts:
The doubler cut, welded, and guts assembled (you can see the midshaft poking out in the middle):
The plate that seals the rear of the doubler:
The clocking ring:
Once I had the doubler parts I wanted to start test fitting things to make sure everything would fit before I got too far into the project. I began by temporarily assembling the two cases. I soon found out that the assembly was almost too long and there was some interferance with the crossmember. Not only did I want the assembly to clear everything, but I knew that I would need some additional room for the drivetrain to "float" and enough clearance to reach the drain and fill plugs.
To clear the crossmember I did two things. The first was to remove this extra tab on the rear case housing:
It ended up looking like this:
Removing the tab helped the assembly to clear the crossmember, but it still didn't give me enough room to access the drain plug. My solution to this was to shorten the entire assembly 1/2" by removing the clocking ring. The clocking ring is a nice feature that is included in the kit, but unfortunately I could not afford the extra length that it caused. I experimented with various clocking positions and finally found one I was happy with. I settled on the final position because it was nicely tucked in above the t-bar without hitting the bottom of the floor.
To eliminate the clocking ring, I marked it's location and used the ring as a template to drill through the rear plate of the doubler case. Once these holes were drilled, I countersunk them so that bolt heads would clear the internals of the doubler case. The result looked like this:
I bolted the assembly together only to find out that once the clocking ring was removed that the rear case would no longer clear the bolt head that holds the rear part of the doubler shifter rail from floating around. I solved this problem by countersinking that bolt head as well:
Once that was completed, I assembled them again for another test fit and was happy with the results.