Advance Adapters' NP/NV231 Slip-Yoke Eliminator
A common cross-type, single U-joint loses strength as the driveline angle increases. The installation of a suspension lift kit prompts the need for a heavy-duty, constant-velocity (CV) rear driveline. While the popular Jeep New Process or New Venture (NP/NV) 231 transfer cases provide for a CV front driveline, these units commonly use a splined output shaft and a slip-yoke driveshaft. For the modified or lifted Jeep Wrangler or XJ Cherokee, the short, stock driveshaft and slip-yoke design will not accommodate the steepened driveline angle.
Advance Adapters manufactures transmission adapters and engine retrofit kits for a variety of popular 4x4 vehicles. Recognizing the demand for CV rear drivelines on Jeep Wrangler and Cherokee models equipped with NP/NV231 transfer cases, Advance Adapters created its Slip-Yoke Eliminator (SYE) kit. The kit eliminates the splined output and slip-yoke rear driveline. In place of the sliding yoke, the kit provides a new, heavy-duty output/mainshaft and a fixed-yoke flange. The new flange accepts a custom-built driveline with a heavy-duty Spicer CV joint.
This well-engineered kit provides several gains. The new 32-spline output shaft and fixed yoke provide a 50 percent increase in output stamina over the stock output shaft and slip-yoke. This enables the NP/NV231 transfer cases to handle more horsepower, larger tires, and steeper driveline angles. For lifts, the shortened output assembly allows the use of a much-needed, longer rear driveline.
Advance Adapters makes this fixed-yoke kit available for popular Spicer 1310 or 1350 CV-type flanges. There is even an option for 1310 or 1350 non-CV yokes, satisfying users who want a single cross-type joint with a longer rear driveline. A longer driveline will reduce U-joint angles and decrease stress to the driveshaft and joints.
An additional gain with the SYE kit is the fixed rear output shaft yoke and seal design. To prevent oil leakage on stock NP/NV231s with a splined output and slip-yoke, the rear driveline must remain in place. Removal of the slip-yoke driveshaft will open the transfer case to a huge fluid leak. The Advance Adapters' SYE fixed-yoke design seals the output and allows the vehicle to be driven with the rear driveline removed (using four-wheel-drive mode to propel the front axle). In a backcountry emergency, a damaged rear driveline will not leave your vehicle stranded. You can remove the rear driveshaft and drive the vehicle in "front-wheel-drive mode," much like a CJ or Rubicon model.
The Advance Adapters SYE fixed-yoke kit fits all Jeep Wrangler and XJ Cherokee models equipped with either the New Process or the New Venture 231 transfer case. The kit serves as a direct bolt-on for the TJ and later XJ applications of the 231. For Wrangler YJ models and earlier XJ Cherokees equipped with the NP231, slight modifications enable the installation of this same kit. The well-detailed instructions included with the kit explain any areas that might require attention or minor changes.
By installing the fixed-yoke SYE conversion, the 231 transfer case gains increased stamina and versatility. Reducing the excessive length of the stock 231 allows the use of a chassis lift, oversized tires, and a CV driveshaft to decrease rear driveline stress. The Advance Adapters' fixed-yoke conversion is a valuable asset for '87-and-later Jeep Wranglers or XJ Cherokees equipped with NP/NV231 transfer cases.
For our '99 XJ Cherokee Project TrailRunner, the kit installation is a straightforward, bolt-on setup, requiring no modifications whatsoever. With a 6-inch lift kit and 33-inch tires on the to-do list, the SYE kit is a necessity. A CV-type rear driveline will be part of the lift package. For builders who have similar plans for a Wrangler or XJ Cherokee, take a close look at this installation. The Advance Adapters fixed-yoke SYE conversion provides the perfect complement for a suspension lift.
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