Simplicity LM-01210-83  SPINDLE, Pinion Differential

The purpose of the Spindle is to control the safe rotation of the mower blade during operation. The spindle is often made of aluminum on residential mowers and steel on commercial mower decks. Much like a shear pin in other applications, the spindle is designed to break if the blade hits a hard object to protect the deck shell from damage. Screws are required for installation and are usually sold separately. Most use self-tapping screws to create the threads when screwed into the holes in the spindle.