Sorry to hear about your probs.

The differance in load on an engine from 4th gear to top is quite alot. Detonation will nearly always take place not because of rpm but because of load. Your plug may well of looked perfect in 4th gear but the engine is not under the kind of load it would of been under had you been in 6th. Im sure you would of found the plug looking alot leaner if you had done the chops in 6th.

Next time try going up a couple more jet sized and doing the chops in 6th. You dont have to hold it there for 10 seconds to start with, not untill your confidant that youre on the safe side of jetting. I would say youre at least 2 sized to small going by the damage you did to the piston.
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If I have to take the carbs off once more...