Advanced Engineering Tech - I am baffled?

Customer calls me to say that he was on his car hard the other night and now there was a knock, well more of a thudding above 2,500 intermittently. I figured that he possibly had a fouled plug or a broken valve spring. I pulled the plugs and they all look great except 5 and 7. Here is the interesting part. The 5 plug electrode strap is pushed into the electrode but with no contact marks and with no damage. The cylinder still has compression by plugging it with my finger (not scientific, but I didn't have my compression tester with me. 7 looked pretty crappy too but not touched. I swapped the plugs to 1,3 and the wires and fired it back up with no issues. Idles fine, drives fine, revs up fine. We are going to do a compression test and a leak down test tommorrow. What in the heck would contact the plug strap if there is nothing broken in the motor? The heads are not that milled and it has atleast 15 dyno passes and some good street hits on it and all has been fine till now. I have only seen a broken or damaged plug when a piston has let go.

Could be one of 2 things. Carbon build up on the piston causing it to get closer to the plug. The second thing it could be is that it has spun a rod bearing and the bearings have piggybacked each other which makes the piston push up higher and it will make a thud against the cylinder head and also contact the strap on the plug.

What plugs were in the car?

Or some badass detonation

Thanks for the replies guys. I did the compression and leak down test today which turned out to be very bad. So I got a bore scope in there and the piston from the intake valve relief to the center top of the piston is broke off at the ring land or eyebrow, however you want to say it. I couldn't see any detonation from the scope. All of the tuning logs don't show any detonation and the wide band was right on for both sides. Plus the Closed loop O2's were within +/- 5 of each other on both sides. If I had a lean side it would have shown on the WB's or the CL02's. There was no detectable knock on the street or dyno and the tune is conservative.

The other thing that is bothering me is that the passanger side all came in with in 3psi of 220 on the compression test. The driver side was 215, 220, 175, and 195 respectivly. I did go with five revolutions instead of the regular four though for consistance. I checked all cylinders four time total and the engine temp was 185* and the battery was jumpered to my truck that was running so no speed variations on the cranking.

So I don't get it. As far as I can tell there was no unmetered air getting in. I didn't get to check the #7 cylinder to see what was up with that one. There was not point after I saw the #5. The other possibility I have heard from some experts is cylinder wash. Or some unburnt fuel got behind the ring and ignited and blew the piston top off. I am going to have the brand new Motron 60lb injectors flowed I guess and check the intake manifold for cracks.

I really don't know what could have happend. Why would it happen after all this time if there was a serious problem? I have answers now but no reasoning or solution. When I get back from Combat Diver school I am going to pull the head and see what the extent of the damage really is. This only about my tenth motor and the 30th or so car I have tuned and this is the first issue.