GAP CAP - Dual Space-Saving Air Gap Drain For 1 1/2" & 2" Standpipes Ideal space-saving air gap Twin inlet port: Larger port is 1/2" slip that also directly accommodates 5/8" O.D. poly tubing. Smaller RO port directly accepts 3/8" O.D.poly tubing. Includes inlet elbow with flow straightener (P/N 8504). 5 to 7 G.P.M. Fits 1 1/2" & 2" standpipes.