Loose spark plugs?
Discussion
Quickie that has always puzzled me.
Why is it that spark plugs are always slightly loose when you undo them? The car is a 944, I've owned it for more than 10 years and serviced it myself in that time. And every time I've taken the plugs out they are not very tight. Tight enough to need a spanner, but no more than gently 'nipped' up.
New ones are put in to the required torque (20Nm) and left there for 5000 miles or so.
5000 miles later they are removed and ... are looser than when they went in.
Why?
Head is alloty, plugs are Bosch and (I presume) steel bodied. Is there some dissimilar-metals witchcraft going on with the heat-cycling?
Why is it that spark plugs are always slightly loose when you undo them? The car is a 944, I've owned it for more than 10 years and serviced it myself in that time. And every time I've taken the plugs out they are not very tight. Tight enough to need a spanner, but no more than gently 'nipped' up.
New ones are put in to the required torque (20Nm) and left there for 5000 miles or so.
5000 miles later they are removed and ... are looser than when they went in.
Why?
Head is alloty, plugs are Bosch and (I presume) steel bodied. Is there some dissimilar-metals witchcraft going on with the heat-cycling?
Interesting replies - thanks. Looks like my experience is uncommon but not unheard of.
Yes I have tried undoing one as soon as it is done up (wondering whether it's something to do with the crush washer being crushed) and it took about as much torque to undo as doing up.
One associated thing that I ponder is how tight a plug has to be to be gas-tight? And how would you know if a plug was leaking? I guess the latter is answered by the fact that the engine would show symptoms of poor compression, so loss of power and poor economy.
Thanks for your help.
Yes I have tried undoing one as soon as it is done up (wondering whether it's something to do with the crush washer being crushed) and it took about as much torque to undo as doing up.
One associated thing that I ponder is how tight a plug has to be to be gas-tight? And how would you know if a plug was leaking? I guess the latter is answered by the fact that the engine would show symptoms of poor compression, so loss of power and poor economy.
Thanks for your help.
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