Spark plugs that haven't been changed for a long time can become one with the cylinder head. Fortunately, it's possible to repair damaged threads instead of scrapping the head.

Damaged threads can be restored to better-than-new condition in spark plug holes, carburetor fuel inlets, transmission and engine castings, oil drain plugs, exhaust and intake manifolds, cylinder ...

This is called "seating the plug." You have to do it by hand or you run the risk of starting the plug crooked and ruining the threads on the plugor the threads in the spark plug hole in the engine ...

Ari mentions that there's no way to keep metal shavings out of a cylinder when cutting threads on something like a spark plug hole. I have a way to do it if you're in a pinch and desperate.

A significant mismatch between the spark plug and hole thread can cause detonation-promoting hot spots. If the plug protrudes too far into the chamber, piston interference is also a possibility.

Not long ago, one of my shop mates salvaged a junkyard engine. The only problem found was that one spark plug hole was cross-threaded. The labor cost to pull the head and repair it with a hardened ...

Damaged threads can be restored to better-than-new condition in spark plug holes, carburetor fuel inlets, transmission and engine castings, oil drain plugs, exhaust and intake manifolds, cylinder ...