Will, My '74 OEM coolant tubes measure 35mm OD (1 3/8 inches) and have a wall thickness of 1.5mm (1/16 inch) 1 1/4 inch hard copper pipe, the type used for plumbing houses, has a 1 3/8 inch OD and a 1/16 inch wall thickness. Have fun, SOBill Taylor sobill at aol.com