Making a “Good” Rig “Better” – Part 1: Transfer-Case

The Achilles heal in any modified Jeep (at least the older ones) will be its transfer-case. This is due to the one-piece rear driveshaft and the slip-yoke output of the case. Once you lift a Jeep more than 2″ you start running into driveline issues caused by the misalignment between the output of the transfer-case and the pinion yoke of the rear axle. This is less noticeable on Jeeps with a longer wheelbase (XJ/MJ, ZJ/WJ, LJ, etc) but it’s only a matter of time before that slip-yoke stops slipping and starts vibrating. This is also not an issue for new(er) Jeeps (JK’s, JL’s, and JT’s) which switched to a fixed flange style output and a two-piece shaft from the factory (they finally learned their lesson).

So many parts.
SYE, overhaul kit, 6-gear planetary and wide-chain kit: JB Conversions
2wd low-range shift kit: Tera Flex

This misalignment issue is compounded by the one-piece rear driveshaft. As the rear suspension cycles up and down the distance between the output of the transfer-case and the rear axle yoke changes. To make up the difference the slip-yoke slides in and out. While this works, in theory, it’s not ideal because the lateral slip movement of the yoke is not within the axis of rotation of the driveshaft. Any binding that might occur due to friction within the slip-yoke or the u-joint between the slip-yoke and the driveshaft will cause a vibration.

For a while I was running a modified transfer-case that lacked a slip-yoke. This was done by installing what is known as a SYE kit or a “slip-yoke eliminator.” There are two types of SYE’s out there. There is the cheap and easy one known as the “hack-and-tap” method which simply involves drilling a hole through the slip yoke (the hack) and then running a bolt (the tap) through it locking the slip yoke to the output shaft of the transfer case. The more expensive, and better, option in my opinion is a full SYE kit that replaces the output shaft of the transfer-case and allows for a standard yoke or flange to be attached. There are a few versions of these base on the length of the SYE. By installing an SYE you can then switch to a two-piece driveshaft that has a built-in slip feature that is within the axis of rotation of the driveshaft. This mitigates driveline vibrations as the suspension cycles during normal on-pavement and off-pavement travel.

SYE (left) vs Slip-Yoke (right)

Sadly my original transfer-case with the SYE installed (pictured above left) starting having problems (wait till you see how bad). More than likely the idiot who rebuilt it (me) missed a step during the rebuild or something else went wrong (spoiler alert – not entirely my fault). This meant I had to pull the case out and replace it and its two-piece driveshaft companion with a stock transfer-case with a one-piece shaft. This was fine for daily driver duty (for awhile) but, as I said before, it was just a matter of time before a vibration showed up.

That is the issue my Jeep has been facing the past few months. At first it was a minor hum, then a minor vibration, and now a not-so-minor vibration. I’m pretty sure I know it’s the transfer-case because I can feel the vibration in the seat of my pants rather than the steering wheel. I can also feel my transmission and transfer-case shift levers vibrating when I hold them. Another clear indication this is not something in the steering or front suspension. In fact, I think the steering and front suspension of the Jeep is one of the “better” aspects of this rig that won’t need fixing.

The first red flag something major was wrong
The magnet was a solid disc of shavings and glitter.
Even the pickup for the transferase pump was clogged with metallic “mud”

Now I have a few options at this point. The fantasy would be swapping the stock 231 series two-speed transfer-case for a four-speed Atlas transfer-case. This would not only give me more gearing options but it would also give me the ability to run “twin sticks” and do cool things like front-wheel-drive in high or low or two-wheel-drive in low. Sadly I don’t have the money — or time — to do an Atlas swap right now. Being stuck with a 231 I can either rebuild the old case with the SYE I took out or install an SYE in the stock case I’m running right now.

Disassembly continues
A parts cleaner, or at least a lot of brake clean, goes a long way in cleaning up the housing halves.

Now, if I’m honest, I’m sure neither transfer-case is 100% at this point. I think I could very easily combine the two of them, plus a few replacement parts (bearings, seals, etc), and come up with single working transfer-case. That would be pretty easy. However, when have you know me to stop at easy? Since the transfer-case is already out, and will be torn apart anyway, the time is right to do a few upgrades.

Upgrade #1: SYE & Overhaul Kit

So the obvious upgrade that has to be (re)done is an SYE. And since both cases are “questionable” might as well get an overhaul kit. This is the minimum starting point for any 231 rebuild. In fact, I would argue that even a stock Jeep could benefit from an SYE and one-piece shaft. Since I have another LJ (the 2006) whatever is left from this rebuild project will get (re)built into a spare for the other LJ.

New SYE output shaft (left) vs old SYE output shaft (right)
A stock slip-yoke shaft would be longer and instead of a threaded tip at the top it would be splined
Probably could have reused the old SYE kit, but opted to use the new one on this case
Will save old SYE parts for the t-case in the 2006 LJ (because why not?)

The overhaul kit includes all the small bits and bobs you need — or should install — when doing any work on a transfer-case. This might be my issue with the first case I did the SYE on because I reused all the internal parts. (Yes kids, do as I say not as I do.) Learned my lesson so this time around I got an overhaul kit to cover what the SYE kit does not. Also, it’s nice to have spare snap-rings for when one inevitably goes shooting across the shop and lands in some dark dusty long forgotten corner to remain for all of eternity and remains untouched as the memory of man begins to fail. Anyway…

Upgrade #2: Wide Chain Kit

There are two upgrades that are easy and a no-brainer for any Jeep with a 231. The first of those is a wide chain kit. The 231 series case was not only in Jeeps but it was also in Dodge trucks. Since Dodges had optional V8’s in both a 5.2L (318 c.i.) and 5.9L (360 c.i.) their transferases needed to be a little more robust. The chain in question connects the input and output shafts of the transfer-case thus allowing for the single input from the transmission to get split 50/50 between the front and rear outputs. It’s rather simple: a wide chain can handle more torque. Most wide chain kits boast over a 100% increase in torque capacity. Since my Jeep now has a 5.3L LS V8 under the hood I deemed this a rather essential upgrade.

Stock Chain (top) vs Wide Chain (bottom)
You can also see that the stock chain is “stretched” compared to the new chain.
This is (probably) what contributed to the chain jumping off the gears
(and not a result of my own incompetice – right?)
This chain was off the gear and you could see where the output gear was chewing up the chain.
Fun fact – without lockout hubs on the front axle the front driveshaft and output of the transferase will spin going down the road even while in 2wd.
Front output gear for the stock chan (left) vs front output gear for the wide chain (right)

Upgrade #3: 6-Gear Planetary

Time for the second essential “easy” upgrade. Within the anatomy of a transfer-case the part that provides the gear reduction for low-range is the planetary gear assembly. In the Jeep version of the 231 this assembly has three planetary gears. In the Dodge version of the 231 the planetary gears are doubled for a total of six. This upgrades the maximum torque the transferase can handle. Twice the gears = twice the torque, right? Maybe not exactly, but even at a 50% upgrade over the stock assembly it would be more than enough for the power my stock 5.3L is putting out.

Stock 3-gear Planetary (left) vs 6-gear Planetary (left)
The input shaft was already transferred over to the 6-gear planetary.

Upgrade #4: 2Low Shift Fork

Now I could stop with the above mentioned upgrades and already be way ahead of where I’d be with a stock transfer-case. However, this is a classic case of “while I’m in there” adding yet another modification/upgrade to the list. One of the main feature of the Atlas transfer-case I’d love to have is the ability to shift into two-wheel-drive low-range. This would get me all of the benefits of the mechanical advantage provided by the transfer-cases gearing (which for a 231 series case is 2.72:1) without engaging the front output which would limit my turning radius. This will come in very handy when I am towing the trailer because I can mitigate the load on my transmission and brakes without the front end binding on me.

TeraFlex 2low shifter (left) vs stock shifter (right).
The slot controls one axle, the outer profile controls the other.
The jagged “teeth” across the top are indents for shift lever.

The upgrade I am *not* doing

There is one other upgrade I could do if I wanted to. There is a 4:1 gear assembly I could install that would replace the stock 2.72:1 gear assembly. While this would be great for rock-crawling it would be murder on the drivability of my Jeep under normal trail riding conditions. 4:1 in both the transferase and axles in my Jeep, combined with the torque of a v8, would just make “slow” trail riding “too slow.” Also, the aftermarket 4:1 kits have an RPM limit to them. Not a big deal within the confines of a small off-road-park (with a speed limit of 10 mph), but become more of an issue when you have a lot of ground to cover (when forest service road speed limits are 35+ mph).

<Not pictured because I didn’t do it – DUH!>

Thoughts on Install

With some help from Peter at Aylestock Automotive we were able to rebuild the transfer-case in a solid day’s work — about 6 hours total. It’s not a challenging process other than paying close attention to the details and remembering where each snap-ring, gasket, and o-ring goes. Each upgrade part comes with detailed instructions. As you go through the rebuild process you just switch to those instructions when it comes time to do that particular part. The nice thing about the four upgrades I chose is they require no modifications to the transfer-case housing halves. Just disassemble and reassemble per the overhaul instructions while substituting the new parts along the way.

Peter making quick work on breaking down the transfer-case.
In this pic the old SYE kit has already been removed.

That being said, for the bulk of the rebuild we followed the instructions for the Teraflex 2low kit. This gave a pretty comprehensive set of teardown instructions since the shift fork is buried pretty deep in the case. The only things not removed during that install is the planetary gear assembly and input gear. At that point we just switched over to the overhaul instructions.

All done. Looks a lot like it did before, right? But the insides are totally different.
And like ma’ma always said, “It’s what’s on the inside that counts.”

Now that rebuild is done all that’s left is to remove the current transfer-case, install the rebuilt transfer-case, reconnect the shift cable, and then reinstall the two-piece driveshaft I was running with the old SYE equipped transfer-case. The next step is to rebuild the left over parts into a working spare for my ’06, but that sounds like another project for another day.


All parts featured in this article were purchased. No affiliation between ECOA or JB Conversions, Teraflex, or Quadratec exists or is implied.

Huge thanks to Peter at Aylestock Automotive for the continued support of the ECOA mission to Educate, Encourage, and Inspire!