Header Diameter qusetion
 

Do the Header Manufacturers gauge the diameter of the header tubes from the OD or the ID? say your buying a set of 1,3/4 headers. is this based on OD? is this the industry standard of measurement? looking to upgrade my 66 Lemans from the hooker comps on it now suppose to be 1 5/8's, going 1 3/4 or bigger.

OD

:)