There are only a limited number of options for driver's-side drop transfer cases, and I needed one to go with the driver's-side differential on my Early Bronco front D44 axle. The case I chose had to be physically small, yet as strong as possible. It only took a little while to decide on the Early Bronco Dana 20 (EB D20). The fact that I found one of them in a local yard made the decision that much easier; these cases are supposedly fairly rare.
The Dana 20 transfer case is a very compact gear-driven cast-iron transfer case. Jeep used a passenger-side drop D20 in the CJ for many years, and it had a 2.0:1 low range. There is, however, a Ford version of the D20. It's a driver's-side drop case and has a 2.46:1 ratio. Ford used this case in the Early (66-74) Bronco, and it's still possible to find them here and there. However, since it's been out of production for 25 years, parts and rebuild kits can be hard to find. Fortunately, there are plenty of EB-oriented parts stores along the West Coast that have stockpiles of this kind of thing.
The Jeep and Ford D20s are quite different, though many of the bearings and seals are the same. The cases, gears, and (I think) the shafts are not interchangeable, for the most part. One really cool feature of the D20 is that it's got separate forks for the front and rear outputs, so with some minor surgery it's possible to put twin-stick shifters on it. This lets you do interesting things like front-wheel drive and 2wd low range (with the hubs locked), etc.
Here's are photos of my EB D20, and the backside of the tcase-tcase adapter:
Again, a rebuild kit went into the D20 as part of the prep work to get it ready. Valley Gear just south of the border in Washington supplied the kit, which went in using little more than basic hand tools. While I was in there, I removed the two interlock pins that live between the shifter rods and stop you from doing fun things like 2wd low range, front-wheel drive only, and high range on one axle with low range on the other. This is necessary to make the twin-stick conversion work.
The protruding snout on the backside of the adapter is a separate piece of aluminum, pressed into the main plate of the adapter. This snout section is basically a copy of the protruding snout on the factory D20 adapter. The factory adapter for the D20 actually holds the input shaft, the input bearing and seal, and the input drive gear, so there are several important features that had to be machined into the snout. Three steps are required inside that bore hole: the first step is a very light press fit for the input bearing to slide in, the second step is the stop for the bearing (and leaves some space for oil to fill behind the bearing), and the third (innermost) step is bored out to the right size to press-fit a seal in place. The seal I used was Federal-Mogul p/n xxxxx, picked off the shelf at the local Lordco because it had the right inner diameter to fit around the NP203 output shaft. There's also a snap ring groove inside the first step, for the snap ring that holds the bearing in place. Anyway, this snout section was made on a lathe and really was quite straightforward, since all that's required is to measure and duplicate the features from the factory adapter.
Don't forget to make an oil channel for oil to flow back from the other side of the bearing. When we pressed the snout into the adapter, we didn't line this channel up properly - it's supposed to be at about 7 o'clock, not 10 like mine.
Aside from the snout, the only other important feature on this face of the adapter is the ring of starter holes around the snout. These are spaced 3 degrees apart (120 dimples) on the same diameter as the D20's input bolt circle. This was a great idea that my machinist friend had: six of these holes ended up being drilled out for the bolts going through to the D20, but I didn't choose and drill them until I was almost ready to bolt up the D20. This let me "clock" the case, adjusting the angle at which it hangs down for optimal clearance of the skid plate. When I found an angle I liked, I marked the position of the case and drilled out the six dimples that lined up closest to the holes in the D20.
The angled slope up to the D20 mating surface let me get rid of a lot of unneeded material on the adapter, and provided more room to get tools and hands in between the transfer cases. Oh, and the bore hole that the snout slides into has a thin shoulder just before the opposite side, that exists to position the snout at the right depth in the hole. Again, don't forget about the gasket between the adapter and D20 when positioning the snout!