Stainless - 1/2 in. MPT x 5/8 in. Barb (1/2 in. ID). Use this barbed fitting with our 1/2" ID Silicone tubing for a better fit. Because Silicone tubing is soft and flexible it actually fits much better on this over sized 5/8" OD barb than on a 1/2" OD barb.