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Tera Low Dana 20 Gears
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By: John Nutter - 7/2000

Project DIY gets Tera Low Transfer case Gears

Project DIY has benifitted from a set of Tera Low gears for it's Dana 20 transfer case for a year now. These gears yeild a 3.15:1 low range while leaving high range at 1:1. The results of this gear swap have been fantastic, even when compared to the previous 2.46:1 low range gears that DIY used to run. With streetable 3.54 axle gears, DIY is still able to acheive a 70:1 crawl ratio.



Tera gear set for Dana 20 transfer cases. The Dana 18 gear set is similar, but needs one less gear.
Tera gears next to factory 2.03:1 low range Dana 20 gears. The shiny new gears are from Tera. Note how much thicker the teeth are on the Tera sliding gears compared to the originals.
Tera gears next to 2.46:1 low range Dana 18 gears. The sliding gears are nearly the same size, but the Tera gears have much thicker teeth and a higher tooth count for greater strength.
2.03:1 low range sliding gear laid over a Tera sliding gear. The Tera gear is significantly larger.
2.46:1 low range sliding gear laid over a Tera sliding gear. The Tera gear is slightly smaller than the 2.46's gear, requiring less grinding on the case itself.
Arrows point to the ridge I had to remove with a grinder. Your particular case may require more or less grinding.
Tera gear slid all the way onto the rear output shaft. The arrow points to area where the ridge was ground away.
No grinding was required on the top of the case, unlike my 2.46:1 conversion.

Low gears are very important for nearly every type of 4 wheeling and they are especially important for rock crawling. Changing the transfer case gears in a 4x4 equipped with a Dana transfer case is probably the easiest and most cost effective way to achieve a substantial gain in crawl ratio without ruining highway drivability, or sacrificing strength in the rear end gears, thanks to Tera Manufacturing.

Tera has a 3.15:1 low range gear set available for all Jeep, Early Bronco and Scout Dana 18 or Dana 20 transfer cases. The gear set replaces all five gears in Dana 20 applications and all four in Dana 18 applications. The kit also includes a complete gasket kit, small parts kit and new 1-1/4" center shaft.

Gear Ratio Comparisons

Trans. (1st gear)Low Range Gearingaxle gearsCrawl Ratio
T18 (6.32)2.035.3869:1
T18 (6.32)3.153.5470:1
T18 (6.32)3.155.38107:1
3 Speed (~3:1)2.035.3833:1
3 Speed (~3:1)3.153.5433:1
3 Speed (~3:1)2.465.3840:1
3 Speed (~3:1)3.155.3851:1

Click here for more numbers

Changing the transfer case gears from 2.03 to 3.15 has the same effect in low range as changing from 3.54 to 5.38 gears while leaving high range unchanged for good freeway driving and without losing strength in the ring and pinion set. Changing from a 2.46:1 low range to 3.15 is nearly the same as going from 4.56 to 5.38 gears. Three digit crawl ratios are possible with the use of a compound low type transmission such as a T-18, the Tera 3.15 gear set and 5.38 axle gears. A very respectable 51:1 crawl ratio is the result of using the Tera 3.15 gear set in an early CJ with 5.38 gears and a three speed transmission.

Installation

The gear set arrives in a well designed shipping box with foam inserts to protect the gears from damage during shipping. A gasket and seal kit, small parts kit, new center shaft and needle bearings, the gears themselves and very detailed instructions are contained in the box. The instructions cover the disassembly and re-assembly of the transfer case and are very thorough. No special tools are need to disassemble the transfer case, but a short length of 1-1/4" OD pipe, tubing or wooden dowel is necessary to hold the needle bearings in place during assembly.

Assembly is similar to rebuilding the stock transfer case except that minor grinding may be required for gear clearance. I found that the Tera 3.15 gears required less grinding on the case than the 2.46 gear swap using Early Bronco and Dana 18 gears. The pictures show where the grinding was required on my particular transfer case. Some cases will need more grinding than mine did, some will need less. The instructions from Tera have a clear diagram showing where to grind for those that are not sure. Remember to do the grinding away from the area where you are working on the transfer case and to clean the case thoroughly before test fitting the gears.

Early Dana 18s from 1966 or earlier with the small index hole will require a large hole Dana 18 or Dana 20 casing becuase there isn't enough room for the intermediate gear in a small hole case. Dana 20 transfer cases are inexpensive to buy used, and all the Dana 18 parts will transfer in. This doesn't add much complication to the gear swap. Disassemble the Dana 20, set the Dana 20 parts aside, and rebuild the Dana 20 using all of your Dana 18 parts and the Tera gears. The 2wd/4wd mode shift rail will require triming to fit past the larger intermediate gear on Dana 18 applications. You will also need a bearing retainer for a large hole Dana 18. This bearing retainer is also availablefrom Advanced Adapters if your Jeep dealer can't locate one.

Driving Impressions

After a year of use I am still very impressed with these gears. I had to drop the bottom cover off of my transfer case when working on the drivetrain of DIY and no gear wear was visible. Street driving is the same as it always has been. The ability to keep my 3.54 axle gears has really made pavement runs to the trail head a much more enjoyable experience. There is no need to creep along at 50 mph like 4x4s with 5.38 axle gears and no overdrive are forced to. I am actually able to run on the freeway for long periods and have driven Project DIY on freeway trips over 100 miles each way with no problems.

Off-road is where the gears really shine. While 70:1 may not be low enough to enter in ARCA contests, it has been plenty for trail riding and rock crawling. Even on trails like the 21 Road and Upper Helldorado, 70:1 was enough. In the Midwest the gears have been great. Second or third gear low range work well for sand or snow running where wheel speed is necessary. So far I've only found one instance where I had to go into high range to get enough wheel speed to back out of a stuck.


Contacts: Related Links:
  • Tera Mfg. Inc.
    Dept ORN
    8385 S. Allen St., #115
    Sandy, UT 84070
    Phone 801.256.9897





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