Who doesn't like the biohazard symbol? The hex is deep enough so a washer can be placed between the gear and bearing. This helps the gear clear any protruding screws.
The attached .skp has the hex head socket oversized by .4mm. It's a little looser than I expected but works well. Printed @ .2 layer height and .4 infill. Printrbot Wood Extruder Gear
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8.98 w x
6.246 d x
3.535 w x
2.459 d x