I am in the middle of doing a front and rear axle swap in my 1950 Willy’s CJ-3A, and I’m trying to figure out what my options are for centered rear output transfer cases. The Jeep is equipped with a T-90 three-speed transmission. I recently pulled the Spicer 18 transfer case off my Jeep T-90, and I have a T-90 with a Dana 300 out of a Scout II. My thought was I could swap the IH Dana 300 onto the Jeep T-90, but I’ve been reading that the input shafts might be different. Any help on this would be greatly appreciated.
For starters, it’s doubtful that you have a factory International Harvester Dana 300 and T-90 combination. While IH used both T-90s and Dana 300s, the two were never used together in a factory application. If you’re certain that you have an IH T-90, then most likely you have a Dana 20 attached to it (which is not a bad thing). If you’re certain that you have an IH Dana 300, then you probably have a different transmission than a T-90 (though it is possible to bolt an IH T-90 to an IH Dana 300; more on that in a minute).
IH Dana 20s and Dana 300s are often mistaken for one another, as they look similar. The best way to tell them apart is knowing that the Dana 300 has an aluminum tailhousing for the rear output while the Dana 20’s is cast iron, and the Dana 20 has a 1-inch-deep sump in the inspection cover while the Dana 300’s is fairly smooth and flat.
If you have an IH Dana 300, then you have a very rare and desirable transfer case. Used for only one year (1980), the Scout Dana 300 uses the same “Texas” bolt pattern and output shaft style as the earlier Dana 20, which allows it to bolt directly to many Jeep and IH transmissions that were originally mated to Dana 20s while also offering a 2.61:1 low-range. This was a major improvement over the Dana 20’s 2.02:1 low-range. Because the Dana 20 and the Spicer 18 also used the same bolt pattern and drive gear style, this opens up the transmission selection even more. Further, the mating adapter can be as short as an inch long depending on the transmission, which is a big help on driveshaft length.
The issue with the IH Dana 300 is availability (both the cases themselves as well as parts), and only the drive gear from manual-equipped IH 300s are adaptable to transmissions that were equipped with Jeep or IH Dana 20s. The drive gears from IH 300s behind TF727s are different and not compatible. Like the cases themselves, the IH 300 drive gears are no longer available new. With such a unique combination of attributes and knowing that just 30,000 1980 Scouts were made (many of which were equipped with automatics), it’s easy to understand why Scout 300s are rare and command high prices.
Back to your project: According to the information we were able to gather on the subject, regardless of whether you have an IH Dana 300 or Dana 20, as long as it was originally behind a manual transmission with a 1 3/8-inch six-spline input, you should be able to use what you have behind the Jeep T-90 without much issue. You could also use the IH T-90, but input shaft stick-out lengths often vary within different Jeep and Scout applications, so you’ll most likely need to swap the input shaft between the two transmissions. We’d just use the Jeep T-90 if it’s in good shape.
We would be a little leery of using the Scout Dana 300 just because parts are scarce, and it would be a lot of work to put it in only to have a possible issue down the road and not be able to locate replacement parts. Alternatively, you could use a Jeep Dana 20. As a bonus, 3:15:1 low-range kits are available for Jeep and IH Dana 20s from Advance Adapters (advanceadapters.com) or TeraFlex (teraflex.com). If you need Dana 20 rebuild kits or shifters for your project, Novak (novak-adapt.com) is a great resource for parts as well as knowledge of vintage Jeep drivetrain combinations.