At one point in the late 70,s or early eighties NGK came out with a new plug, may have been the U tip.
I put them in my 900cc single cam Honda, and the pistons hit the plugs closing the gap, put normal plugs back in, and it was fine..
the ?900? turned out to be an 836 using cut down CB 350 pistons, they do have high domes.
Just mentioning it ..if clearance is quite close (due to parts used) shimming or changing plugs might make a difference..