these are custom slack spacers for BMX freecoaster hubs as the fit OEM and KHE-Style clones.

they have a 19.5 mm inner diameter and 27mm outer diameter, 

each spacer is 1 mm thick,allowing you to stack to fine-tune your slack-test with different combinations to get the engagement feel you want.

tip: once you've found your perfect slack amount, you can combine multiple spacer  or change the z perimeter on your slicer file  and print a solid single-piece version

i personally like 5.5 mm tick