The gasket is actually a "crush washer". It's there to transfer heat as well as to seal pressure. I've been putting the flat side (larger diameter) toward the plug for 50+ years. It was explained to me that the radiused side is supposed to flatten and conform to the shape of the sealing surface of the head, and should be crushed about halfway by the tightening force, not flattened out completely.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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