1987-1999 XJ NP231s use 23-spline input. 2000-2001 use 23-spline still (varies by year). Most aftermarket SYEs work with 23-spline. Verify before ordering.
The NP231 input shaft splining depends on year and transmission combination:
For XJ owners doing an SYE or rebuild, virtually all parts are 23-spline. When pulling a donor NP231 from a 4WD Ram or a TJ Rubicon, verify the input spline count before installing.
If running a V8 conversion behind a 4L60E or other GM trans, you'll likely use a 27-spline input shaft from an applicable Mopar truck. Advance Adapters and Novak sell spline conversion kits.
Why it matters: a 27-spline input is stronger than 23. For built drivetrains over 400 lb-ft, the 27-spline upgrade is worth doing during a rebuild. For stock 4.0L or 4.6 stroker, 23-spline is fine.
Verify spline count by removing the input shaft and counting, or check the casting marks. Some t-case rebuild kits list compatibility by year — always cross-check the actual hardware.
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.