This is an idler pulley to suit a 624 bearing (4x13x5mm). The pulley outer diameter is 18mm, internal diameter 13mm and overall height is 8.4mm. The pulley outer channel is 6.4mm high by 1.5mm deep.
Assemble with a 624 bearing, M4 machine screw and an M4 washer to give some clearance for the bearing to rotate.
I made this pulley to stop the belt from slipping off the bearings in this drawing robot:
There's also a spacer included with 8mm OD x 4mm ID x 3.5mm height, which was needed with the drawing robot, to add a bit of clearance to fit the new pulleys.
0.2mm layer height, 0.8mm shell thickness