Is the given diameter the inner diameter of the tubefitting or the diameter of the congruent tube?
All tubefittings are marked with a size that corresponds to the dedicated tube diameter. For example, in the case of D - 42 mm tubefittings, the diameter of the tube dedicated is 42 mm. It is worth noting that each tubefitting features a certain clearance, as the tube is assembled inside the tubefitting with the use of a dedicated set screw. Therefore, a slightly smaller or larger tube should be about fine.