Spindle Nut
The nut that secures the wheel bearing and hub assembly onto the steering spindle or axle shaft.
A spindle nut (also known as a hub nut) is a heavy-duty locking nut threaded onto the outer end of a wheel spindle or CV axle shaft. Its primary function is to secure the wheel hub assembly and wheel bearings onto the spindle shaft, keeping the rotating assembly firmly clamped to the vehicle's suspension.
In tapered roller bearing setups (common on older or RWD vehicles), the spindle nut is adjusted to set the precise bearing preload or endplay. The nut is tightened to seat the bearings, backed off slightly, and then locked in place using a cotter pin through a castle nut or a nut retainer cage. In modern sealed hub unit bearings, the spindle nut (often called an axle nut) is torqued to a very high, fixed specification to clamp the bearing assembly split inner races together.
Because it holds the wheel on the vehicle, spindle nut security is critical. A loose spindle nut will cause wheel play, steering wheel shake, rapid bearing failure, and can lead to wheel detachment. When servicing wheel bearings, the spindle nut should be replaced if it is a crimp-style or single-use locknut, and always torqued with a calibrated torque wrench.