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First you may be asking "What IS a slip yoke eliminator kit?" also commonly referred to as a SYE kit. The name is self explanatory, it eliminates the stock slip yoke on the Jeep NV231j transfer case. So the question is, what is a slip yoke? To understand what the slip yoke does, I'm going to explain a few other parts on your Jeep. First the drive shaft. The rear drive shaft connects your transfer case to your rear axle, which essentially drives your jeep while in 2 wheel drive. If you remove this shaft, your jeep would go nowhere (once again, we are talking 2 wheel drive). Since your rear axle is live, meaning it is supported by springs and can move, this poses a problem for a solid rear drive shaft. The drive shaft needs to be able to get longer and shorter depending on the position of the axle. For example, when you hit a speed bump, the axle moves up in relation to the frame of the Jeep causing the rear axle to move further from the transfer case necessitating a longer rear drive shaft.
On a stock Jeep, this is where a slip yoke comes in. The yoke is the part of the drive shaft assembly that holds the u-joint. Look at the picture to the left (click on it to make it larger). I circled the slip yoke in two colors green and red. The combined green and red is the whole slip yoke, the green part is the "slip" part and the red part is the "yoke" part. The green part is covered with a rubber boot so that dirt does not get into the splines of the shaft and also keeps the grease in so that it can move freely.
Scroll down and look at the picture in Step 3 and click on it. Do you see the splined shaft sticking out of the case? Now look at the picture of the two drive shafts on the right. The top drive shaft is the stock one. Notice where I drew a red line? That is the slip yoke without the boot. That yoke slides over top of the splines in the picture from Step 3. This is what gives your Jeep's drive shaft the ability to accommodate the up and down motion of your suspension. If you look at the lower drive shaft (the white one) you will notice once again the red line and a black rubber boot. In this drive shaft the "slip" ability has moved to the middle of the drive shaft itself. This allows us to have a longer drive shaft with better operating angles and a decreased chance of vibration. "Vibration?" you say "but I don't have a vibration." A stock Jeep does not have any problems, and this setup works fine. However, if you lift your Jeep, you will find that by increasing those drive shaft u-joint angles, you will start to incur vibrations while driving. These are caused by the improper u-joint operating angles. Whether or not your Jeep will have these vibrations depends on a few factors: year and model of the Jeep, the engine, the transmission, the rear axle and finally the amount of lift. The best person by far to talk to about your specific Jeep and operating angles is Tom Wood of Tom Wood's Custom Drive shafts. You can also check out his web site for a more detailed explanation of these angles.
Now with the basics out of the way, let's talk about Tom's kit and how it differs from other kits on the market. In my opinion there are two basic types of slip yoke eliminators. First there are those that simply eliminate the slip yoke and give you a longer working drive shaft. Then there are kits that eliminate the slip yoke but also strengthen the output shaft of the transfer case by replacing it with stronger parts. This type of kit is considerably more expensive than the first type. Tom's kit falls into the first category. He uses the stock output shaft of the 231, cuts it down, splines it and finally taps the end for the retaining bolt. While his setup is probably slightly stronger than stock, it does not significantly improve the strength. That being the case, even Tom does not recommend using a 4to1 low range with his kit.
There are two other kits that fit into the first category, the Rubicon Express RE1807 SYE kit for the TJ and the Currie "Tail Shaft Kit." All of these kits are intended for the same purpose, to inexpensively reduce the rear drive shaft length. So what I would like to do is compare these kits and see what we come up with.
First RE's (Rubicon Express) kit is the least expensive and currently runs about $99, followed by Tom Wood's kit for $150 and finally the Currie kit at $179.95. Note none of these prices include core charges, this would be the final cost for installing the kit at the time of this writing. All these kits use a stock main shaft. Tom Wood's and Currie's come pre-cut and drilled/tapped. RE's require you cut, drill and tap yourself. Each kit comes with a replacement yoke. Rubicon Expresses and Curries come with an aluminum flanged style yoke and Tom Wood's comes with a cast Dana yoke. I would choose the yoke with Tom's kit over the flange style, however they are both plenty strong. For overall length, Tom Wood's kit will provide you with about 1/8" shorter over the Currie kit, I don't have a newer style RE to compare lengths, so I can't comment on that. However I would imagine you would get a longer drive shaft, shorter setup with either the Tom Wood kit or the Currie kit over the RE kit since the yoke is flushed into the tail housing.
The installation for these three kits is essentially the same. However, some have (including me) installed the RE kit without opening the transfer case, but I would not recommend this. To install the RE kit properly, you should remove the output shaft from the case, cut it properly (I.E. using a lathe), or something that will keep the cut straight. Then be able to drill and tap it using a drill press.
Ok, enough with all the spec talk right? I think the main difference in the 3 kits is the engineering of how the yoke is bolted to the shaft. They all use the stock shaft, and as long as care is taken to cut, drill and tap the stock shaft for the RE kit, they will all be equal in strength. Tom Wood's kit and Curries kit do have a slight advantage that their yoke is preloaded against the bearing in the tail cone. You won't go wrong with choosing either Tom Wood's kit or Curries kit. In my view they are nearly identical and accomplish the same task with equal strength, so for $30 less I would choose Tom Wood's kit. As for the RE kit, the $50 cost savings did not seem to make up for having to do the cutting, drilling and tapping of the shaft yourself, which if done improperly can definitely cause you problems.
With all of that out of the way, this is a review for the new Tom Wood's kit. After installing the kit, I took a long 4 hour trip the following weekend. I noticed that fluid was slowly leaking from the rear output seal. At my destination, I took the rear output yoke down and inspected the seal. I had noticed while installing the rear output seal the fit did not seem very tight. I called Tom and told him of what happened and my thoughts on the seal. I immediately pulled out a seal and tail cone and inspected the fit. He then had me put some RTV around the outside of the seal and re-install it. It has yet to leak. You will notice in my instructions below, I added the RTV to the install. Since there is not much to "review" meaning I installed it and it didn't break. I thought a better test would be to see how strong these stock output shaft setups are. While installing the SYE kit I also installed a 2 low kit that allows me to run in 2 wheel drive low range. By transferring all of the torque from the engine to just the rear axle rather than splitting 50% front and 50% rear, I have essentially doubled the torque going through the rear yoke over what a stock transfer case has. This allowed me to test the strength of the rear output shaft with more torque than possible in 4 wheel drive. I hate to admit this, because the thought of it was not entertaining, but I TRYED to break it. Yes, I did the opposite of what we try not to do, break things. I ran some trails in 2 low exclusively, being heavy on the throttle on purpose. I also went to a parking lot with good traction, and while in 2 low mashed the gas repeatedly leaving some nice black marks, but no breaks, no leaks.
Does this mean this kit is as strong as an Advanced Adapters or JB conversions kit? No.
Does this mean this output shaft will never break while riding on a trail or the street? No.
Of course I could have abused this setup much harder, probably one of the easiest ways to get a drive line component to break, is to spin a tire at high RPM, then have that tire grab traction instantly. That is the most sure fire way to see sparks and parts flying. However I don't subscribe to that kind of driving be it on the trail or street, so I saw no reason to unnecessarily abuse the Jeep in this manner. I have had this kit on the Jeep for over three months now, and wanted to make sure no leaks appeared and I had a chance to thoroughly test it's strength. If there are problems with this kit in the future, I will post them here.
I think Tom did a good job of producing a quality product that is affordable and works well. Also I would like to add that Tom Wood has probably the best customer service that I have dealt with, he gave me his cell phone number over the weekend that I was installing the kit just in case I had any questions or problems. Customer service is extremely important these days, and Tom is at the top in that department.
Below is my detailed set of instructions. Tom Wood also has instructions available on his web site at http://www.4xshaft.com
Click on pictures for larger ones with more detail.
Tools you will need for installation:
Step 1
Secure vehicle - while dismantling the transfer case, you will be disconnecting both drive shafts. This will render both "P" Park in an automatic and leaving your manual transmission in gear, useless to keep the vehicle from rolling. Please block all tires before starting the installation. Drain the fluid from the transfer case. Remove front drive shaft and front yoke from the transfer case.
Step 2
You need to move the transmission/transfer case mount out of the way to gain access to the transfer case. Position the jack just underneath the bell housing. Bring the jack up far enough to just barely put pressure on the bell housing. Loosen and remove the 4 nuts that attach the transmission mount to the skid plate. These should be near the center of the plate and recessed. Next remove the 6 bolts that hold the skid plate/mount to the frame. Place the jack stand underneath the transmission mount and slowly lower the jack so the transmission mount is resting on the stand. Next remove the rear drive shaft. Remove the two metal bands that hold the rubber boot on the slip yoke (visible in picture). Then remove the 4 bolts and 2 straps holding the u-joint to the rear pinion yoke.
Step 3
Yours should now look like the picture to the right. You now need to remove anything that is on the outer part of the rear output shaft.
*Note Different vehicles have different setups and this particular 2001 TJ only had this 1 stamped piece(slinger) to remove.
These parts may be difficult to remove, but be careful and do not damage the tail cone while removing them.
Step 4
Next you will need to remove the output shafts rear most seal. This is tricky, but if done correctly is not difficult. I have pasted 2 pictures together to try and show this as best as possible. These pictures are of the tail cone removed during this process and of the seal at the end of the installation. This is just for clarity. As you can see there is an outer lip on the metal seal. Carefully remove the seal using a pry or flat head screwdriver. You can see from the picture the bends in the seal from where the screwdriver was used. Once again, be very careful not to damage the tail cone while removing this seal.
Step 5
Remove any retaining rings that you may find during this process on the outer most part of the shaft. NP231s are different, some have 2 and some only 1. You will discard these rings. Leave the snap ring that retains the bearing in the tail housing. There is no need to remove the bearing or snap ring.
Step 6
Next you will remove the tail housing. First remove the speedometer gear housing. Before you remove it, make sure to mark how it is aligned on the case. You may want to make a mark with tape or a marker. There are 4 positions it can go in. The bolt is shown with a green rectangle around it. Remove the bolt, then pull the speedometer gear housing out and away from the transfer case. Be careful not to damage the o-ring. (Note: The picture shows a later stage of the transfer case, this is just for clarity) Remove all surrounding bolts (3 are circled in red in the picture) and gently pry the tail cone away from the case. This can be difficult and time consuming. Please take your time and try not to damage either the tail housing or transfer case. Be patient and pry slowly.
Step 7
When you click on the picture to the right, you will see a red circle. This shows how large the pump is that will be on the shaft when you remove the tail housing. Also you will see a pointer to the pickup line for the pump. Be very careful when removing the pump to disconnect the pickup line by gently pulling it out. Remember this, because you will be reinstalling this pickup line into the pump upon re assembly.
Step 8
Next you will separate the rear transfer case half from the front. Remove all the bolts shown in red on the picture to the left. One of the bolts is a 12point 10mm bolt. The fingers are pointing to the two locations that you can insert your screwdriver to help pry the case halves apart. Also take note of the green circle, it is drawn around a spring on the shift rail. Do not lose it and make sure it goes back on before re-assembly. Also you will notice a green rectangle. Inside this rectangle is a circular magnet. You can slide it out of the case and clean it. This sits next to the pickup filter for the pump and helps to collect small metal particles in the case. Clean it and replace it.
Step 9
Slowly remove the rear tail shaft, front shaft and chain. Be careful and observant of where the parts are in the transfer case. You will have one shift fork attached to the shift rail that is not visible in this picture. You do NOT need to remove it. It is best to leave all parts in the case. If you do remove it, pay attention to how the parts fit together and their orientation in the case, it is very important. I then replaced the front shaft and looped the chain over it.
Step 10
We will now take the output shaft to a bench where we can work on it. Here the old shaft is shown in a vice. We have just removed the retaining ring that holds the gears to the shaft. Slowly lift off each of the two gears and set them down in their proper orientation. Slide the two gears the same way they came off the old shaft, onto the new shaft. Replace the retaining ring. We had to slightly compress it so that it fit snugly around the shaft when replacing it.
Step 11
With the tail housing now removed from the transfer case, you can install the new seal. Install it in the orientation shown in the picture. Be sure to use some grease on the inside of the seal(shown in red), and use some RTV around the outside of the seal(shown in blue). Also clean off any remaining RTV from the back side of the tail housing.
Step 12
Remove all remaining RTV from the 2 case halves and mating surfaces. Installation is the reverse of the removal.
