Where are headers measured?
 

It is the inside diameter that is measured, but where?.....at the collectors or the pipe that goes to muffler? Should these measurements be the same?

Thanks for the replies
JC

Actually, many people specify header size by the outside diameter of the primary tubing, including manufacturers like George Narbel of European Racing Headers. If you specify 1-3/4" headers with him, the ID will be 1-3/4" minus twice the wall thickness of the tubing, or approx. 1-5/8".

The secondary tubing, after the collectors, is always larger than the primary tubing.

TT

Perfectly answered...Thanks.