A timing pulley has five decisions: profile, tooth count, width, material, and mounting. Walk through them in order.
Profile must match the belt — XL for XL belts, HTD 8M for HTD 8M belts, AT10 for AT10 belts. Mixing profiles is the most common cause of premature wear.
Tooth count comes from the speed ratio and the belt pitch length: N = L / (π × pitch). Always round to the nearest standard stock count.
Width must match or exceed the belt width. A wider pulley on a narrow belt is fine; a narrower pulley on a wide belt will wear the belt edges.
Material — aluminum for light / high-speed drives, steel for heavy / industrial drives, cast iron for taper-bushed heavy drives, stainless for food / medical.
Mounting — pilot bore for prototypes and small batches; finished bore (with keyway) for production; taper bushed for flexible shaft sizing on heavy industrial drives.