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How-to: Rebuilding a Jeep 231 Transfer Case with Advance Adapters SYE!

Rebuilt NP-NV231 transfer case with SYE kit installed.

     Eliminating the slip yoke rear driveline and upgrading to Advance Adapters’ Fixed-Yoke (SYE) kit increases the rear output stamina of this NP/NV231 transfer case. The unit will now handle a six-inch chassis lift and 33” diameter tires. A "Slip Yoke Eliminator Kit" allows use of a longer, constant velocity (CV) driveline.

See the following two pages for Part 1 and Part 2 of the NP/NV231 step-by-step rebuild!

     The NP- and NV231 transfer case is the most widely used design in the world. These units were popularized in the XJ Cherokee, YJ/TJ Wrangler, Grand Cherokee and various truck models built by Jeep and a variety of other manufacturers. This is a rugged and reliable, chain-drive transfer case that can handle off-pavement chores and reasonable (33"-35") diameter tires. Like any other automotive mechanism, the NP and NVG231 transfer case will require rebuilding at higher mileage or if abused.

     Owners of Jeep YJ and TJ Wranglers, Jeep XJ Cherokees and Jeep Grand Cherokees quite often perform a chassis lift. The increased driveline angles make vibration more likely and decrease the strength of the U-joints. By design, a common cross-type, single Cardan style U-joint loses strength as the driveline angle or slope increases.

     The popular solution is a heavy-duty, constant velocity (CV) rear driveline. Jeep NP/NV231 transfer cases typically operate with a CV front driveline. However, most 231 units use a splined output shaft with a slip yoke driveshaft. For the modified or lifted Jeep Wrangler, Grand Cherokee or XJ Cherokee, the relatively short, stock driveshaft and slip yoke design create problems when the driveline angle steepens. The more chassis lift, the more trouble! 

Moses Ludel's '99 XJ Cherokee with fresh 6-inch lift kit

     Advance Adapters is a manufacturer of transmission adapters and engine retrofit kits for a variety of popular 4x4 vehicles. Recognizing the demand for a CV rear driveline on Jeep Wrangler and Cherokee models equipped with New Process or New Venture 231 type transfer cases, Advance Adapters created its “Slip Yoke Eliminator” (SYE) kit. The kit eliminates the splined output shaft and slip yoke style rear driveline. In place of the sliding yoke, the SYE kit provides a new, heavy-duty output/mainshaft and a fixed-yoke flange. The new flange will accept 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% increase in output stamina over the stock output shaft and slip yoke. This enables the NP/NV231 transfer case 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 (Cardan) 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 a stock NP/NV231 with a splined output and slip yoke, the rear driveline must remain in place. Removal of the slip-yoke driveshaft opens the transfer case to a huge fluid leak.

Slip yoke eliminator and short tailhousing

     The Advance Adapters’ SYE fixed-yoke approach seals the output and permits driving the vehicle 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 Jeep CJ or TJ Wrangler Rubicon model! You cannot do this with a slip yoke driveline...

     An Advance Adapters ‘SYE’ fixed-yoke kit fits all Jeep Wrangler and XJ Cherokee models equipped with either the New Process or New Venture 231 transfer case. The kit serves as a direct bolt-in for the TJ Wrangler and later XJ Cherokee applications of the 231. For Wrangler YJ models and earlier XJ Cherokees equipped with the NP231, slight modifications enable installation of this same kit. The well-detailed instructions that come with the Advance Adapters kit explain which areas might require attention or minor changes.

     By installing the Fixed-Yoke ‘SYE’ conversion, the 231 transfer case has increased stamina and versatility. Reducing the excessive length of the stock 231 transfer case allows use of a chassis lift, oversized tires and a CV driveshaft to decrease rear driveline stress. If your Jeep Wrangler, Grand Cherokee or XJ Cherokee has an NP or NV 231 transfer case, the Advance Adapters’ Fixed Yoke conversion is a valuable asset for any degree of chassis lift.

     The transfer case rebuild that I have illustrated in this 'how-to' is a 1999 XJ Cherokee NV231 application. The installation of an SYE from Advance Adapters is straightforward, actually a bolt-on setup that requires no additional modifications to the transfer case. With a six-inch lift kit and 33” diameter tires on the agenda, the SYE kit and a constant velocity (CV) rear driveshaft is a necessity.

SYE plus the CV rear driveshaft

     Consider a CV-type rear driveline as part of your chassis lift plans. For builders who have a YJ or TJ Wrangler, a Grand Cherokee or an XJ Cherokee with the NP231 or NV231, review this complete rebuild and SYE installation. Advance Adapters' Slip Yoke Eliminator or ‘SYE’ conversion is a much needed part of any suspension lift!

     Note: More information about Advance Adapters is available at 1-800-350-2223 or the website: www.advanceadapters.com.

     For details on the rear CV driveline component, see the suspension lift article within this XJ Cherokee Workshop section of the website. A six-inch long arm suspension lift, 33" diameter tires and an SYE rebuild of the NV231 transfer case have been trail tested—substantially—for several years now. Simply put, the '99 XJ Cherokee holds its own on hardcore trails...Moses Ludel

     Part 1 of this 'how-to' is the disassembly and inspection of an NP/NV231 transfer case. Part 2 is the assembly and installation of an SYE kit. The work necessary to perform an SYE kit makes rebuilding the transfer case practical.