A common theme with flatfender Jeep builds is that there simply isn’t much space to work with. The entire Jeep becomes a jigsaw puzzle. One major component out of place will start a domino effect that will cause all sorts of other issues further along during your build.
When selecting a transfer case for a flatfender project, you have to consider axle gearing, overall weight, drivetrain length, driveshaft angles, shifter location, width, and so on. Some transfer cases are simply too large to fit between the framerails of a flattie. Others that do fit may significantly reduce ground clearance by hanging down too far below the framerails. We’ve tried a couple of different transfer cases on flatfender projects over the years. We always seem to come back to the Spicer 18, which is the transfer case that most early Jeeps originally came with. From 1941 to 1971, there were several changes made to the Spicer 18. There are three main versions that can be identified by the diameter of the intermediate shaft. The early Jeeps have a dinky and undesirable 3⁄4-inch-diameter intermediate shaft. The ’46-’53 Spicer 18 transfer cases feature a larger 11⁄8-inch intermediate shaft, and late’53-’71 Spicer 18 transfer cases have an even larger 11⁄4-inch intermediate shaft. The change over the years wasn’t to increase the sheer strength of the shaft but rather to improve the bearing design by increasing the surface area that the bearings ride on. The intermediate shafts in all versions of the Spicer 18 transfer case are very wear-prone, especially if the oil becomes contaminated. In this case, bigger is better.
When looking for a Spicer 18 for your project Jeep, inspect the gears and case closely for cracks and rust. An open transfer case left outside will likely have rust on the gears, possibly making them unusable. Try to find a transfer case that has been indoors or one that is still bolted to a transmission, sealing out the elements.
The ’66-’71 Spicer 18 transfer cases found in V-6 Jeeps are often considered the most desirable. They feature the large 11⁄4-inch intermediate shaft, a 2.46:1 low range, a sturdier cast-iron case, and a larger input bearing. You can build your own heavy-duty version of this case by installing some of your Spicer 18 guts and external bits on a Dana 20 case.
After considering our options, we decided our lightweight Jeep didn’t need much more than the strength of a Spicer 18. This choice allows us to add an Advance Adapters Saturn Overdrive. The Saturn decreases top-gear engine rpm by 25 percent and works as a gear splitter, effectively turning our four-speed transmission into an eight-speed. We also like that both the front and rear outputs of the Spicer 18 are on the passenger side, giving us lots of axle ground clearance on the driver side.
We’ve have had good luck with the 11⁄8-inch and 11⁄4-inch Spicer 18 transfer cases in the past. We happened upon a free 11⁄8-inch Spicer 18, so we decided to put it to good use in our flatfender project.
Step By Step
Start by removing the oil pan, yokes, and the rear bearing retainer. Save the bearing shims and avoid damaging them—you’ll need to reuse them.
Remove the shift detent bolts, springs, and balls. The springs may come out in multiple pieces. A small magnet simplifies removal.
With the detent balls removed, you can unbolt the front output assembly. A lot of grunge builds up in here so don’t be surprised if it’s a gooey mess. You’ll need to clean all of the parts thoroughly. The shift interlock pill can be removed for two-wheel-drive low-range operation. We have found that this sometimes causes the transfer case to pop out of low range when decelerating, so we leave it in.
Remove the intermediate gear by sliding the intermediate shaft out from the rear of the transfer case. The rear output shaft is also pulled from the rear of the case. A retaining clip holds the gears in place and needs to be removed prior to pulling the shaft. A few taps with a soft-faced hammer will free up the shaft and the tapered bearing races.
Every Spicer 18 we have ever pulled apart has had a worn intermediate shaft and at least one broken shift detent spring. Order these parts, along with a rebuild kit prior, to dismantling your early Jeep transfer case. We purchased our rebuild kit and other miscellaneous Spicer 18 parts from Herm The Overdrive Guy. The company specializes in new and used older Jeep parts, as well as completely rebuilt Spicer 18 transfer cases, among other things.
Clean the empty case and inspect it carefully for wear and damage. Cracks often start around the oil pan area and on the mounting surface where the transfer case mates up to the transmission.
Use grease to stick the intermediate thrust washers to the side of the case. We also use a small piece of tubing (left) and the new intermediate shaft (right) to hold them in place when installing the intermediate gear loaded with new bearings. If you have a Spicer 18 with a 11⁄4-inch intermediate shaft, you’ll need to assemble the individual rollers in the gear and keep them in place with thick assembly lube.
Install the new rear output bearing on the shaft. It’s only a light press fit so it can be done with the correct sized tubing and a hammer. As you insert the shaft, you can install the gears, thrust washer, retainer clip, remaining bearing, and both bearing races.
Tap in the front output bearing and yoke seal with the properly sized tubing or a large socket and a hammer. Grease the seal to prevent damage when installing the yoke. The front output assembly can then be bolted to the housing.
File down any sharp edges on the shift rails. Grease the shift rails and seals before carefully sliding them in place. You can tap them home with a hammer and a socket or short piece of tubing.
The gaskets are needed on a Spicer 18 to maintain proper fitment and spacing. Since the Spicer 18 is a notorious leaker, we like to smear grey gasket maker on the mating surfaces, in addition to using the gaskets. Many of the bolts are wet, so we use blue or red thread-locking compound on all bolt threads to stop potential oil leaks.
Install the rear output bearing retainer using the original shims. You may need to remove or add a few so don’t use any sealant just yet. Torque the mounting bolts to 30 ft-lb.
Install the oil pan using the supplied gasket. You can use gasket maker alone here if you prefer, but we used both. Some older beat-up pans may need to be massaged with a hammer to make the mounting surface flat.
We removed the factory emergency brake and replaced the original flange yoke with a regular 1310 yoke. This allows for more driveshaft angularity. It’s the same coarse-spline yoke found on Dana 20 transfer cases and Dana 25, 27, and older 44 axles. You need to grind the retainer a bit and add a 1⁄16-inch spacer (washer) under the yoke for proper clearance. Torque the yoke nut to 100 ft-lb.
Using a dial indicator, check the endplay of the rear output shaft. Set it to between 0.004 and 0.008 inch by adding or removing shims. It may take several tries so be patient.
Once you have the proper endplay setting, you have to pull the rear output off one last time to seal it up. Some people have good luck sealing the transfer case by leaving the shims dry. We seem to never have any luck, so we use either messy gasket sealant (right) or spray gasket (left) on both sides of the shims for a sure seal. Smear gasket maker on the splines of the yoke and grease the seal before slamming everything home.
Many different adapters are available for the Spicer 18. We’re installing a GM SM420 four-speed manual truck transmission in our Jeep, so we opted for this Advance Adapters unit (PN 50-2401). It can be installed without disassembling the transmission.
No matter if you are adapting your Spicer 18 or bolting it up to the stock transmission, never pull it into place with the mounting bolts. This can cause the mounting ears of the transfer case to snap off if there is interference. Instead, use a few long bolts or studs to align the transfer case and (if needed) a dead blow hammer to help seat it in place.
If you aren’t installing an overdrive unit, the input gear is inserted through the back of the transfer case. The nut is torqued to 75 ft-lb.
We installed an Advance Adapters Saturn Overdrive on our transfer case. It comes in two main pieces. The bowl gear assembly is installed in place of the standard input gear. A 1⁄2-inch-drive ratchet is used to torque the internal nut to 75 ft-lb. A locking tab and a special snap ring are used to keep the nut from loosening.
The Advance Adapters Saturn Overdrive aluminum outer housing is then installed on the back of the transfer case. The 25 percent gear reduction will make our 5.38 axle gears much more livable on the highway. The effective overall top-gear ratio becomes 4.04:1, which isn’t bad for a 33 to 37-inch tire on longer freeway jaunts.