* Chas H is correct. With heat, the aluminum hole will present a slightly looser fit, not a tighter fit. That's why you apply heat (torch) around a threaded hole to work a seized stud/bolt out of it.
* The problem with replacing plugs in a hot alloy head is more of a caution than a problem. It's more likely to damage cyl head threads with overtorqueing when the threads are hot. Overtorqueing is still a problem cold but the threads are less likely to damage.
* Needless to say, the best way to tighten plugs is with a torque wrench. And the most accurate torque wrench scale in this case would be an inch-pound scale (where 18 ft-lbs would, for example, be 216 inch-lbs).
* But if you don't own an in-lb, or even ft-lb, torque wrench, work with a cold engine, put a drop or two motor oil on the plug threads and tighten the plug until you feel the crush-ring (if the plug has one) collapse. If the plug just has a tapered seat (and no crush ring), and you don't have a torque wrench, you'll just have to approximate it and tighten it until it feels gas-tight. Unfortunately, leaving a plug too loose causes a another, different set of problems.

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ASE Master Tech
Quality Enterprises
Mercedes Svc. & Restoration
Sheridan, IN
317.769.3536

Hey guys.
I got the spark plug out with a #6 ez-out.
The advantage I had was the plug sheared off just below the nut, so the whole core and electrode came out in one piece. No need to drill.
I treated the "thread body" with penetrating fluid, tapped the ez-out lightly with a hammer to get it to bite and then gently used a 1/2 inch drive breaker bar to crack it loose.
Worked slick.
Went from a $1400 head removal estimate down to $8.95 and some hair loss. Whew!
Again thanks to all who offered their help.
C'

The hole for the spark increases in size when hot. I have no idea where you got the information it decreases, but it's wrong.

Every machine shop guy and tech I have ever talked to including my father who has 35 years experience as a mechanic. Heat up some aluminum and tell me if it doesn't expand. Since it expands (swells) it will constrict inside a hole.

I have more than 35 years experience and aluminum does expand when heated. Aluminum housing are heated when bearings are pressed in or out. Sometimes a manufacturer specifies heating the head to remove or install valve guides. Your information is incorrect.

I'm glad it worked out well!

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1993 190E 2.6 135k
1989 Ford Thunderbird SC 5 Spd 79K