A cap system for enclosing a metallic fastener assembly extending through a structure includes a first securement mechanism positioned about a periphery of a washer of the metallic fastener assembly wherein the first securement mechanism includes threads. The cap system further includes a cap member including a sidewall having an inner surface which defines a cavity dimensioned to receive the washer and defines a second securement mechanism complementary configured to engage the first securement mechanism positioned about the periphery of the washer. An end of the sidewall of the cap member defines an opening which provides the washer to have access into the cavity.