i monkeyed around with replacing a 25 year old noisy chain garage door
opener's plastic gear and it was a waste of $25 and hours and hours in
a hot garage in summer.
in the equal amount of time for $60 labor the new replacement garage
door opener i picked out with a quiet belt drive was installed.
for me it wasn't just the gear gone bad, the chewed up gear was a
symptom of the whole system binding.
look at home depot.