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I replaced all eight spark plugs with Motorcraft AGSF22WM (SP-479) during a tune up. I discovered cheap threaded inserts in cylinders 1, 2, & 3 (inserts came out with the plug, no evidence of thread locker). I opted to replace with nearly identical OEM Tool brand inserts while I had a CalVan kit on order. Started the van after plug install and was having misfires/rough idle. FORScan helped me narrow down the misfires to cyl 1 & 7. I pulled the #1 plug and found that the ground electrode was smashed and bent to the side. Clear impact marks on the ground electrode. Center electrode was marred but not broken off - looked like a fairly "gentle" squish. No obvious marks on the top of the piston aside from one small scuff on the carbon deposits that may have been the location of contact.
My assumption is that with who knows what the previous owner was doing with these inserts, and the not-so-great-feeling I had when I torqued the new plug down to 20 lb-ft - that the spark plug/insert assembly landed too deep and barely made contact with the piston head. When I received my CalVan insert kit, I went ahead and repaired the #1 plug well. While that was curing I pulled the #7 plug and found the same smashed ground electrode - but NO evidence of an insert (confirmed with borescope, only ~4-5 factory threads). This plug was also torqued to 20 lb-ft, and there was a CLEAR mark in top of the piston where contact was made (see attached photo, "white" oval in picture where carbon was chipped off). Using the borescope I found no evidence of any objects in either cylinder, no additional scuffs or pits in the carbon deposits, and no abnormal sounds when the engine had been running, even with the misfires.
I have come across several other posts on various forums of people having similar issues and they always seem to be dismissed by the communities as either impossible, or "something in the cylinder". The only thing I can come up with is that the shoulder where the spark plug bottoms out has been worn out from previous wrenching, possibly from over torqueing, leading to the spark plug sitting deeper in the cylinder than designed, allowing contact with the piston. The old and new non-smashed plugs measure within ~0.004" (0.102 mm) of each other tip to tip when properly gapped. The damaged ones are ~0.04" (1.0 mm) shorter due to the smash. With the plug metric thread pitch of 1.25 (1.25 mm between threads) - that's not even one whole "extra thread". This seems to confirm an "over tightening" situation. Is there a documented point where the this normally occurs? Seems like it might be one of those manufacturing tolerance issues where some heads might be more prone than others.
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