The title

It's just possible that the hero's name was chosen not because of his enviable sexual apparatus and -performance but because of an ironic double-meaning. The U.S.slang for a W.C. is (a) "John" as in 'John the baptist'(<'flush').The 'Shaft' bit may allude to the plumbing that leads away from the toilet-bowl ( as in 'John-shaft'), pointing up the nature of the slimy ghetto world in which the hero lives and moves (etc.):a social "s-bend", in fact.