One of my LiftMaster garage door openers (1/2HP, Model 1205A, MFT 1999) stopped working after 10 years. Upon first glance, I saw white dust had accumulated in the Up/Down travel adjustment holes. Upon removing the casing, the nylon drive gears had failed (the drive gear that turns the sprocket had stripped).
This past weekend I fixed the opener in about three hours and I highly recommend the: Liftmaster Gear and Sprocket Assembly Kit Part Number 41C4220A.
I also bought Sears Craftsman Liftmaster Garage Door Opener Gear Kit – Part #41A2817 as well because my house has two LiftMaster openers. But I found the sprocket solution to be better in this case as I could just drop it in. I now have the extra stripped sprocket which I can use with the extra part kit to replace the stripped gear and reuse it on my second opener when and if that fails.
The Liftmaster Gear and Sprocket Assembly Kit Part Number 41C4220A comes with a decent checklist of the steps needed to replace the failed nylon gears. However this checklist was missing key pictures or diagrams that could have sped along the repair.
Here are some photos which should add more detail to those directions. The one in particular is the break down and order of parts when dismantling the engine assembly (which has the spinning RPM limiter plastic piece).