18mm and 19mm inner diameter. This ring made me go down a mathematical rabbit hole, to find the rule by which new layers are added, and to find what shape it creates with infinite surfaces. I found a rule for series (next layer by last layer),but could not find the rest. it seems to always close perfectly with enough layers. Would love to hear if someone manages to prove it. for anyone wishing to change the size i would recommend scaling it, because to make it I had to use a 3d sketch from hell as well as to solve the layer rules (can sketch without but the sketch tore itself apart when I tried).