Liftmaster/Chamberlain/Craftsman Gear and Sprocket Sets:

Residential, overhead Liftmaster (et al) garage door operators use a gear and sprocket drive gear set to transmit motion from the motor to the chain or belt that lifts the door. The drive gear inside is nylon, and is designed as a 'sacrificial' part. Should your door hit an obstruction while traveling, the door tends to suddenly stop before the motor has a chance to react. When (if) that should happen, if that drive gear were not in the drive train to absorb the shock, serious damage might occur to the motor, the belt/chain, or the rail; maybe even the door itself. All these parts are much more expensive to replace than the gear set. Instead of all this damage happening, the nylon drive gear is designed to shear the teeth off, preventing the shock from harming other components. While this may seem a major inconvenience to the homeowner, it is a relatively cheap insurance policy built into the machine.

Liftmaster 41C4220A
Gear and Sprocket set for most
chain drive machines

While there are gear (only) sets available, Garage Door Supply Company (garagedoorsupplyco.com) recommends replacing the entire gear and sprocket set. The cost is about $10 more, but you get the new shaft and sprocket, new top cap and critical top bearing that you do not get in the cheaper gear (only) set, and best of all, it comes pre-assembled, saving probably an hour or two of dis- and re-assembly.

I estimate that ~90% of Liftmaster (et al) chain drive openers use the 41C4220A gear and sprocket set, BUT if you are not certain, you should find the exact model number of your gdo, and call Liftmaster-store.com at 888-880-8821, for expert advice on purchasing the correct gear and sprocket set for your machine. Installation of the replacement set can be very easy. Next post I will give some valuable hints and tips about these assemblies.

Liftmaster 41A4885-2
belt drive gear and sprocket set