i've battled this same issue - total pain because you don't want to over torque the caps and risk damaging the plug by twisting the shaft in the insulator. my solution was to pull the caps off and just slightly crush the top of the cap with some vice grips - just bring it slightly oval at the top, but leave the bottom round so it threads on easy most of the way and acts like a locking nut when the top contacts the threads. it takes some finesse to not overdo - you don't need a lot, just enough to get it to grip.