question about installing spark plugs

Correct

Wrong. I don't know why there is so much misinformation on aluminum heads. Spark plugs will not be "siezed" in an aluminum head when it is hot. I have remove plugs from aluminum heads when hot all the time and have never had a problem. In fact, it sometimes help to get the engine up to operating temp when removing a stubborn plug.

I have also seen plugs back themselves out of an aluminum head while the engine was running because they were not torqued properly. There was no damage to the threads, the plug was reinstalled and properly torqued and no more problems. Plug was easily removed at next plug change.

I have removed and installed spark plugs from cold, warm and hot engines over the years and I have never had a difficult time removing any plug. Be it with a iron or aluminum heads. Even with Japanese motorcycles I have never had a problem. I have always used a torque wrench when dealing with spark plugs.

Correct, aluminum generally expands more that steel (i.e. the coefficient of thermal expansion is greater) and if a material with a hole in it gets hot, it all expands (including the hole). I always use a bit an anti-seize on my plug threads (careful to not get it on the center insulator) to prevent galling of the aluminum and steel threads.

In my experience with precision measuring of holes in parts of various materials tells me that as the material warms up, the holes get larger.

"kend" sez:

Dang, reminds me of a blind date I had in Philly one time ...

Yup! Good clarification.

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After you've picked up that last part for the weekend tune-up

Would you care to explain how the above can happen ?? The spark plug is surrounded by the cylinder head which is full of hot coolant. The cylinder head has much greater mass than the tiny spark plug and is heated by the hot coolant. So just how does the cylinder head cool off while the spark plug stays so hot ??