A threaded fastener assembly for use in an environment in which threads exposed to the ambient must be avoided, comprising: a bolt member having a head and a shaft that includes a first smooth shaft portion extending from said head having a smooth unthreaded wall surface and a first predetermined diameter, and a male-threaded tip portion extending from said smooth shaft portion and coaxial with it, with a male spiral thread thereon, and having a second diameter; a nut member configured for threadably engaging said bolt member and having a blind bore extending from a first end of the nut and opening only at said first end, the bore having a first smooth wall portion of a first predetermined bore diameter to admit entry of the first smooth shaft portion of said bolt member, and a second female-threaded bore portion extending coaxial with the first smooth wall bore portion and having a female spiral thread adapted for threaded engagement with the tip portion of said bolt member; wherein a seal member is disposed within said first smooth wall bore portion of said nut member adjacent said first end; the bore of said nut member is a stepped bore wherein said first diameter thereof is larger than the second diameter thereof and wherein said bolt member is stepped such that the first diameter thereof is larger than said second diameter of said male-threaded tip portion, and wherein said seal member is an annular seal which is fitted into an annular channel formed in the first smooth wall bore portion, such that once said nut member is fastened onto said bolt member, said seal member closes off a space defined between said the tip portion of said bolt member and the female threaded bore portion, such that the male and female spiral threads of the bolt member and the nut member are entirely concealed. a bolt member having a head and a shaft that includes a first smooth shaft portion extending from said head having a smooth unthreaded wall surface and a first predetermined diameter, and a male-threaded tip portion extending from said smooth shaft portion and coaxial with it, with a male spiral thread thereon, and having a second diameter; a nut member configured for threadably engaging said bolt member and having a blind bore extending from a first end of the nut and opening only at said first end, the bore having a first smooth wall portion of a first predetermined bore diameter to admit entry of the first smooth shaft portion of said bolt member, and a second female-threaded bore portion extending coaxial with the first smooth wall bore portion and having a female spiral thread adapted for threaded engagement with the tip portion of said bolt member; wherein a seal member is disposed within said first smooth wall bore portion of said nut member adjacent said first end; the bore of said nut member is a stepped bore wherein said first diameter thereof is larger than the second diameter thereof and wherein said bolt member is stepped such that the first diameter thereof is larger than said second diameter of said male-threaded tip portion, and wherein said seal member is an annular seal which is fitted into an annular channel formed in the first smooth wall bore portion, such that once said nut member is fastened onto said bolt member, said seal member closes off a space defined between said the tip portion of said bolt member and the female threaded bore portion, such that the male and female spiral threads of the bolt member and the nut member are entirely concealed.