Initially, there was only going to be one transfer case in my Jeep. The 2wd NP435 has a fairly long 28-spline output shaft that mates perfectly with the input coupler shaft of the EB D20 transfer case, so plan A was just to mate them together using a custom adapter, likely made by welding most of the NP435 tailhousing to most of the factory D20 adapter. Trouble was, the length of the shaft dictated that the adapter would be about 7" long! I thought about all that space being taken up by an adapter in my short-wheelbase Jeep and decided that there had to be something better that could be done with all that room. So in stepped the notion of putting another gearbox in there, and it wasn't long before the NP203 gear reduction box was suggested as the perfect candidate!
The New Process 203 transfer case has to be one of the heaviest, beefiest cases ever used in a commercial vehicle. It consists of three separate parts: the gear reduction section, the chain drive section, and the differential section. The cool thing is that the gear reduction box of this otherwise useless case can be removed and used as a self-contained switchable gearbox in projects like this one. Make planters and paperweights out of the rest of the case.
This chunk of the NP203 has been used in dual t-case projects for a long time, though usually with the NP205. Here's a link to Gord Pritchard's excellent dual-case writeup using the NP205, for instance.
Anyway, the reduction box is ideal for this sort of thing. It's got bearings at the input and output, a shifter shaft on the right hand side, a PTO port on the left side, and in some cases it's got its own drain plug. The "output" shaft, which is actually the intermediate shaft in the NP203, is nice and beefy, perfect for cutting and resplining for whatever you bolt to the other side of it. All you need to add is a fill plug, a vent fitting, and adapters!
The NP203 reduction box is almost exactly 6" long, front to back. Add a 2.25" thick adapter in front, and a 2.5" thick adapter behind, and you end up with a bolt-in package that's just under 11" in length. That's less than 3" longer than the Plan A powertrain - not too shabby, considering my crawl ratio just went from 67:1 up to 135:1! The trade-off, of course, is that Plan B added no small amount to the project cost and complexity, but hey - you only live once, right?
My NP203 came from a long-deceased Dodge truck. The Dodge version has a 27-spline female input gear and (in my case at least) the drain plug boss in the bottom was already drilled and fitted with a plug. There are Ford (28-spline I think) and GM (10-spline) versions of the NP203. The GM version is rare, and not the best choice for mating to the NP435; fine splined setups like the Ford and Dodge are generally accepted as being superior, since they don't wear as quickly. The GM case is best used with an SM465 tranny (and a Chev 350 engine), since it'll bolt up with a factory adapter.
Here's a photo of my NP203 reduction box, and the backside of the tranny-tcase adapter:
Again, I made a point of replacing the two big roller bearings in this gearbox, and the over 70 loose roller bearings on the countershaft as well. I made the gaskets on both sides from some 1/16" sheet gasket material.
As you can see, the adapter has a single large step in the bore hole that houses the NP203's input bearing retainer. This step was sized so that the retainer lightly press-fits into it, and this ensures accurate alignment of the input gear to the NP435's output shaft. Its depth isn't important, since the NP203's retainer is bolted on. Note that some of the snout of the NP203's input gear had to be cut off. This was because the two cases didn't fit as close together as I needed them to, otherwise. Thanks to the large radius of Moser's spline cutter, the splines in the NP435 shaft bottomed out too soon, so cutting the input gear was the only other solution. I still have about 1.5" of spline engagement, which ought to be plenty.
The only other important features of this face of the adapter are the recesses for the four bolts that go into the transmission. The bolt heads have to be sunk that far in to avoid interfering with the 203's bearing retainer. Then there's the five holes for the bolts that go into the 203. I added one hole to the 203 - the one just below and to the right of the input shaft, in the picture. This let me avoid using the three holes below there. The walls of the 203 case are plenty thick enough there to allow this kind of on-the-fly modification. I ended up plugging the lowest hole in the case, and using the other two to hold on a retainer plate that covers the notch in the end of the countershaft, holding it in place.
Chad Koehn, a coworker of mine who happens to be a machanical engineer, is planning on putting an NP203 reduction box in front of his Landcruiser's factory transfer case. He made a drawing of the reduction box's front and rear bolt patterns after precisely measuring the hole coordinates using a precision measurement tool at our office. This drawing is available here, in DXF and DWG formats. Email me for a TIFF-format version (it's over a megabyte in size). Note that only the holes are shown precisely - the outlines in the drawing are not representative of the case outline!
There's a seal in the NP203's input bearing retainer, so it's not necessary to have one inside the adapter. Oil from the tranny will get all over the place inside the adapter, but so what? It'll keep the bearing nicely lubed...
Again, don't forget to allow for a gasket when adding up your depth numbers!
Here's the other side of the NP203, and that side of the second adapter, with the modified output shaft:
There are two steps in the bore hole this time. The first one is sized to be a mild press fit over the aluminum plate, since this is how shaft alignment is ensured in this case. This step also has to be just deep enough that it holds the plate in place in the NP203. The second (large) step surrounds the plate, without touching it. It has to be deep enough that the heads of the bolts going into the D20 don't touch the plate. Note that I cut the tab off above the oil drainback notch in the plate, so that it would fit properly inside the adapter.
My adapter has been anodized blue, since I was able to get it done gratis. That explains the colour. It was made from a chunk of 2.5" thick plate stock that I got cheap at a local metal recycling place. It was cheap for aluminum, anyway. The machining was done by a very generous (and capable) CNC machinist Jeeper friend at his workplace. He and I designed the adapter on the fly (cut, test, cut, test, etc) over the course of several Saturdays. Sorry, I don't have any detailed drawings or machine files for it, but with some patience you ought to be able to do the same thing.
Again, don't forget about the gasket!
As for the NP203's output shaft, Moser modified it, basically copying all the features from the D20's input coupler onto it, so that it mates properly with the D20's input gear. Figuring out exactly where to put those features takes some very careful measurement! Here's the Word 97 document I sent to Moser with the modification instructions for the shaft and the gear.
I had the shaft case-hardened along with the NP435 output shaft. This is described in detail in the previous section.
For a fill plug, I just drilled a half-inch hole in the PTO cover plate, a little more than halfway up, so the countershaft is mostly immersed in oil and can fling it around inside the case. To plug the hole, I used a 1/2" freeze plug insert I found at Lordco - this is a rubber plug with a screw through it. Put it in the hole and tighten the screw, and the plug expands to lodge itself in the hole - pretty handy. For a vent, I drilled and tapped a 1/4-NPT hole up near the top of the case, poking out horizontally on the passenger side. In went a brass 1/4-NPT fitting with a hose barb for the vent line, and it was ready to go! Ford and GM 203's may not have a drain plug in their bottoms, in which case these will need to be added as well.