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Jeep Transfer Case Upgrades - Transfer Case Bible

Posted in How To on December 20, 2006 Comment (0)
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Contributors: JP Staff

You've got a transfer case in your Jeep, and this article is about transfer cases. That should be enough to get you interested, right?

OK, OK, what we've got here is all the information we've learned over the years and over the many, many Jeeps we've had. In this article you'll find the model number of the case, what kind of Jeep it is likely to come in, what kind of fluid it needs, a brief description of it, what ratios it has, what modifications we like to make, and how to flat tow it.

Well, what we are going to do is give a rundown of the cases commonly found in Jeeps-both the original Jeep cases and common swapped-in transfer cases. This is part one of two, where we cover the gear-driven cases. The second part is going to be about the newer chain-driven transfer cases. Without further ado, here is Jp Magazine's Transfer Case Bible.

Spicer/Dana 18*
This is the case that started it all. From the very first Bantam, Ford, or Willys prototype all the way through the war and into civilian life with the flatfenders and then back to war again with the M-38 and M-38A1s, this is the most-used transfer case in Jeep history. It was stuck in Jeeps for about 30 years of production, and as you would expect from a part put in a vehicle for three decades, there were a lot of changes made.

The one thing that didn't change was the offset output of the case. The rear output was offset to the passenger side, which meant that the pumpkins were in line under the Jeep, making for easier avoidance of trail obstacles.

The biggest two changes over the years are the ratios and the intermediate shaft sizes. Most early military cases are going to be found with a 1.97:1 low range, while the civilian and later M-38-era Jeeps got a 2.46:1 low range.

Since this is an offset case design, the intermediate shaft is constantly under load. The power comes through the transmission on the input shaft and then needs to be transferred to the offset front and rear outputs. If the gears of the input meshed directly with the output, the outputs would be spinning in the wrong direction, so an intermediate shaft is needed with an associated intermediate gear. This gear and shaft are constantly spinning, even in two-wheel drive on the highway.

With a pitifully weak flathead four-cylinder engine, the diameter of the intermediate shaft didn't matter as much as in later years when the power output started to climb. The military got a measly 3/4-inch shaft. When it went into civilian production, the size was kicked up to 1 1/8 inch with later units getting a 1 1/4-inch shaft. Obviously, the larger the shaft, the more power the case will live with. All cases can be modified to accept that later large shaft, but it can be a costly conversion.

At A Glance
Spicer/Dana 18

Design: Part-time, gear-driven, iron-case, front and rear passenger-side offset output, two sticks for range/front output selection

Found in: Virtually all '71-and-earlier short-wheelbase Jeeps, Willys wagons and trucks, forward control Jeeps

Feed it: 85W-90 gear oil

Available stock ratios: 1.97:1, 2.46:1

Use it with: Light Jeeps with up to moderate V-8 power (keep in mind, intermediate shaft size)

Flat tow: Put the transfer case in Neutral, front output in the "out" position, range selector in the "neutral" position. Put the transmission in gear or Park and leave the driveshafts attached

Modifications: Saturn Overdrive, Novak Intermediate Shaft kit, Tera Low 3.15:1 low range

Identify: If the offset wasn't enough, nor the twin sticks, look for the very common transfer case-mounted emergency brake and an oil pan

*We refer to it as Spicer/Dana due to the fact that in 1946 the Spicer Manufacturing Company changed its name to Dana Corporation

**The Ford GPW used both the Spicer 18 and a Ford copy of it, the Ford GPW 7700 with a 2.46:1 low range

Dana 20Just a small step beyond the Dana 18 is the Dana 20 transfer case. With a 111/44-inch intermediate shaft, a centered rear output, and a single shift lever, Dana Corporation didn't mess with much else.

The centered output meant it ran cooler on the road at the modern speeds of the 1960s and 1970s highways, thus improving reliability. However, it still had coarse spline output that proved to be the driveline fuse with swapped-in engines.

Along with the improvements came one step backward for the Jeeps this case was found in. From the 2.46:1 enjoyed in the Dana 18, the Dana 20 got a measly 2.03:1 low range.

At A Glance
Dana 20

Design: Part-time, gear-driven, iron-case, centered rear output, passenger-side front output

Found in: '72-'79 CJs, '67-'73 Jeepsters, '63-'70 Wagoneer/Gladiator/J-Trucks (and '71-'79 with manual transmissions)

Feed it: 85W-90 gear oil

Available stock ratio: 2.03:1

Use it with: Smaller Jeeps with up to a decent small block or fullsize Jeep with stock powerplants

Flat tow: Put the transfer case in Neutral, put the transmission in gear or Park, leave the driveshafts attached

Modifications: Tera Low 3.15:1 low range, twin stick with old Dana 18 or aftermarket parts, 32-spline Advance Adapters rear output

Identify: Cast-iron rear tailhousing and an oil pan

Dana 300
Come 1980, AMC was making its version of the Jeep more and more plush with car-like transmissions, carpeting, A/C, automatic transmissions, and so on. So it is a surprise that among all these things with nothing to do with off-road ability, the company also came up with the Dana 300.

Basically, it is a beefed up, updated version of the Dana 20; the 300 was bigger and had fine spline output shafts, yet retained the gear-driven, iron-case durability of the past designs.

The 1980 unit had a shorter rear tailhousing than later units, which is nice for driveshaft angularity until you learn that the speedometer mount is different than later Jeeps and the bolt pattern of the tailshaft housing is different, which precludes some heavy-duty upgrades.

At A Glance
Dana 300

Design: Part-time, gear-driven, iron-case, front passenger-side offset output, rear centered output, one stick for range and 2WD/4WD selection

Found in: '80-'86 CJs

Feed it: 85W-90 gear oil

Available stock ratio: 2.62:1

Use it with: Depending on how it's built, almost any Jeep. Wouldn't advise it for the heavier ones (5,500 pounds and above) with any combination of aftermarket parts

Flat tow: If your case is in the stock orientation, remove your rear driveshaft from the axle and fasten it out of the way. If you've clocked it flatter than stock, you may get away with the same method mentioned for the Dana 20. Basically, it needs to have at least one gear on the rear output in the pool of oil so it slings the oil around when the Jeep is moving. In the stock clocking setup, this doesn't happen

Modifications: Twin stick setup, JB Conversions Lo-Max 4.0:1 low range for the increase in strength where needed, and heavy-duty 32-spline front and rear outputs

Identify: Its got a centered rear output, oil pan, and an aluminum tailhousing

New Process 200
It's a rare one, that's for sure. The NP200 is a divorced case that is a direct predecessor to the venerable NP205. If you stumble across one, your first clue will be the yoke where the female input should be. The next clue will be that it's big and heavy. After that will be the owner giving it away because it tends to overheat.

Mainly found in the M-715s, it was also seen in DRW Jeep trucks in the 1960s and early 1970s. Neither one is a particularly common or high-mileage vehicle, but they are still revered in the Jeeping community. These trucks tend to sit in fields and next to garages until someone digs them out and bribes the current owner enough to let go of their "project."

Fast forward 30-40 years, and you've got a divorced transfer case that, while near bulletproof, only has a 1.96:1 low range and, in about 60 percent of them, there are overheating issues when driven on-road at or near the top speed of 55 mph for any distance.

This overheating is commonly attributed to straight-cut gears, but this is a misnomer. There are, in fact, helical-cut gears in this transfer case. Their pitch isn't quite what the NP205 has, but they are indeed curved. The overheating could possibly be attributed to lack of use and pitting and galling of the internal components, courtesy of the environment and lack of usage.

At A Glance
Model: New Process 200

Design: Part-time, gear-driven, iron-case, front and rear passenger-side offset output, twin stick operation with front axle engage/disengage and high/low range options

Found in: '67-'69 M-715, M-724, M-725, M-726, and circa '60s and '70s DRW J-Trucks

Feed it: 85W-90 gear oil or heavier

Available stock ratio: 1.96:1

Use it with: If you can get over the meager low range and luck upon one that doesn't overheat or don't care about on-road manners, you can run this case in any size FSJ you want with a swapped-in 2WD transmission. Because it is divorced, it won't fit SWB Jeeps (under 105-inch wheelbase or so) well at all.

Flat tow: Transmission in gear or Park, transfer case in Neutral. Note: If your particular case doesn't handle high speeds when being driven, it won't when being towed either. Besides, what would you flat tow an M-715 with? A 5-ton?

Modifications: Swap the center-mounted parking brake with the offset rear output for some relief from the heat problem. Put in lower axle gears to decrease the speed of the case

Identify: It's big. It's divorced-mounted. It's leaking. It's got a band-style parking brake on it. It's cast iron. You try to lift it and worry about medical repercussions. Fill all these, and you've got yourself an NP200

New Process 205
This is the big Maha transfer case. Cast -iron, gear-driven, used in 1/2-, 3/4-, and 1-ton trucks from Chevy, Dodge, and Ford for more than two decades, this case can take whatever beating your little Jeep can throw at it.

It can be easily mated to many Jeep transmissions with shade tree ingenuity or aftermarket support. If you are having issues blowing up driveshaft yokes, 1410-style yokes can easily go on this case, it can be twin-sticked without difficulty, and common speedometer senders fit all of them. They are available with any number of input spline counts and configurations, driver-side and passenger-side drops, and divorced and married styles. This is the bulletproof transfer case of your dreams-you can't destroy it.

The problem? The low range is only 1.96:1, its heavy (like really, really heavy), and fitment can be a problem with earlier Jeeps because it's a physically big case.

At A Glance
New Process 205

Design: Part-time, gear-driven, iron-case, aluminum output and input retainers, front passenger- and driver-side offset output available, rear center output, stock single-stick operation with 2WD, 4WD high, and 4WD low options

Found in: Early '70s to early '90s Chevy, Dodge, and Ford pickups

Feed it: 85W-90 gear oil or heavier

Available stock ratio: 1.96:1

Use it with: If you can fit it and don't mind the added weight and the huge overkill factor. Chances are, you won't break it

Flat tow: Transmission in gear or Park, transfer case in Neutral

Modifications: JB Conversions Lo-Max 205. It's got a 3.0:1 low range, even heavier duty than the big-block handling stock gears, and relatively conservative pricing. Again, if it'll fit in your Jeep and mate with your transmission, this is the last transfer case you'll ever need

Identify: Look for the identification tag on the back of the case first. If you can't find that, look between the cover on the rear of the case for the input shaft and the rear output for a small, round cover. This cover will be equidistant between the two and covers the intermediate shaft. It'll have three bolts holding it in. If it's got a cover there with three bolts holding it on, snag it


Advance Adapters
Paso Robles, CA 93446
JB Conversions
Sulphur, LA 70664
Novak Adapters


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