Sync Wheel Pulley drive normally attached on the stepper motor to move actively with the timing belt, that’s why sync wheel drive without bearing. And the number of teeth is matter and affecting the movement speed, accurate of print result, so if you change different size , different number of teeth sync wheel remember to do the calibration. Also need to remember the lock securely on the stepper motor with the small screw, otherwise it may have movement skipped.
16 / 20 Teeth idler pulley
H-type synchronous pulley wheel
Tooth width : 6mm
Tooth: 16 / 20 (T)
Belt Width: 6 (mm)
Center hole diameter : 5mm / 8mm