A carb "stone" assembly is used to inject a gas into your beer for carbonation. The standard configuration is: 1-1/2 in. TC cap x 3/8 in. FNPT, however, any size TC cap is available and can be ordered with a male or female threaded fitting for your gas hook-up. 1/2 in. and 1 in. diameter carb "stones" (sintered SS) are available in a variety of sizes.