Some information provided by JB Conversions
According to JB Conversions there are 3 main reasons to install the Heavy Duty Slip Yoke Eliminator kit in your Jeep:
With the exception of the stock output shaft found in the “J” model, the NP231 can handle considerable abuse without exhibiting any signs of distress. The planetary gear assembly, drive sprockets, and chain in the 231 are very strong and rarely have problems. When there is a problem, low oil level is usually the cause of failure. The original 27 spline output shaft in the 231 is the only real weak point in the entire unit. The stock output shaft features a spiral cut speedometer gear just past the rear output bearing. The minor diameter of the speedometer gear teeth is fairly small. When the shaft fails it will almost always break in this area.
The rear output “slip-yoke” design of the NP231 is notorious for leaking. The rear driveshaft yoke used with a slip-yoke style transfer case is designed to slide back and forth across the splines of the output shaft. Under normal driving conditions and vehicle payload, the slip yoke will remain within a narrow range of movement across the splines and seal surface. The surface area past this narrow margin is exposed to the elements and will form a ruff surface due to rust and dirt. Once the vehicle becomes loaded down with trail gear, or the suspension is cycled heavily, the slip yoke will travel even more therefore dragging the corroded surface across the rubber output seal. This is where the leaks start. Over time, a low oil level could be reached within the transfer case and the result could be catastrophic failure.
Another problem with the slip yoke design is that the rear driveshaft yoke cannot be removed from the vehicle without loss of transfer case fluid. Broken u-joints can often require the vehicle to be driven back to civilization on the front shaft only. Without a fixed yoke on the rear output, this task would be impossible without experiencing major transfer case damage due to loss of oil.
Kit Upgrade: The HD slip yoke kit utilizes a standard Dana Spicer end yoke. This “bolt-on” yoke eliminates the leak prone slip yoke entirely. With the addition of the fixed yoke, the rear output seal runs in one location on the yoke therefore seal life is maximized.
3) Driveline angles:
On any short wheel base vehicle, driveline angle is critical and easily influenced with the addition of a suspension lift. Typically, Jeep Wranglers come factory equipped with a rear driveshaft that is 13.5″ or less. With a running length this short, every inch of added suspension lift has a drastic effect on the angle of the driveline.
Kit Upgrade: The only way to DECREASE the working angle of the U-joint is to lengthen the driveshaft. Once installed, the HD yoke kit will add 4.5″ (YJ’s) to 6.5″ (TJ’s) of length to the rear driveline therefore flattening the overall slope of the shaft.
Gains in driveshaft length are based on measurements taken from the center-of-u-joint on the original driveshaft to the front of the case. Be careful in buying a slip-yoke kit based on the claimed length “number” in inches. It is better to look at the design of the new rear housing in the fixed yoke kit as opposed to that of the rear housing of the stock tcase. Remember this; the stock NP231 has an output bearing at the rear of the speedo housing as does the new slip-yoke eliminator kit. These bearings are within 0.25″ of the same location as measured from the front of the case. The shortest design for a yoke kit would be to place the yoke immediately after the rear bearing. The JB kit does exactly that. Anything shorter would require a shorter end yoke or more shallow rear housing and that is not possible due to the size of the speedometer gear bore. What this means is that the JB kit is designed to be the shortest overall length possible regardless of where you measure from.
After the installation of the JB kit, the improved operating angles of the u-joints will greatly increase the life of the driveline. The new HD yoke kit is shipped with a C.V. compatible end yoke and requires a 1310 series u-joint. Other end yokes are available to accommodate larger u-joints however the 1310 is the preferred joint.
Read more about Transfer Case Upgrades and Options