Damaged Spark Plug

[4sfed]

Finished the weekend with no other issues. Cranking compression was down on that cylinder (80 psi vs 220) but never got worse. The replacement plug started showing the same type of damage after one 30 minute session. After removing the head, the cause was pretty obvious ...
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1300.jpg

[Roundybout]

Does that plug hole have the threads repaired with an insert?

[Roundybout]

Sorry- double post.

[4sfed]

Does that plug hole have the threads repaired with an insert?

Yes, a Helicoil. Not particularly well done.

[MadBill]

Never would have imagined that cause, but it makes perfect sense now! I suppose the larger OD of the Helicoil left such a thin wall between it and the (oversize?) valve seat recess that it cracked and blew through..

PS: What's holding the valve open?

[Roundybout]

Does that plug hole have the threads repaired with an insert?

Yes, a Helicoil. Not particularly well done.

I worked on a mower (flat head Briggs) that had a helicoil repair and the insert was too short. It didn't reach down far enough leaving the last thread or two exposed on the plug in the chamber. It torched the plug pretty good and damaged the head similar to your pics. I assume from not having enough contact to dissapate the heat. Almost like using way too hot of a plug.

[4sfed]

Never would have imagined that cause, but it makes perfect sense now! I suppose the larger OD of the Helicoil left such a thin wall between it and the (oversize?) valve seat recess that it cracked and blew through..

PS: What's holding the valve open?

The distance from plug to valve seat was already thin. The too short Helicoil just exaggerated that situation.

The cams are still in the head.

[MadBill]

Cams in the head? Oh, wait, yeah, I've heard of those...