NP203 Transfer Case Locking Hub Conversion Kit

This is the hub conversion service kit Mile Marker supplies (part number 501) for full-time four-wheel-drive trucks running the New Process NP203 transfer case. The NP203 is a chain-drive case with a center differential, so the front axle turns all the time and the front hubs, bearings and axle shafts keep spinning even on dry pavement. That constant rotation is why NP203 trucks chew through front hub and bearing parts and give up fuel economy compared with a part-time case.

The kit supplies the drive plate, bearing and gasket hardware that lets a set of manual locking hubs mount and engage correctly on an NP203-equipped front axle, so the front shafts and driveline stop turning once the hubs are unlocked. It is a service kit, not a complete hub - it goes in behind the locking hub body. Mile Marker has built locking hubs and the matching conversion hardware for this application for decades, and the 501 is their listed service kit for it.

Bench notes: pull the wheel and rotor, remove the factory drive flange and its snap ring, clean the spindle threads and the wheel bearing adjusting nut, and check the bearing preload before you assemble anything new. Replace the gasket rather than reusing it; that surface is the seal between the hub body and the spindle, and a weeping gasket lets grit into a bearing you just set. The parts are metal, so no plastic ring is carrying the load. Confirm you actually have an NP203 case (73-79 era GM, Ford and Dodge full-time four-wheel drive) and not the part-time NP205 before ordering - the two look similar under a truck but take different front-end hardware.