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”) & require a single gasket that is installed on the inside of the tank. Available gaskets are cross-linked polyethylene. EPDM or Viton (4” fitting comes standard with a cross-linked polyethylene gasket).