Ceramic Crack in Plug shielding barrel

What would cause this in Champion (mass electrodes), other than dropping them?

Noticed EGT #2 running higher than usual in cruise, LOP. Did mag check and #2 went cold or intermittent. Plug fired fine at richer mixtures or lower power settings (like during my mag check on the ground). Ended up being the plug and noticed a crack in ceramic installation on the inside of barrel, ie where your ignition lead cigarette end screws into.

Plug is only through about 50% of it's useful life...disappointing considering the price of these suckers.

Over tightening the high tension lead retainer nut will do it. It brunnels the barrel, pushing it down onto the ceramic, and a crack results. Those nuts should not be tightened more than 1/8 past finger tight.