Turning down the OD of a gear

[hrench]

This is very case-dependent. Just remember that the teeth are designed to be in contact at the pitch-circle diameter, not at the outside diameter. So cutting the outside will have no effect on many systems.

So as long as you have 'regular' gears, not some kind of ten-tooth pinion or something, the outside two percent of the tooth should never be needed as the next tooth is engaged long before it disengages.

Also, technically, involute teeth aren't sliding, they're 100% rolling contact, but if you change the center-to-center distance, even by adding a measley 3 thousandths, you'll have some sliding contact and less gear efficiency, more wear.

But as someone above mentioned, what is the gear pitch?--if the teeth are very small, 0.020 could be alot. if they're big teeth, 0.020 is less concern.