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Jeep Transfer Case Upgrades - Transfer Case Bible

Used Case
JP Staff | Writer
Posted December 20, 2006

All You Need To Know

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.


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