Although I do not know the size unless I try to open it without parts list, I think that it is convenient when I do not want to stop production as much as possible. Just the same size is many. Because there are many types of O-rings, I want you to reduce the price by about a half to make it easier to buy other standards and different size sets. And the movement of the case hinge is bad. Although it is a precious case, it seems to break hinges before becoming familiar and smooth.