Wow, that's pretty gunked up!
Thanks for sharing the image though.
I suspect if #7 electrode is getting hit, it is probably hitting something harder than carbon. Thinking your about to have catastrophic failure in #7 from an extremely worn crank of rod bearing.
Do 273 pistons tend to he down in the hole, zero, or above deck?
Anyway, watching to see further teardown and to see which way you proceed with the building.