Bolts are threaded into the back plate of the fitting so there are no welds or bolt holes that can be potential points of leakage. These fittings come standard without a gasket (with the exception of the 4") and require a single gasket that is installed on the inside of the tank. Available gaskets are cross-linked polyethylene, EPDM and Viton. (4" fitting comes standard with a cross-linked polyethylene gasket.)