Every year about this time we see a sharp increase in the number of drive gear and sprocket failures in garage door openers. We have customers asking "Can I get just the large plastic gear by itself?" Most of these customers have seen "cheap" aftermarket gears on the internet. Many of these gears are made in Venezuela or other aftermarket sources and include just the large gear and not the drive gear on the motor shaft. These will work for awhile but if put any value on your time you should always change both gears at the same time and be sure to lubricate them well.