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A good rule of thumb for setting valves is go ahead and do it if your hands won't freeze in the cold or burn on the engine.

The linear coefficient of thermal expansion for aluminum is 22.2E-06 m/mC and for steel 13E-06m/mC. So aluminum expands about twice as much as steel for a given temperature change. To understand these coefficients, a 1 meter aluminum rod will lengthen 22.2 microns (millionth of a meter) for an increase of one Celcius degree. There is probably less than 10 cm (0.1 meter) of aluminum around your valves that will affect clearance, so 1 C means 2 microns. And there are 25 microns per thou of an inch. So to be within a thou of an inch the temperature can vary about 13 Celcius. Most valve specs give a range of a few thou, so your engine could be 30 to 40 Celcius either direction from room temerature, and you would still be on spec.

Hopefully somebody will double check my math and logic.
Engineer or Physicist
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