Fixing a stripped knob on a dryer requires some troubleshooting to determine why the knob is stripped. The knob is the portion of the dryer timer that you see. It turns a shaft on the timer, which rotates on a cam. Although the plastic sleeve on the knob does simply just wear out, sometimes a defective timer is the cause of knob failure. Once you determine the knob is at fault, fixing a stripped knob on a dryer is simply a matter of replacing the knob. Once the plastic cracks or weakens, the knob is useless.
Pull the dryer knob off the timer shaft. Some dryer manufacturers have a small c-clip securing the knob to the timer shaft. Grab the clip behind the dryer knob with a pair of needle-nose pliers. Pull the clip up until it disengages from the shaft, and pull the knob off the unit. The clip is part of the dryer knob.
Check the timer shaft for wear. The shaft is flat on one side. If the shaft is rounded around the edges of the flat side, the timer requires replacement. Turn the shaft clockwise with a pair of pliers. The shaft should turn with little resistance. If the shaft spins or will not turn, the timer requires replacement.
Take the stripped timer knob to an appliance repair shop, if the timer shaft appears to be in good working order. The plastic sleeve on the inside of the timer knob is worn, requiring complete replacement of the knob.
Align the flat portion of the dryer knob sleeve with the flat portion of the dryer timer shaft. Push the knob over the shaft. If your dryer uses a clip to secure the knob, push the clip down with a flat-head screwdriver once the knob is on the shaft.