Buying guide

Timing Pulley Bore Size: How to Choose the Right One

Match the bore to the shaft, check the teeth and belt width, and use a flat on the shaft for the set screw.

The bore is the hole that goes on the shaft. Common GT2 bores are 3, 4, 5, 6, 6.35 and 8 mm; 5 mm fits the usual NEMA 17 motor shaft and 8 mm fits many lead screws.

Choose in this order

1. Shaft diameter: measure it with calipers. The bore should be the same size, with a close fit.
2. Profile: GT2, HTD 3M or HTD 5M, to match your belt.
3. Teeth: more teeth give a larger pulley and finer wrap; 16 or 20 teeth are the usual GT2 sizes on 3D printers.
4. Belt width: the pulley width must cover the belt; for a 6 mm belt use a pulley made for 6 mm.

Fixing the pulley

Tighten the set screw onto a flat on the shaft. A pulley that slips on a round shaft is a common cause of lost steps.

See also: choosing a pulley · belt types · all products