is this car a california emmisions car? if it is it will have an extra pipe in the trunk like the gas neck. it is for fuel vapors. 1971 cars have a 1 year only fuel liller neck that has a different pattern gas cap. only a 1971 cap will fit tightly. any other will be loose. chrome aftermarket or painted originals. the only cure is to get a proper 71 cap, or replace the fuel neck w/ a 72-76 a bodie filler neck. dusters and demons and dartsports work as well as swingers and scamps. the necks are the same. i had to do the same thing to mine. 73-76 have a bend for added trunk room as well....