Yea in sure they have, but just by looking at them, they grind off half the threads so when one turnes it 90 degrees it can be removed. I remember a few years back when some ford pickups, 5.4's i think, only had 4 threads holding in the spark plugs. Well needless to say many had problems with spark plugs shooting out of the heads! i am sure they're safe but its always in the back of my head