2 Pack - Drive Gear for Sears Crafsman Liftmaster Chamberlain Garage Door Openers 1984-Current

will this fit model 139.53990D craftsman opener

Is there a difference between the gears for Sears and the regular Chamberlain models?

I just replaced the gear at the end of april and it just broke at the end of september. i know i did something wrong, but any ideas as to what i did?

Will this fit a Liftmaster 3850?