Why did my 30 yo maytag de212 stop tumblilng when the timer dial became stripped?

Because the timer consists of a small motor connected to a series of gears that rotate cams, which turn electric contacts off and on inside the timer. The timer contacts control the dryer motor as well as the heat circuit and the timer motor itself. A defective timer motor or a defect in the timer's electrical contacts can cause the timer motor to fail and the timer will need to be replaced. The contacts can be checked for continuity with a multi-meter. This test should only be made with the power removed from the dryer.