If a thin ring is heated, it may grow in diameter, including the ID. That’s why hot water makes a jar top expand. But a hole drilled in a block of metal won’t behave that way. It is constrained in all directions away from the hole, so it has to decrease the diameter. I remember when I changed plugs in my first car with aluminum heads. The instructions said to make sure the engine was cold.
@jtsanders, that’s always been my thought too. Conversely, I e seen bulldozer accessory attachment bushings replaced by torching the hole in the 2" plate steel mound, while simultaneously cooling the new bushing insert with nitrogen. They didn’t torch the whole rear end of the dozed to make it work.
It occurred to me (and to a friend with a strong background in physics and an obsession for car repair) that the aluminum head and steel spark plug will expand differently when heated. The effect might distort the match of their mating surfaces of the threads. If that happened, the threads in the soft aluminum block would be sacrificed to the disposable steel spark plug.
So I’m going to remove them cold.
And besides, especially for the rear bank, there’s no way to reach the plugs without getting severe burns.
Absolutely correct that what matters when you heat it is if the aluminum hole gets bigger than the steel spark plug. From this web site http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html I got these values for coefficient of linear thermal expansion (micro-inches per inch per deg-F): aluminum, 12.3; steel 7.3.
So, the hole in the aluminum head expands more than does the steel thread on the spark plug.
BTW, I wondered about the relative expansion of the concentric parts of the spark plug. That web site says coeff for industrial porcelain is 3.6. Looks like there could be some differences between the inner electrode, the insulating material, and the outer metal case/thread. But people have made spark plugs for, what, 100 years. I’ll trust that they have worked that part out.
jt, I don’t know where you came up with your theory but its simply not true. The hole will expand with the block IF the block is heated uniformly. If only a part of the block is heated, then things happen differently, but there are way too many cases of that which would make a practical discussion on a web site impractical.
Anyway, I can also appreciate the part about avoidance of pain and burns. There will be enough pains getting to the back plugs that you don’t really want to add the burns as well. So when are you going to do this and let us know how it went?
Well, I thought I’d have done it already…my mechanic friend has a lift but he lives a couple hours away and I haven’t had time to get there. I may just pay someone locally to do the job to save time. I drive so little lately that it’s not a pressing issue right now, but I’d guess I’d get it done sometime in the next two weeks.
Soak with PB Blast and change with a cold engine.
PB Blast has been tested and its proven to be more powerful than mom’s spit. And mom’s licking a thumb and wiping crap from the corners of little mouths is powerful stuff.