6mm
Crouzet 81513001 Intermediate Supply Module
This model is made with Seamless 316 SS, so corrosion is hardly ever a problem.
GT2 Aluminum 36 Teeth 12mm Bore Synchronous Wheel Idler Pulley for 3D Printer DI
This unbranded generic tool is a reliable choice for basic industrial tasks.
Aluminum 30 Teeth 8mm Bore 2mm Pitch Timing Belt Pulley
You will see the metal mesh allows for airflow easily.