In order to prevent the common mode of the equipment, I bought it by considering insulation sleeve and this washer to insulate the metal bolt. Although it was the deciding factor of purchase that the inner diameter was as large as 19 mm at M16, the actual inner diameter was only slightly less than 17 mm. Since the written dimensions are not available, the satisfaction level is unsatisfactory. It's hard to get money with this. The material was glass epoxy. I feel that there are many burrs on the inner diameter side (barrels).