* 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.

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