Extra light, temper hardened stainless steel loop. 5.3 mm horizontal loop I.D. An industry standard.
Size 3.5 - Tube I.D. - 1.4mm
Size 4 - Tube I.D. - 1.6mm
Size 4.5 - Tube I.D. - 1.8mm
Size 5 - Tube I.D. - 2.0mm
Size 5.5 - Tube I.D. - 2.2mm
Size 6 - Tube I.D. - 2.4mm
Size 6.5 - Tube I.D. - 2.6mm