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Considering a Transfer Case Upgrade?

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Trent Riddle | Writer
Posted June 1, 2001
Photographers: The Manufacturers, Verne Simons, Christian Hazel

No, You’re Not Stuck With the Factory T-case. You Have Options

Step By Step

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  • The Atlas II T-case from Advance Adapters is a super-strong, gear-drive T-case that can be matched to almost any application. The units are available in three low-range ratios: 3.03:1, 3.80:1, and 4.30:1.

  • The Dana 300 was offered in Jeeps from 1980-1986 as well as some Scouts in late 1979-1980. This T-case is a gear-drive unit with a 2.61:1 low-range. It is a stout unit when used in light rigs, but should be avoided if you own a fullsize truck. Aftermarket upgrades include ultra-low 4:1 gearing and heavy-duty output conversions.

  • The New Process 208 replaced the New Process 203 and 205 T-cases in the ’80s. These units have a low-range ratio of 2.6:1 and can be shifted on-the-fly from 2WD to 4WD high. They’re not quite as strong as the 203 or 205 but they do hold up well to moderate use.

  • Land Cruiser owners can upgrade to the late-model split transfer case used by Toyota from August 1980, and on. Those who choose this option can benefit from the 2.280 low-range of the later unit. However, the 19-spline input will require using a later four-speed (again, from August 1980 or later, or a five-speed, used beginning October 1982 (seen bolted to the T-case here). This will cost more but you’ll get both a lower first gear and possibly even an overdrive in the bargain.

  • The New Venture line of T-cases offers a vast array of possibilities. All have a 2.72:1 low-range ratio. The Dodge 1/2-ton 231 is the most desirable for light-truck applications, while the 241 from the 3/4-ton is better for heavy trucks. These T-cases offer only part-time 4WD. Another option is the NV242, which offers both full-time high as well as part-time high and 4WD-low. The Hummer version of the 242 is the strongest but it doesn’t offer a 2WD selection. Aftermarket upgrades for these units include ultra-low 4:1 gearing and slip-to-fixed-yoke output conversions.

  • The Klune-V crawler box can be mated to just about any transmission/transfer case combination and adds a second low-range. The units come with either a 2.72:1 or 4:1 reduction.

  • The New Process 205 T-case is considered to be the hot swap for fullsize pickups. This gear-drive unit sports a cast-iron housing and can take a serious beating. Ford, GM, Dodge, and International all used the 205. Careful, though: Each manufacturer used a slightly different 205. The good news is they are plentiful in the salvage yards. The down side is that the low-range ratio is a measly 1.96:1.

  • The NP207 and the NV231 and 242 T-cases came from the factory with a slip-yoke rear output. Kits are available to upgrade the output to a fixed-yoke output, which allows the use of a CV rear driveshaft to help eliminate vibration. Pictured here is an HD kit for the NV231, which also upgrades the shaft diameter for added strength. Currently, only standard-duty kits are offered for the NP207 and NV242.

  • Owners of Toyota pickups and 4Runners can upgrade to lower gears in their stock T-cases, add a secondary crawler box, or both. Here you see a comparison between 2.28:1 and 4.70:1 crawler gears.

  • Suzuki owners are not forgotten when it comes to added low-range. Kits are available to reduce the low-range to as much as 4.89:1 for these Lilliputian four-wheelers.

  • Even Isuzu owners can benefit from aftermarket upgrades to the low-range gearing. Here you see a set of Tera-Low gears that ups the reduction from the stock 2.05:1 to an improved 3.07:1. (See Four Wheeler, April 2001, for a story on how to install this gearset.)

  • For those of you who own or are considering an NP205 T-case, Offroad Design offers a solution to the unit’s limited low-range. By adding the low-range section from an NP203 T-case you can double down your 205 and get a crawl reduction of 3.96:1. Kits are also available for the NP203 and 208 T-cases.

  • One weakness in the Dana 300 is the output shaft. The stock Dana 300 rear output shaft (left) has been known to break when the unit is used in heavy trucks with large tires. The stock shaft is only 11/8-inch-diameter with 26 splines. Advance Adapters offers an upgrade to a new, larger 13/8-inch-diameter, 32-spline output shaft. The kit comes with a new tailshaft housing, output shaft, taper roller bearings, and speedometer drive.

  • Land Cruiser owners can upgrade from the marginal factory T-case housing to this billet unit from Warden’s Automotive. Upgrading to this T-case housing virtually eliminates cracked housings. One trick is to build a new unit using the gears from a T-case that was mated to a three-speed transmission. These gears offer a 2.31:1 low rather than the 1.99:1 low found on T-cases mated to the four-speed transmission. In addition, this T-case housing is big enough inside to allow the use of larger, lower-ratio, aftermarket gears as they become available.

If you have a four-wheel-drive truck, you’ve got one of these. It’s the heart of every 4x4, and the best of them come with a low-range gear. That’s right, we’re talking about the transfer case, or T-case. The T-case is what splits your driveline torque and transmits it to both axles. It also allows you to drive down the highway at a reasonable speed and then slip into low-range and crawl down the trail. For the average Jack (or Jill) the stock T-case is just fine, but those of us who really take the sport to heart soon find that T-case upgrades often are needed to improve our 4x4’s fun factor off the pavement.

T-cases with a low-range ratio of 2.00:1 or higher (numerically lower) are just not suited to rough four-wheeling. Factory T-cases with a low-range ratio of 2.5 to 2.7:1 are better, but really difficult rockcrawling may find you wishing for a T-case with a low-range ratio of 3:1 to 4:1. Another consideration in the T-case department is this: Just how strong is your factory T-case? Bigger tires, motors that are more powerful than stock, and tough trail use can punish some T-cases to the breaking point. If you’ve broken your T-case or are worried that you might, then an upgrade is called for.

Finally, there is the concern over slip-yoke driveshafts on trail rigs. The slip-yoke-style T-case output is similar to that found on 2WD trucks and cars with automatic transmissions. A slip-yoke is a splined sleeve that floats over a gearbox output shaft allowing for drivetrain length variations that come with suspension travel. A fixed-yoke also has slip joints, but these are in the center of the shaft, not at one end.

A slip-yoke is fine for street use but can be a nightmare on your trail truck. Consider this: If you break a rear driveshaft, you’ll be stuck unless you have a second driveshaft. This is because removing the driveshaft takes the slip-yoke out of the T-case and allows the fluid to drain out, which will burn up your T-case if you drive. Because of its different configuration, a fixed-yoke does not present this problem. Further, a slip-yoke is typically not compatible with a driveshaft that uses constant-velocity (CV) joints. This means that you may have driveline vibrations you can’t cure if you lift a truck with a slip-yoke shaft system.

The solutions to these problems can be found in both the salvage yard and the aftermarket. On the salvage yard side, owners of early Ford pickups with single-speed Dana 21 T-cases can swap in the Dana 20, used in Broncos. Owners of trucks with aluminum-case, chain-drive 208 T-cases can swap in cast-iron, gear-drive 205s. If you have a ½-ton 231 T-case you can upgrade to the ¾-ton 241. All you have to do is find the right piece at your favorite salvage yard.

There are also many options available from the aftermarket. Here the solution may be to upgrade your existing T-case or swap it out. On the Jeep, Dodge, Chevy, Scout, and Bronco front, the aftermarket offers low-gear sets for the Dana 18, 20, and 300 T-cases, as well as for the NV231, 241 and 242 T-cases. Low gears also are available for Toyota pickups, 4Runners, Suzukis, and Isuzus.

Another option that will yield lower gearing is the doubler, or crawler box. This is basically a secondary low-range bolted into the driveline between the transmission and T-case. A doubler gives you two to three low-ranges, use of which gives you better control on rugged trails. (How three low-ranges, you ask? Well, if you have a 2.72:1 stock ratio and a 4.0:1 crawler box you can get both those ratios as well as 10.88:1 by doubling down.) To improve durability you also can upgrade your T-case output shaft and swap the slip-yoke output for a fixed-yoke system. There are even billet aftermarket T-case housings that will add strength to your truck driveline. Almost anything you can think of to improve or upgrade your T-case is available today, and if it isn’t, you can just swap in an Atlas II T-case in one of the three low-range ratios available. Your options are endless when it comes to T-case upgrades. The only questions are: How strong, and how low, do you want to go?