2.6 Inch / 65mm

There are just a couple of acronyms that are used in the descriptions, so any references to BT is regarding Body Tubes and CT refers to Coupler Tubes.  54mm is used as a general term for the basic size rather than an absolute measurement.  The parts here are available in thin (Mach1), normal (Wildman) and heavy (MadCow) wall thicknesses for coupler tubes.  If you have any questions or a special size requirement, please contact SMT Designs on the Contact Us page.  I will adjust diameters of parts for you given your measurements for no charge other than the normal product price (only caveat - non-returnable unless I made them wrong).