Q on header primary tube dimensions

[67ragtp]

When a header is advertised with a specific primary tube diameter, does that dimension reflect the inside diameter of the tube at the flange or the outside diameter of the primary tubes specifically stainless headers? e.g.1-7/8" primary tube diameter, how do you interpret that?

Thanks Rich

[jst8a5.0]

Pipe is measured inside and tubing is outside.