1" (25mm) smooth bore insert "Six Pack" contains 6 smooth bore inserts (1/8", 3/16", 1/4", 5/16", 3/8", & 7/16" orifice sizes) (3, 5, 6, 8, 10 & 11mm) and an NH threaded coupling with gasket. The Six Pack allows you to change smooth bore orifice sizes by simply removing and replacing a rubber gasket from a hard coat anodized aluminum coupling. There are 6 different orifice sizes included with each order. These inserts can be easily interchanged depending on the situation. The inserts are made from a lightweight and durable black nylon. 1" NH (25mm) thread is standard.