well just crusing at about 2500 rpm and bam, knock,knock,knock. . so i pulled #8 plug and the insulator was gone electrode bent up and gap part sheared off. never seen this one before. my guess is top ring came apart and a piece got wedged between piston and cilinder wall and guilitened the plug. will be pulling head tonight.

sound reasonable ? any one seen this before ?

 

I've seen insulators broken from detonation, but this sounds worse.

 

that was my first thaught but all other plugs looked good, so i blew out that cylinder and put in new plug and wham off with its head.