Section 1a - Q & A about SYE’s, what they are, and when they should be used.
The Following Was Written By Balloo93, and Used With His Permission (JF Moderator, Member Since 2002)
Balloo93’s Website - http://www.cardomain.com/ride/325707/1
1. What is a SYE and why is it used?
SYE stands for Slip Yoke Eliminator. The Slip yoke is the yoke that slides over the output shaft of the transfer case. This is the yoke that the rear driveshaft connects to at the Transfer case. The 84-95 design uses an output housing that contains a seal at the end that holds fluid in the housing and lubes the yoke as it skids on the output shaft. This fluid is the Transfer Case fluid. The 96 and later models use a sealed housing and the yoke is lubed with grease from the factory. A small rubber boot is attached to the housing and the yoke to keep the grease from contaminants. The downside is that this setup is longer from the rear of the case to the end of the yoke.
The SYE kit replaces the stock slip yoke and housing with a sealed housing and a fixed flange or yoke. This fixed flange or yoke is what a new CV style driveshaft will bolt up to. A CV (constant Velocity) shaft uses two U joints instead of a single one and this allows for much steeper angles. These shafts also utilize a two piece design that allows for suspension travel by allowing the shaft itself to compress and extend verses the yoke sliding on the output shaft. The sealed output now also allows for the rear driveshaft to be removed from the vehicle and not allow the transfer case fluid to empty out of the transfer case (84-95 transfer cases).
2. When should a SYE be used?
The stock rear driveshaft is a fixed length unit that relies on the slip yoke to adjust when the suspension travel. When lifting an XJ, the angle between the rear axle pinion and the slip yoke become much greater. The factory setup was not intended for these variations in lift. The angle difference will be most noticeable in the fact the slip yoke will be pulled further out of the housing than before the lift.
The differences in angle can be attributed to several things, but they all revolve around this center point, the length of the rear driveshaft. You will hear various things from various people about what size lift can be ran without a SYE. The key here is to recognize the differences between their XJ and yours.
The worst possible setup you can have is a 96+ with an auto transmission, a NP242 transfer case, and an 8.25 rear axle. This combo makes for a long drivetrain and the 8.25 pinion housing is also quite long. These factors will leave you with a short rear driveshaft and even a 2" lift will see negative affects from it.
The best setup for less noticeable vibration would be a 84-95 with a manual trans, a NP231 transfer case and a D35 rear axle. The 84-95 slip yoke was shorter than the 96+. The Manual transmissions are shorter than the auto transmissions. The NP231 is shorter than a NP242 transfer case. The D35 has the shortest pinion housing, followed by the D44 and the longest is the 8.25.
3. What are some of the harmful effects of not using a SYE?
The difference in the angles put a strain on the output shaft. This places load on the transfer case bearings and the seals on the output shaft. The 84-95 slip yoke housing is more prone to showing obvious signs by leaking transfer case fluid.
Another, more obvious problem, are the U joint ears of the Driveshaft making contact with the slip yoke as the driveshaft spins. This will make for a noticeable clanking noise and hard vibration. A transfer case drop will clear that problem up, as well as a high clearance extended slip yoke, but it will not correct the strain on the output shaft.
4. What are other misc. positives does the use of a SYE provide?
As mentioned before the ability to remove the rear driveshaft if damaged and drive off a trail under the power of the front axle is a major plus. If you run a stock front driveshaft with a SYE, you can carry a spare for both the front and the rear driveshafts.
5. Which SYE/Driveshaft combo do you recommend?
There are several different configurations for a NP231 transfer case, but only one or two for the NP207 and NP242. These options are both offered from Rubicon Express in the form of their Hack and Tap SYE. It requires you to cut the rear output shaft down to 1" and tap the end so that a flange can be slid over the splines and bolted to the output shaft. The 96+ H-n-T kits for the NP242 are much cheaper than the older style ones. The reason behind this is that the older units have a flange and a housing that must be fitted. The 96+ are sealed housings, so it only requires the flange.
The NP231 offerings are plentiful. There are a couple of flange style Hack and tap designs (RE and Currie) and even more with a shorter and tapped output shaft that use a yoke or flange. There are also Heavy Duty units that use a 32pline output shaft and a Yoke. The replacement output shaft types are more involved as far as installing them, but they also allow the easy use of a stock front driveshaft to work.
A stock front driveshaft from a 91+ auto transmission and NP231 equipped XJ can be used with either style SYE. The flange style setup will require an adapter to work, but most yoke style SYE use the same 1310 U joint that the front shafts use stock. This setup can be used for lifts ranging from 3-6" depending on which type of SYE you use. The yoke style SYE's tend to be shorter than the flange and adapter type, so a taller lift may not be possible since the stock shaft has only so much travel to it. The longer type SYE can use a stock driveshaft with more length.