Recently I posted a thread asking for help in installing an HEI in my 72 Buick GS 455 and received a lot of assistance from forum members that led to successfully accomplishing the mission. My car is an original 455 by SN but not a Stage 1. However the prior owner rebuild the engine to Stage 1 specs - so i was told. But all I can really see are the roller rockers installed in what appears to be the stock heads. After getting the HEI to work and the timing set. I decided to install new spark plugs and cut my spark plug wires to a better length since they were all over the engine compartment despite being what was called custom cut wires. When I selected the plugs, I read several posts on this forum and settled on an NKG plug whose PN I cannot recall because my car is in the shop and I don't have access to the plugs. There were 2-3 members that concurred with the use of these plugs or the similar plug that was one notch colder or hotter, I cannot recall which. So I bought the NKG plus, installed them and cut the SP wires to length and started the car. Engine started fine but soon began to make a tapping noise and I had no clue as to what was wrong. I checked the wire connections - all looked good and pulled a couple SPs and they looked fine. I finally gave up and took the car to the shop. Ends up the tapping noise was the pistons intermittently hitting four of the eight plugs - obviously the ones I did not look at. My shop did a compression check on all 8 cylinders to check for piston damage and all were good. The four damaged plugs were only damaged on the very tip of the plug. The ground electrode was either touching or almost touching the center electrode thus reducing the gap to nothing or almost nothing. What did I do wrong and what is the probability of the pistons being damaged even though the compression is good?