This is a slightly improved version of the PrintrBot LC extruder small herringbone gear. I removed artifacts from a defunct set screw hole, and replaced some missing vertices in the teeth. Same gear, better mesh, less defects.
The hole for the shaft is purposely slightly undersized, as to allow for reaming/drilling for a precise fit to the motor shaft. You will need either a 5mm reamer, or drill, or a #9 reamer or drill, or you could also use a #8 reamer or drill. In order to make this gear turn smoothly, you should perform this on a drill press or lathe. Or, do it by hand carefully. ALWAYS WEAR SAFETY GLASSES WHEN DRILLING ANYTHING.