Round Gas B Vent Pipe 3" to 12" pipe and fitting couplers overlap 13/8" when joined together; therefore, assembled length is 13/8" less than listed length. The allowable minimum clearance to combustibles is 1", except for 4RPX on which clearance is provided by locking band on pipe only when installed in a 2 x 6 stud wall. Outside Diameter (OD) Sizes 3" through 6" is size plus 1/2". Sizes 7" through 12" is size plus 1" A full set of installation instructions are supplied with caps and supports