in 07 Fit engine the spark plug hole is located at the junction point of the individual exhaust ports (right between 2 exhaust valves). I would not be surprised if detailed examination will find a hairline crack between exhaust port and the spark plug threaded hole. If such crack is present , the plug will be coming loose over the time.

 

Interesting thought!

Our #3 loosens over time. I've blamed it on improper pitch angle of the threads or not tight enough tolerances of the threads, but your idea could be true.

Of note also though is that the plugs our motors call for have much less total threads than any of my other late model vehicles with COP design. Ecoboost motors plugs have literally 50% or more threads on the plugs.

 

Loose plugs have been a AL head issue for years,, not just honda,, Miata's MR2's , heck my 69 VW bug spit them out..
You cant EVER over torque one,, once you do the threads get torqued out of shape and they never stay tight again.

All I've ever done is cleaned the threads in the head with a nylon gun brush and a gun swab with carb cleaner on it.
Put the plugs in dry and since I change them about every 20 to 30 K miles, never had a problem.

 

That doesn't explain how my friends GE blew the OEM, never changed, plug out of the head. I seriously doubt it was over torqued at the factory.