Doesn't aluminum expand with heat? Won't that make the thread hole tighter? Now, making the spark plug thread metal colder, ought to shrink that away from the thread groves. Dry ice? Cold spray?
two tired - i was a thermodynamics major in college and we did a calculation that shows that adding heat to a hole, in metal materials causes expansion, not contraction. i'd use heat - and no anti-seize as the plug is supposed to be grounded to the metal casing.
frank
Frank,
No sir. You would have to heat the entire head to get that hole to expand and those temps would likely char any oil inside. The threaded hole is constrained by the surrounding mass of the head casting. If you heat just the thread hole, the aluminum expands in that area but the only place it has to expand into the where the steel spark plug remains are, making it grip even tighter. Aluminum expands faster and at greater volume than steel which would make the interface between them even tighter. Further, the head is designed to move heat from the hole to the cooling fins. Thereby, the entire mass of aluminum of the head will try to expand in all directions. Such expansion forces will tend to make the hole diameter smaller, as that hole is a stress reliever for the surrounding mass of metal (barring the steel now embedded there). I simply don't believe your calculations ever addressed a metal mass as complex as a cylinder head. If the metal being heated was a simple unconstrained ring, then yes, the hole and the outer diameter would expand. But, if you hold the outer diameter in place physically, the inner diameter, being the only area to expand into, will diminish. A metallurgist and some welders know these things, if not by training, then by experience, which is where the college boys find their calculations have missed several variables that should have been included in the equations. Materials science and thermodynamics are related, but NOT the same thing.
Anyway, it's the relative temperature between the steel (now stuck in the aluminum head), and the head itself that is the goal. Dry ice/cold spray, if applied quickly enough to the inner steel, WILL contract/shrink the steel. Then it is a matter of the conduction rate between steel and aluminum as to which one contracts faster. It's a pity that an easy-out will exert outward pressure on the steel, increasing the friction between steel and aluminum.
Heat on the other hand, may just change any glue properties bonding the steel to the aluminum threads (like burnt oil). Turning that glue to ash may well make extraction easier. But, I'd let it cool before the extraction attempt.
I agree wholeheartedly about the anti-seize, btw. But, that's a preventive, not a cure at this point.
Cheers,
