The pulleys are .40” inside the flanges to allow for belts up to 3/8” in width. Two set screws using 1.5mm hex key.
This GT2 (also called 2GT) timing belt has a 2mm Pitch and 6mm width
Specifications:
GT2 Pulley
Φ22.0mm*13.0mm thick
2*M4 thread for M4 set screws
Φ6.0mm D hole
AL6061-T6