Teraflex Manufacturing Dana 20 Low Range Kit

Part 2 of the Uber-20 project
(Building the ultimate low geared transfer case at home)
By BillaVista

Page 7 - Checking Case Operation, Testing, Results & Contact Info

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Checking the case operation

To check the case operation and ensure that you have all the parts in the right place, proceed as follows:

  1. Assemble the case completely, except for the bottom inspection cover.
  2. Place the case upside down on the bench with the shift rods towards you.
  3. Place some sort of support (a chunk of 2x4 worked for me) under the rear output housing so that the rear output flange can turn freely.
  4. Arrange the case so that you can turn the main drive gear (input gear).  I accomplished this by bolting my doubler adapter to the case and turning the NP 203 output gears by hand - thus driving the input gear of the transfer case.
  5. Dana 20 shift positions are controlled, via the shift rods and shift forks, by the positions of the front and rear sliding gears. For both sliding gears, towards the front of the case is high gear, in the middle is Neutral, and to the rear of the case is Low gear.  I deleted the interlock pill on reassembly, enabling "twin stick" operation - i.e. the ability to shift the front and rear outputs independently.
  6. With the transfer case installed the right way up in the vehicle, the following are the shift lever, shift rod, and sliding gear positions for each gear possibility (interlock pill removed).  Note that these listings are for the case "as installed" in the vehicle, with the CAN/US drivers position the left of the vehicle. Note that because the fulcrum of the shift levers is above the shift rods when the transfer case is installed in the vehicle (i.e. the shift levers pivot about a point that is above the shift rods), the shift lever positions are the OPPOSITE of the shift rod positions for high and low gears.  With the transfer case on the bench, make sure you can smoothly shift between all of the following positions, with no binding, ratcheting, grinding, etc.  It shouldn't be hard to shift - you should be able to do it with strong hand pressure on a punch or lever inserted into the front shift linkage hole of each shift rod.  You may have to rotate the gears slightly to get the teeth to line up as you shift.
Gear Rear (left) shift rod & sliding gear position Rear (left) shift lever position Front (right) shift rod & sliding gear position Front (right) shift lever position Picture
Rear Low rear forward middle middle
Rear High forward rear middle middle
Front Low middle middle rear forward
Front High middle middle forward rear
4 Low rear forward rear forward
4 High forward rear forward rear
Neutral middle middle middle middle


Once you are done with checking, install the lower cover using RTV and torque the cover bolts to 15 ft lbs.
Detail of my home-brew shift linkage.  This pic illustrates how, when you shift the lever forward, you are actually shifting the shift rod to the rear.



From left to right are:

  • NP 203 low-range box shifter
  • Dana 20 rear axle shifter
  • Dana 20 front axle shifter

Note that if you remove the interlock pill and twin-stick your case, there is the possibility to shift one axle into low gear and the other into high at the same time.  This will quickly cause serious drivetrain damage (although I did try it briefly on a soft, loose surface, just to see what it was like.)  Some folks may decide to rig up some sort of extra mechanism to allow twin -stick shifting while preventing the levers from being more than one position apart.  I didn't bother, as the shift pattern arrangement would mean that to inadvertently shift into low-high or high-low, one stick has to be all the way forward, and one all the way to the rear - which is really obvious, and hard to do accidentally.  Also - if the rig is in low-high or high-low, it's VERY obvious as it hops and spins and churns in a very unusual manner.

Installing the case in the vehicle

This was probably the hardest part of the whole project.  Here you can see my complete doubler transfer case assembly ready to be installed - it's one heavy SOB, and pretty tricky to get in place, lined up, and the bolts all started without cross-threading!. 

As with all the other mating surfaces, you must gasket the transfer case to transmission joint.  This was the one place where a decent gasket would be helpful, as it would prevent inadvertent smearing of RTV as you jack, strap, and struggle to get the case installed and lined up.


 I ended up having to cut a hole in the floor...
...then putting one strap around the roof of the Wolf, with another ratcheting strap extending vertically from there, through the hole in the floor, to a chain wrapped around the transfer case.
Then with a floor jack on the output housing for additional support and to help clock the case to the right position, I was able to get it all lined up and installed.


The most important part - what are the results of this upgrade?

What you gain is:

But the greatest improvement relates back to what I was saying at the beginning about "gear stepping" or "ratio spanning".

Recall my original gear table:

Final ratio % Jump
6:1 50
8:1 33
13:1 63
15:1 15
26:1 73
29:1 12
54:1 86 !!!
59:1 9
107:1 81 !!!


What I was really missing was a gear between my low / low and the next lowest gear. That is, something between 59 and 107, as well as something between the next 2 gears i.e. (29 and 54). I knew this just from driving the rig - even before charting the math involved.  In other words, my lowest gears (deeper than about 25:1 which is what I use most on the trail) used to be

27, 29, 54, 59, 107.

Now I have

27, 29, 41, 46, 54, 85, 92, 169

You just don’t get choices like that with a fancy aftermarket transfer case. If I had, for example a 4.3 low ratio Atlas II, I would have:

27, 28, 63, 115, or in effect, only 3 really usable gear ratios, for twice the price. To me, that is far from the ideal setup.

In my opinion, it’s all about 3 things:

- Options for different terrains
- Ratio spanning or % jump between ratios; and
- The lowest low you can get

Judged by that criteria, and by real world budget constraints, with the option of building over time, the low buck doubler with aftermarket gears in one transfer case comes out as an awesome option - and one I am extremely pleased with and don't regret one bit.


The rig is now much more of a pleasure to drive.  Whether you're driving a performance car, a rock buggy, or pedalling a bicycle - huge jumps in gear ratio suck!

In the kind of rocky, slippery terrain I prefer, shown here, the extra gears, and especially the way low 169:1 give me great improvement in control and drivability

Time will tell with regards to durability - but going by the apparent quality of the components, as well as my initial testing - I don't expect any problems.

As these pictures show - I got pretty wedged-up trying to climb this waterfall.  I tried virtually every gear combination as well as tactics ranging from hammering down in high range, to crawling in low/low.

In fact - I hammered hard enough to tear chunks in my Michelin XML military tires like this (and anyone with experience with these tires can tell you how hard it is to do that - they are seriously tough!), tear one completely off the rim, and even broke a rocker arm and spun the main and con-rod bearings in my engine trying to get free.

My drivetrain suffered no ill affects at all (except the motor) - Based on this - I'm pretty confident the Uber-20 can hold up to my abuse.

To get your Teraflex Low20 kit, contact Teraflex Manufacturing:

Contact Info:

Tera Manufacturing, Inc.

5251 South Commerce Dr.
Murray, UT 84107-4711
phone/801.288.2585 -- fax/801.288.2571


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