If you're approaching the melting point of the aluminum alloy, the threads will be softer, but your head is destroyed. Anything in the normal operating range will not soften the threads measurably.

Hot is a good idea for 606 glowplugs since they sometimes are a bugger. The aluminum has a much higher thermal coefficient of expansion than does the steel glowplug, theoretically decreasing the annular pressure & friction.

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Gone to the dark side

- Jeff