Why is there sometimes clearance between the tubefitting and the tube?

This is due to the fact that the tubes may have a minimum tolerance of inner and outer diameter. This also applies to tubefittings. Since the tubefittings are made of cast iron and then hot-dip galvanized, there may be a slight difference between them. This difference is ultimately necessary to ensure that the tube and tubefitting can be connected. This clearance does not affect the strength of the structure.