Spark plugs closing (Protege5)

Bought my first p5 a couple years ago, and got it at a great price since it had a misfire. Resolved the issue by regapping the plugs, as one had closed.

Since I've owned the car, I've had the plug in cylinder 4 close twice, and recently cylinder 2.

Inspecting the cylinders and piston heads with a scope reveals no damage from foreign objects. My only thought is the cylinders are getting hot enough for the compression to close the plugs.

Is this normal? I'm running standard NGK plugs.

Recently dropped a grip on injectors and coils, since I thought an injector might be bad, but it was a closed and damaged spark plug in 2.

Any help and advice would be appreciated.

First thought is....are they the correct plugs? If nothing else is issue then too long of plug would be my first guess as to how this could happen.
I'm fortunate enough to work in the service department of a Mazda dealer, and access to parts. I've got NGKs from parts based on the VIN. Not sure if they're alternative parts, or if they're the exact ones.
I'm just speculating but if your car is burning oil (like just about every P5) then you could have thick carbon buildup on your pistons.


If a piece of carbon breaks off and gets caught between the piston and spark plug, it could close the gap. That stuff is rock hard.

Maybe try and look down the sparkplug hole with a scope and see what your pistons look like.