Pinion Alignment with Axle Shims on a Jeep Cherokee

Pinion Alignment with Axle Shims on a Jeep Cherokee

When lifting a Jeep Cherokee, the pinion angle changes relative to the transfer case. The alignment must be within the limits of the universal joints to prevent driveline vibrations.

One method of adjusting the pinion angle is to use angled shims. These shims are placed between the spring and the spring perch to rotate the pinion to match the drive shaft.

When a Jeep is lifted three inches or more, the drive shaft angle exceeds the recommended limits for the universal joints. A common method to correct the alignment while retaining the stock drive shaft is to drop the transfer case one inch and use two degree shims or use angled lift blocks like the Rough Country Lift kit includes.

In order to eliminate the misalignment, a Slip Yoke Eliminator kit is often used on the rear of the transfer case. Several versions are available for the NP231 that replace the output shaft with a stronger, shorter shaft giving multiple advantages for one simple modification.

With a Slip Yoke Eliminator kit installed, the standard drive shaft is replaced with a double cardon type like the one used in the front of the Cherokee. In fact, if you have an automatic transmission, a stock front shaft can be used in the rear with a SYE conversion.

The double cardon type shaft however uses a very different alignment from the standard shaft. Quite a bit of axle rotation is needed to align the pinion.

In the standard configuration, the pinion and the transfer case are aligned to be parallel to each other. The idea is to have the same operating angle for both universal joints on the drive shaft.

The double cardon shaft uses a special joint with two universal joints at one end and a singe joint at the other end. The correct alignment is to have the drive shaft parallel to the pinion and the entire angle is made up in the two joints on the opposite end.

For a three inch lift, I found about 8 degrees of shim was needed. This Jeep was fitted with two degree blocks so adding six degrees of angle to the spring pack brought it into alignment.

To calculate the shims needed, you will have to use trigonometry. However it is not too bad using this simple method.

With the transfer case and axle in place, I mounted the drive shaft. The misalignment was immediately evident. I placed a straight edge along the bottom of the pinion and parallel to the pinion shaft. I then measured the distance from the drive shaft center to the top of the straight edge. In my case, I measured to a point 30 inches up the shaft and found it need to be raised three inches to make it line up.

Now divide the distance it is off by the distance up the shaft. In my case it was 3 divided by 30 or 0.1.

To convert this number to the angle, you need to take the arctangent of the number. This is easily done on a scientific calculator or even easier to do using Google. In my case I typed arctan(3/30) in degrees into the Google search line. Google returns this result: arctan(3 / 30) = 5.71059314 degrees. Round it to the nearest degree and try that shim. In my case a six degree shim was really close and worked fine.

To install the shim, you have two options. If you get a steel shim, you can weld it to the spring perch making it a permanent part of the axle. I chose to add it to the spring pack so that it can be easily changed if I want to add longer shackles or different lift blocks.

To install the shim in the spring pack, you will have to disassemble the spring pack. It is generally a good idea to replace the center pin and the shim kit came with new center pins. So you can just cut off the nuts from the center pins. They are often impossible to unbolt due to dirt and corrosion.

Begin by removing the U bolts that hold the axle to the spring. Separate the spring from the perch. Next, use two large C clamps to hold the spring pack together while you remove the center bolt.

Place the new bolt through the shim and then pass the bolt through the spring pack. Tighten the nut. Be sure the fat end of the shim is to the rear to rotate the pinion up for the Cherokee. Align the pin with the block or spring perch and press it back together with a floor jack under the axle. Cut off the excess bolt length on the center pin.

Reinstall the U bolts and tighten them to the proper torque. Give it a good whack with a hammer to make sure every thing seated right and recheck the torque. It is good to recheck the U bolts after the first run as they sometimes move around on the axle tube and loosen up.

Verify the alignment visually or with a straight edge. If it looks right, congratulations! Put the wheels back on and give it a test drive.

Rough Country Suspension Suspension Lift Kit

Installing a Slip Yoke Eliminator (SYE) in a Jeep Cherokee

Installing a Slip Yoke Eliminator (SYE) in a Jeep Cherokee

This weekend I installed in Jenny’s Jeep Cherokee the SYE equipped 231 that I built last fall. Our plan was to use a stock front drive shaft in the rear and to remove the transfer case lowering blocks that had to be used to align the drive shaft previously.

Before I began, I coated my hands with Market America Clear shield.

I began by removing the original transfer case. As described in a previous article I installed the SYE in a spare 231 case. The Slip Yoke Eliminator kit can be installed with the transfer case still in place under Jeep.

I began by draining the fluid from the transfer case. Then I removed the rear drive shaft and disconnecting the front shaft from the T case. To remove the transfer case, I supported the transmission and dropped the cross member. I also disconnected the exhaust that the cat flange to get a little more working room.

Next I used a 9/16 box end wrench to remove four of the six bolts that hold the transfer case to the transmission. I used a socket and long extension to get to the two behind the linkage bracket.

With the old transfer case out of the way, I put the modified case up. I had an assistant turn the output shaft to align the splines while I pushed the case into place. Then I tightened the six nuts that hold the case in place.

I lifted the transmission back into place and reinstalled the cross member. Next I reconnected the exhaust and the shift linkage. Then I filled the case with fluid.

The trouble began when I went to hook up the rear drive shaft. The shaft that I had pulled for this purpose would not reach. It was simply not long enough. It also had trouble sliding on the slip joint. So, I rummaged around the barn and found another front shaft. This one slipped in and out easily and reached just fine.

We bolted up the front shaft and took it for a short test run. A very short test run. As soon as we backed it out of the barn, it was obvious that the advice we had read in an internet forum about what shims to use was completely wrong. The drive shaft angle was completely wrong.

I made a few measurements and found we needed 7 degree shims. A call to our local 4wd shop revealed they did not stock them. We chased a few leads they gave us only to find dead ends. As we were ready to give up for the day and possibly the weekend, Jenny used her new Droid phone to research and found Summit racing, 142 miles away, had them. She called and found they had six and eight degree shims in stock but no sevens.

I expected our trip to Golden Mountain was postponed again, but she jumped in my truck and was waiting for them to open the next morning and had shims back at the shop by noon. Determined to go off roading this one is.

I unbolted the spring pack and installed the six degree shims. With it all back together and better aligned, we took it for another test drive. Much better, but there was still a nasty vibration.

Back in the shop I found that the shaft I had grabbed had a lot of play in the double cardon part. So we swapped her front shaft to the rear and made a test run. This time it was vibe free.

I hammered the other shaft apart and greased the splines to get it to slide. Once it was flexible enough I installed it in the front. A quick test run showed some noise but the vibes were at an acceptable level.

Monday we packed up and drove 111 miles to Golden Mountain Off Road Park in Sparta, TN. She had no problem going seventy miles per hour on the interstate following me with my rig on the trailer.

The SYE equipped Jeep handled very well on the trails. Jenny enjoyed the extra clearance that she got from removing the spacers as well.

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Installing a Slip Yoke Eliminator SYE in a NP231

Slip Yoke Eliminator Installation
Well written instructions came with the kit

First I removed the transfer case from the transmission. This was pretty easy since they were both laying on my shop floor. I did have to jack up the transfer case to get to the bottom bolt. That seems a bit backwards as the top bolts are the hard one to get to with it installed in the Jeep.

Next I used my pressure washer to remove most of the dirt and grease form the outside of the case. There was all colors of mud. I am sure there was some from Harlan, Aetna Mountain and Golden Mountain as well as some other places still on there.

sye install NP231
NP231 transfer case ready for SYE install

Next, per the directions, I placed the transfer case in 4 Low. Next I used a 1 1/8 socket on my air wrench to remove the nut on the front yoke and then removed the front yoke.

Front yoke removed
Front yoke removed

Next, using a 9/16 wrench I removed the selector lever. If I had not moved it to 4 low before removing, turning the nut would have shifted it there any way.

SYE install np231
Shift lever removed

Next I removed the three 15 mm bolts that held the tail housing in place and tapped the tali housing off. This gave access to the snap ring that holds the main shaft to the rear bearing.

tail cone off
Removing the tail cone exposes the snap ring.

With that snap ring off, I removed the 10mm bolts that hold the output housing in place and tapped it off. The speedometer drive had been previously removed.

Output housing removed exposing the oil pump
Output housing removed exposing the oil pump
Stock 231 output housing
Stock 231 output housing

Next I noted the locations of the three different types of fasteners that hold the two case halves together. All but one has a 15 mm head. Two of these have washers and go over the alignment dowels. For some strange reason there is one bolt that has a 10mm 12 pt head and is longer than the rest. It is on the top.

ready to seperate case halves
Note the location of the three different types of fasteners.

With all the bolts out, I used a flat blade screw driver to pry the case apart using the pry slots. I was careful not to pry anywhere else so I would not scratch the sealing surfaces.

At this point, some more fluid came out even though I had drained it earlier. I carefully separated the oil pump from the pick up tube and removed it.

After removing the rear case half, I inspected the chain and the magnet. Even though this Jeep shows over 220,000 miles on the odometer, the chain looked great and there was very little metal on the magnet.

Rear case off
View inside the NP231 with the rear case off

Next I pushed the front output shaft out and removed it with the chain. I inspected both the sprockets and the chain further. Still no visible wear.

Front output shaft and chain
Front output shaft and chain

Next I removed the main shaft from the case. I photographed the shaft so I would know the relationship of the mode selector and drive gear.

Main shaft
Main shaft with mode hub and drive sprocket

The snap ring was just a hair too big for my Craftsman snap ring pliers to open. So I modified the pliers by grinding off the stops just a bit so they would open wider. The snap ring then slid off easily. I was then able to slip off the mode hub and sprocket.

This being an early 231, I had to remove the caged bearing from the sprocket before I could reuse it on the new shaft. I used my ball joint press to press out the two bearings.

pressing out the roller bearings
pressing out the roller bearings

I pre lubed the new shaft and slid the sprocket in place and then the mode hub. I installed the new snap ring from the kit.

Next, I slipped the new main shaft into position in the case. I then removed the old spline seal form the front input shaft and installed it with the chain. I decided to wait until the front yoke is ready to go on to install the new spline seal.

New main shaft in place
New main shaft in place

I then test fit the rear case half to see if the shift rail needed to be modified. Sure enough this is one of the ones that required the rail to be cut as it extends 1/2 inch too far.

shift rail too long
Need to cut 1/2 inch off the shift rail.

I reinstalled the shift lever and moved the case through all its positions to see how the shift rail moved and saw that it only interfered in 4 low. I cut the extra shaft length with a cut off wheel on my grinder after marking the spot with tape.

Cutting the shift rail
Cutting the shift rail

I also inspected the oil pump and saw the pump housing seal was worn and needed to be replaced. I got a new one from Crown along with the proper RTV to seal the case.

New oil pump seal
New oil pump seal

After a final inspection, I applied a bead of RTV to the case and after letting it cure I put the two case halves together.

RTV curing onthe rear case half
RTV curing onthe rear case half

Here is where I hit my first snag of the installation. The oil pump needs to be installed at the same time as the rear case so that the pickup up tube can be inserted into the pump. On a stock transfer case, you can slip the pup off the drive gear and get enough wiggle room to install the tube after the case halves are together. However, on the SYE shaft, the rear section of the shaft is bigger and stronger making it very hard to get enough room to move the oil pump.

There was a bur on the output splines that the oil pump has to slide across and working by myself, I could not hold the case steady and guide the pump down the sticky splines. I went ahead and assembled the case halves, thinking I could install the dip tube later as I always have on stock cases.

I had already torqued the case bolts to 25 ft lbs before I realized I was not able to move the pump like I was accustomed to. However, with some fishing with a dental pick and a small screwdriver, I was able to install the pick up tube and eventually get it seated in the pump. Next time I will get help if necessary to make sure the oil pump and case go together as a unit.

Oil pump in place
Oil pump in place

Next I placed one of the snap rings for the speedometer drive gear on the shaft, then slipped on the blue drive gear followed by the other snap ring.

Speedometer drive gear in place
Speedometer drive gear in place

Next I applied a bead of RTV to the new output housing. The Crown Kit comes with eh bearing and seal preassembled in the new housing. I torqued the bolts to 17 ft lbs.

New Output Housing
New Output Housing

The output housing is drilled an tapped for a vacuum switch on the shift rail. However, the Jeep this case is going into does not use one and this kit comes with a handy plug. I noticed before screwing in the plug that the shit rail looked very close to the end of the hole. Sure enough on a test fit, the plug hit the end of the rail about one thread form being tight. I shifted the case around and found that if the plug was all the way in, the case would not fully shift into 4 Low.

I must not have cut the shaft short enough buy 1/16 inch of so. Rather than pull the output housing and cut the shaft again, I simply ground off the end of the plug. I had to remove about on thread to get it to clear.

Shift rail plug
Shift rail plug

Next I installed the two yokes and called it done for now.

Rear yoke installed
Rear yoke installed

I used a spare speedometer drive gear to plug the hole until the case is ready to be installed in Jenny’s Jeep.

Slip Yoke Eliminated!
Slip Yoke Eliminated!

The next step will be to determine the best way to set the pinion angle as the pinion now has to pint at the transfer case instead of being parallel to the out put shaft. Our plan is to use a stock front drive shaft in the rear position.

Overall, I was very pleased with the quality of the Crown Kit and the well written instructions. I am not sure how much time I spent on this project as the time was spread over several days. The new main shaft is much larger than the original in the tail shaft and speedometer drive areas so in addition to being shorter, it also looks much stronger.

Note how much bigger the SYE shaft is than the original
Note how much bigger the SYE shaft is than the original

Update: The case is now installed in Jenny’s XJ. It took approximately 8 degrees of shims to align the pinion with her three inch lift. The shift rail needed to have been cut shorter as the plug will bind the shifting into 4 low if screwed all the way in. I added a second O ring to cure it. Other wise it works great. See the write up for installing the SYE in the Jeep Cherokee here.