Project Jeep CJ-7
TeraLow Gears
in the Dana 20 Transfer Case

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The Need for less Speed - With 3.07 gears in my Dana 44 axles and 2.03 gears in the Dana 20 transfer case, one or both had to be swapped to improve the offroad gearing.  The T-18 with a 6.32 to 1 1st gear made 4 wheeling bearable, especially with a hand throttle controlling a strong AMC 360 V8, but rock crawling was sometimes a little harsh and I knew lower gears would mean better control. 

So there was going to be a gear ratio swap in either the Transfer case or the differentials, or eventually both.   But money is always an issue.  An axle gear swap would definitely be the more costly upgrade.  So I brought up the handy little crawl ratio calculator and did a few gear Ratio comparisons. 

Changing the transfer case gears from 2.03 to 3.15 has a better net gain towards a better crawl ratio than lowering my axle gears to 3.73.  3.73 because that is the lowest I can go and keep my Detroit Locker.   Additionally, I could keep a decent high range for good highway driving and without losing strength in the ring and pinion set.  For my type of offroading, which is Mountain trails, mud and occasional rock crawling, I actually didn't want to go too low.  So the math came out like this:

As the math figures, the Teralow yielded a better crawl ratio than an axle gear swap.  When I eventually reduce the gears in the axles to 3.73, I'll have a very nice 74:1 ratio.  But for now, the Teralow seems to be the most cost effective way to get a better crawl ratio.  On top of that, I can do install the new gearset myself.

The following are my notes on this Teralow Dana 20 gear swap.

IMPORTANT: I'm not an expert!  I'm an average backyard mechanic.  These pages are not meant to replace the manufacturer's instructions, rather are just my account of the installation for your amusement only just like the rest of the project and the website for that matter.  You might find a few tips in here and and maybe even a few blunders.  If you choose to engage in these activities or undertake any project, it is by your own free will and at your own volition. I assume no liability for how any particular individual chooses to use the information presented here or anywhere else in the pages of this project.

Whenever I get a new toy, I like to inspect the parts and take inventory.  The kit comes with a set of 5 new gears, needle bearings, a new center shaft, and new seals and gaskets.  The gear set is impressive when compared to the stock gears and are a little more beefy than the stock gears.  I also noticed that the gear teeth are a little more finer with more teeth than stock, and thus how the kit attains a lower ratio.
Before you get started, it's a good idea to look over the exploded view diagram and familiarize yourself with the parts and their names.  Post the sheet in plain view while you're working on it so you can glance up to see the diagram when you need to.
TIP: Use plastic Ziplock bags to store your bolts and small parts in and label each bag with what the part is and any other important note you may think of.  It might be weeks before you get it all re-assembled and people tend to forget the details.  I know I do.    ANOTHER TIP: Take Pictures!  I can't tell you how many times I've gone back to refer to pictures (digital pics are nice, no film) when I can't remember how something went back together or what part in the diagram was what part in reality.   Transfer Case Removal Irony - Though I had planned to do the Teralow kit eventually, ironically, the whole reason that prompted me to get the Teralow kit was because I had a transmission problem, or so I thought.  On my last trip the sound of gears grinding on gears was coming out of the transmission.  After removing the top of the T-18, then removing the transfer case, I found that the adapter between the two was coming loose.  Bolts were backing out and the T-18 upper gear set was drifting backwards, causing the sound of grinding gears.  With the top of the transmission off, I could see this occurring.  I tightened the adapter and the sound went away.  So now that I have the Dana 20 out, I might as well do the gears as I've wanted to do for a while.  I called Mike at OK 4 Wheel Drive, who gave me a great deal on a Teralow Dana 20 kit.  If you are looking for anything 4 wheel drive related it's worth it to at least give these guys a call for a price. (OK4WD - 908.454.6973).  The following pages document the installation of the Teralow Dana 20 gear set.
Removed the Dana 20    1.   All 4 of the Jeeps tires were chocked very well so there was not possibility of it drifting.  Also the Transfer case lub was drained.    2.    As I said, I had the transfer case out already but in the process of doing so I had removed the gear shift cover plate in the transmission hump, the shift lever assembly.    I found a nice way to support the transmission while the drive train skid plate was removed.  Using a piece of 1/4 inch angle iron across two floor jacks it supported the T-18 transmission directly so I could remove the Dana 20.  During the process when it wasn't in the way I kept a floor jack under the transmission mount just in case.  Redundant support is a good idea for safety reasons.    3.   I removed the drive shafts.    4.   I disconnected the speedometer cable at transfer case.    5.   I removed transfer-case-to-transmission attaching bolts and muscled the transfer case out and onto the floor.  It's a good idea to have eye protection on while under a vehicles, especially my Jeep with all the mud and crud that falls out when I'm working on it.    6.   While I was down there, I remove transmission-to-transfer case gasket.  I had also discovered that the transfer case to transmission adapter was loose, thus the cause of the noise.  When I tighten it up again, I'll clean the bolts and treat it to some Locktite so they don't back out again.       With the transfer case on the bench it was time to break it down.   Disassembly NOTE: Refer to exploded view for part identification. 1.   Removed bottom cover and gaskets. 2.   Removed bolts attaching rear bearing cap assembly to transfer case and remove assembly. 3.   Remove the main shaft gear through front of case.      4.   Remove intermediate shaft lock plate. 5.   They recommend using brass drift and a plastic mallet to drive intermediate shaft out of rear of case.  I used a block of wood and a framing hammer.  When the wood started shredding, I finished the job with a long bolt making sure not to nick the case where the shaft goes through.  The kit has a new intermediate shaft so I wasn't worried about it.    6.   Then I removed the intermediate gear assembly through bottom of case collecting all the bearings and washers.  These will be replaced. 7.   I removed the front output shaft nut and washer.  I had to use a piece of wood to stop the gears from turning. 8.   I removed the front output shaft yoke.      9.   I think removes the cover plate attaching screws and removed the cover.      CAUTION: When removing cover plate, take care to avoid damage to the shims between the cover and case.      10.   I positioned both shift rods in neutral.   11.   I then removed the rear output shaft shift fork setscrew.    12.   Removed the plugs that cover access hole to the poppet ball and spring.    13.   Inserted a punch through pin hole in rear shift rod (the one closer to the center of the case) and rotate rear shift rod 1/4-turn counterclockwise and pull rod out of case.   NOTE: When shift fork is free of rod, use hand to catch poppet ball and spring under shift rod.    14.   Removed the front shift rod housing attaching screws and slide housing from remaining shift rail.   NOTE: When housing is free of the rod, use hand to catch poppet ball and spring under shift rail.    15.   Using hammer and brass drift, drive front output shaft towards rear of case.  Teralow's instructions say "rear output shaft" but if you look at the diagram, you'll see it the front output shaft.  I had to make sure I was doing what they suggested for disassembly. It helps if the terms are correct in the instructions. I found that the I would be driving the gears against the case so I put a few pieces of wood between the gears and the case to I wouldn't be damaging anything by driving the output shaft back.       16.   Remove gears, spacer, and thrust washer from inside case and rotate shift rod to expose the setscrew.    17.   The next step is to remove setscrew and pull out the remaining shift rod.  I had a hell of a time trying to remove the setscrew.  I decided to leave the shiftrod in the case and just work around it rather than risk breaking something trying to get the screw out.  Having the shiftrod there turned out to not be a problem.  I just had to clean around it and work with it there.  So you'll see shots with the rod still in the case when it shouldn't be.  If you can get yours out, do it.
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Items underlined and bold below are included in the Tera Low20 low range gear kit