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Dana Spicer 18 - The Best Transfer Case Ever?

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John Cappa | Writer
Posted June 1, 2000

Can't Stand the Suspense? OK, It's the Dana Spicer 18, But Click Here to Find Out Why Plus: Easy Rebuild Tips

Step By Step

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  • Here’s a big advantage the 18 has over more modern ’cases. Notice how the boulder can pass under the axles just under the driver seat. No more weaving and backing to clear centered and wrong-sided differentials.

  • The Spicer 18 was available with three different-sized intermediate shafts. You can measure them without ’case disassembly. This one measures 1 1/8 inches (’46-’53), but a 1 1/4-inch (’53-’79) and a weak 3/4-inch (MB and GPW only) were available.

  • When looking for an 18, be sure to check the housing for cracks. The Texas-shaped flange is prone to cracking if it has been installed improperly. Also check the intermediate shaft bores for damage and cracks.

  • Two different locating bores were used on the 18s. The early models used a 3.15-inch bore (shown); later versions had a 4.00-inch locating-bore.

  • An Advance Adapters Saturn overdrive (installed) bolts to the P.T.O. output of the 18 and replaces the output gear and cover plate (below). No driveshaft modifications are required.

  • For those with longer wheelbases or strange drivetrains, an adapter is available from Advance Adapters to turn the 18 into a divorced transfer case. Advance has adapters to mate just about any transmission to the Spicer 18.

  • The intermediate gears are interchangeable (except the 3/4-inch) as long as you use the rest of the related gears. On the right are the bearings and shaft of an 1 1/8-inch ’case, and on the left are the 1 1/4-inch shaft and bearings. The individual rollers on the left are held in place with grease during assembly (shown inside gear).

  • At least three different P.T.O.s were offered for the 18. They are no longer manufactured but they can be found in wrecking yards if you plan to do some work. Running a winch off of the P.T.O. isn’t a good idea since the transfer case can’t be in gear and run the winch at the same time.

  • The interlock pill is found between the shift rails. To get to it you have to remove the front bearing retainer. Removing the pill allows the use of two-wheel-drive low-range.

  • Here’s a little secret for you. You can replace the rear flange yoke (right) with a normal one (left). It offers more angularity, but a small 1/8-inch spacer (lower left) is needed along with some grinding if you use the e-brake type of rear bearing-retainer. The Dana Spicer yoke (PN 2-4-2851X) is for a 1310 U-joint. It’s actually the rear yoke for a Dana 20. Others are available up to a 1350, but the transfer case then becomes the weak link.

  • Talk about a theft prevention device. Most thieves wouldn’t know what to do with four shifters. One shifter is for the tranny, one is for the overdrive, and two are for the transfer case.

We constantly hear and often find ourselves saying, “They don’t build ’em like they used to.” This generic phrase can also be applied to transfer cases. Long gone are the days of heavy-duty, iron-cased, geardriven transfer cases like the NP205, Rockwell, Dana 20, Dana 300, and the best transfer case ever, the Dana Spicer 18.

Oh sure, there are stronger ’cases than the 18, but no ’case has as many desirable off-road features. We commend the NP205 for being bulletproof, but is its extra weight needed when you’re only throwing pebbles at it with your stock V-8, six-cylinder, or four-popper? It’s also too big to use in most short-wheelbase applications and has a measly low-range of 1.96:1.

The 205 has its place in fullsizes where the 18 probably has no business because of the corpulence of such vehicles. Those that need the 205 accept that the low-range leaves something to be desired, or they add a Doubler kit if there is enough space. Interestingly, the gears in the 18 and the 205 are similar in size, hinting at the strength of the smaller Spicer. Manufacturers build aftermarket parts to make lightweights like the NP231 and others more like the early Spicer, with items such as twin-stick shifters, true neutrals, part time kits, 2-low conversions, slip-yoke eliminator kits, and so on. All of this and more is achieved on the 18 without buying these often expensive parts. Come to think of it, the best way to make the 231 off-road ready might be to swap it out for some real iron, the Spicer 18.

The 18 may be a far cry from modern but it does have its advantages. It is perfectly suited in lighter-weight rigs (although it can be found in some larger trucks and wagons), and we have seen versions of the 18 survive in 1,000hp sand drag Jeeps.

For crawling around sharp corners, two-wheel-drive low-range can be obtained simply by removing an interlock pellet (we’ll show how).

Most 18s came with twin-sticks from the factory so there’s no need to purchase them separately like on the 300, or dream about them if you have a Dana 20, NP205 or 231. Adding an overdrive to your 18 is a simple bolt-on procedure, no tranny swap or driveshaft modifications necessary. The overdrive effectively turns your four-speed transmission into an eight-speed with a 25 percent overdrive. Also, if a P.T.O. is your bag then try and put one on an NP231, Dana 300, or 20. Good luck. While P.T.O. units for the 18 are no longer manufactured, used ones can still be found by roaming wrecking yards.

Looking for something to cure vibrations and binding coming from your rear driveshaft? Look no further. The Spicer 18 has a rear output yoke 5 inches lower than any of the other common ’cases. The 18 is also shorter than most transfer cases, allowing a longer driveshaft. No need for expensive CV shafts. Sure, both outputs are to the passenger side, but this is an advantage when traversing the rough stuff. A friend tells us, “Just keep the rocks under the driver seat.” With both differentials on the right side of the vehicle, the rocks pass snag free on the left. That’s much easier than trying to avoid the rock with the front diff only to have it smack your centered rear differential. Besides, if you use your newer Jeep hard then you’ll be replacing the axles anyway. Might as well do it right the first time.

Worried about low-range? Well, the 18 has a low of 2.46. That’s lower than the Dana 20 and close to the 2.62 of the Dana 300. It doesn’t quite compare with the 2.72 of the 231, but 4:1 low-range gears are available for the 18 at a much cheaper price than a 4:1 planetary and new front case half for the 231.

The 18 came in several versions. If you are looking to swap one in then it is important to know what you have so you can get the right rebuild kit and adapters. Early Jeeps (’42-’45) used a weaker version of the 18. It had a ¾-inch intermediate shaft. Few of the internals are interchangeable with other 18s. These ’cases should be avoided unless you are putting together a restoration project.

The 1 1/8-inch intermediate shaft ’cases can be found in ’46-’53 Jeeps and some Internationals. It’s not the strongest version but it is adequate for most applications. It has a 26-tooth, six-spline input gear. It can be converted to the 29-tooth, 10-spline version if the accompanying gears (input, intermediate, low-range output, and high-range output) are used as well.

The 1 ¼-inch version can be found in ’53-’79 Jeeps and some Scouts. This ’case has a 29-tooth, six-spline input gear. The 18s mated to the T14 transmission from the factory have a 29-tooth, 10-spline input gear. The 1 ¼-inch Spicer 18 is the most desirable from a strength standpoint. Later versions of this case have a 4.00-inch locating bore and a sturdier design. These versions are interchangeable with Dana 20 housings. All of the 18 parts will bolt onto the Dana 20 case. The internals from a 1 1/8-inch ’case can be used in the 1¼-inch ’case with the exception of the intermediate shaft and bearings. Low-range gearsets are only available for the 4.00-inch bore version of the 18.

If you have a Wrangler or later CJ, the Spicer 18 may be the perfect conversion. Add up the prices of all the parts you’ll need to make your current Jeep ’case off-road ready. You just might find out that it’s less expensive and more beneficial to ditch that new stuff and replace it with the old. Here are a few captions that might sway you to the “right” side. We also threw in a rebuild for those in-the-know.


National Drivetrain
Chicago, IL 60609
Advance Adapters
Paso Robles, CA 93446