I don't really think they were put in that tight at the factory. I think
that when the engine is first "breaking in", it expands and contracts quite
a bit, and that is what causes the plugs to tighten up.
For this same reason, plugs can get pretty tight in aluminum motors. I left
some plugs in my motorcycle for nearly 15,000 miles and I actually broke one
off at the threads trying to remove them - had to get it out with an
'easy-out'. That's why they recommend changing plugs so often on
motorcycles (like once a year or every 3-5k miles). I also learned to put
anti-seize on the plugs ;-)
Along this same line of reasoning, you want to remove (or loosen) plugs from
(aluminum) engines when they are hot, since the metal is expanded; however,
let the engine cool down before installing and tightening the new plugs.
-- '92 Dak CC 2wd 318 3.55 '84 GoldWing Interstate
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