Click for Coverage
Due to the EU’s Global Data Protection Regulation, our website is currently unavailable to visitors from most European countries. We apologize for this inconvenience and encourage you to visit for the latest on new cars, car reviews and news, concept cars and auto show coverage, awards and much more.MOTORTREND.COM
  • JP Magazine
  • Dirt Sports + Off-Road
  • 4-Wheel & Off-Road
  • Four Wheeler

Freeway Flattie Part 1

Posted in How To: Transmission Drivetrain on January 1, 2012
Share this

Okay, see if you can follow this. When I first built my ’53 DJ-3A I used low-pinion Ford 9-inch axles with 5.83s and stock 2.46:1 gears in a Spicer 18 T-case. With the junkyard Warn Overdrive engaged, I could turn 2,850 rpm at 65 mph on the street, and the SM420’s 7.05:1 First gear gave a 101:1 crawl ratio, which was just about perfect for my relatively low-torque 3.5L Shortstar DOHC V-6. Then the hardcore bug bit and I replaced the Spicer 18 with a built Dana 300 running stock 2.62:1 gears and 32-spline front and rear output shafts. The Dana 300 has a much higher rear output location than the Spicer 18, so to get a rear driveshaft in the thing I had to build a new centered rear axle with a high-pinion True Hi9 centersection. The axle build required a gear change to 5.38s since that’s the lowest ratio offered for the True Hi9. But even with the high-pinion rear diff, to prevent U-joint binding at full droop I had to run offset-trunion U-joints in the Tom Wood’s 1350 rear driveshaft. The crawl ratio of the new setup was nearly identical at 99:1, but without an overdrive, my engine speed was a whopping 3,500 rpm at 65 mph. Not that I could ever reach that speed since the crazy rear driveshaft U-joints caused the Jeep to feel like you were riding a paint shaker even at modest road speeds.

If you read my Trail Head editorial this month, you know all about why I want to go back to an Overdrive-equipped Spicer 18 to make my ’53 DJ-3A highway-capable once again. It turns out that I had it right the first time. But unfortunately I’d sold off my genie Warn Overdrive and Spicer 18 to help pay for the Dana 300 swap. However, companies like ATV Manufacturing and TeraFlex are still supporting these 70-year-old T-cases. And that’s where I found the solution to my dilemma of how to add the lower rear output location and overdrive ability of the Spicer 18 without killing my crawl ratio.

If I used stock 2.46:1 Spicer 18 gears with my 5.38 axle gears, my new crawl would be around 93:1. That’s not terrible. In fact, it’s just about ideal for most normal V-6 and V-8 powered Jeeps. But my little Shortstar needs all the torque multiplication it can get. By using 3.15:1 Low18 T-case gears from TeraFlex, I’d have a 119:1 crawl ratio, which will make the Shortstar much happier and help its resistance to stalling when idling over obstacles. The 3.15:1 gears are also a great solution for a good crawl ratio if you’re not running a granny-gear T-case or deep axle gears. Plus, I could engage the Overdrive for a more-modest 89:1 crawl when the obstacles aren’t as gnarly. Done! A quick call to ATV Manufacturing in Brush Prairie, Washington was all it took to line up the ultimate Spicer 18 build using a Dana 20 case, Low18 gears, and ATV’s own ATV Mfg. Overdrive. I hopped a plane and stood over the shoulder of vintage Jeep parts guru and ATV’s owner Herm Tillford as he assembled my new T-case and ran through some of the finer points of successfully rebuilding and running Spicer 18s. Tune in next time for a more in-depth look at ATV’s Overdrive and the T-case installation into the Jeep using an adapter from Novak Conversions. Then, how about a good old-fashioned vintage Jeep road trip?

PhotosView Slideshow

Spicer 18 Through the Years
1- The standardized ’41-‘45 WWII-era flatfender and very early ’45 CJs used the T-84 transmission with shift rails that protruded through the rear of the tranny case, requiring corresponding holes in the mounting face of the Spicer 18. These holes can be a source of leaks when not used with a T-84. These first Spicer 18s ran a small 3⁄4-inch intermediate shaft with bronze bushings and only had 1.97:1 low range with a special 27-tooth x 6-spline input gear and are considered to be somewhat weak. All Spicer 18s with the exception of the late V-6 models have a 3.150-inch input bore diameter and are referred to as “small hole” cases.

2- Early ’46 models increased the intermediate shaft to 11⁄8-inch with caged needle bearings on the intermediate gear. The low range was upped to 2.43:1 and the input gear was 26-tooth x 6-spline. Note, early 11⁄8-inch cases still have vestigial T-84 shift rail holes, so Tillford speculates Spicer merely bored leftover 3⁄4-inch hole MB cases.

3- The later ’46-’55 models were essentially the same as the early ’46 models, but the T-84 shift holes were gone and a solid T-case to transmission mounting face helped prevent leaks when bolted to a T-90 or other transmission.

4- The late ’55-’71 models received a larger 11⁄4-inch intermediate shaft that utilized free roller bearings inside the intermediate gear for more strength and longer service life. The input bore diameter remained at 3.150-inch and input gear was 29-tooth, 6-spline, although pickups and many later four-cylinder CJs received the “large hole” 4-inch bore case. Low range for the 29-tooth cases is 2.46:1.

5- All ’66-’71 V-6 models got the “large hole” 4-inch input bore cases with slightly heavier construction more akin to a Dana 20 case and a 11⁄4-inch intermediate shaft. The input gear is a 29-tooth, 6-spline with the T-86 and 29-tooth, 10-spline unit with the T-14 transmissions.


Murray, UT 84107
Novak Conversions
Logan, UT 84321
Crown Automotive Sales
Marshfield, MA 02050
ATV Manufacturing

Connect With Us

Newsletter Sign Up

Subscribe to the Magazine

Browse Articles By Vehicle

See Results