What would cause a spark plug to crack above the nut?

[Red Shift]

This is my brother's subaru legacy. Three seperate (read different brands of) spark plugs in the same cylinder have formed hairline cracks above the nut. I'm talking about the part outside of the cylinder. It's a single coil setup.

Any ideas?

 

[ShelbyGuy]

if the porcelain cracks, its because something is hurking on it in the wrong direction. (why are you asking here and not on a subaru forum?)

 

[redmach1]

I have seen it happen when a hot plug gets hit with water or oil.

 

[ac427cobra]

This is my brother's subaru legacy. Three seperate (read different brands of) spark plugs in the same cylinder have formed hairline cracks above the nut. I'm talking about the part outside of the cylinder. It's a single coil setup.

Any ideas?

Years ago I used to notice this when changing plugs on engines that you could not see what you were doing like a small block cheby. (I still hate 'em) Particularly with Champion plugs.

In a vast majority of circumstances it was due to two main contributing factors.

# 1 was not using a spark plug socket that had a rubber insert in it to prevent busting the porcelain outer body.

# 2 was having excessive mis-aligned pressure when doing the final torque on the plug therefore cracking the porcelain on the body of the plug with the socket.

I can say with 99% certainty that they are NOT cracking on their own!

YMMV

:thumbsup::coolman::beer:

 

[Red Shift]

Thanks guys, I think you are right. I talked to him again and he said two are cracked in the same spot in the same pattern. He said you can barely even see the crack. I would think you guys are right about it being mechanically and not thermally stressed.

He also said that it was the easiest plug to get in though, no odd angles. I recommended for him to remove the rubber boot from the socket and use a 3/8ths hose to retrieve/rethread the plug. The rubber insert would be the most likely thing to stress he top of a plug.