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?
Probably a bit of heat cycling, the crush washers are always steel and once done-up as advised - are essentially a one-shot deal.
Plugs really only do need to be nipped-up, so long as gas-tight, it is good.
NB I'm always happy when plugs come straight out easily. Put it this way - the worst possible thing is a sparkplug that refuses to come undone easily: it means anything from huge overtorque on install, or cross-threaded, to massive carbon build-up (engine on the way out somewhere) to ..head -off just to unf*ck it...
None of those options are good!
Plugs really only do need to be nipped-up, so long as gas-tight, it is good.
NB I'm always happy when plugs come straight out easily. Put it this way - the worst possible thing is a sparkplug that refuses to come undone easily: it means anything from huge overtorque on install, or cross-threaded, to massive carbon build-up (engine on the way out somewhere) to ..head -off just to unf*ck it...
None of those options are good!
Edited by Huff on Wednesday 14th June 20:37
2Btoo said:
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.
Have you ever tried removing a plug immediately after you have fitted it? Maybe the issue is that removing the plug simply takes less torque than fitting it, rather than aging? Must admit it's not an effect I notice on mine, although mine rely on a tightening angle rather than a torque limit (and note the tightening angle is substantially smaller if fitting a used plug).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.
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.
2Btoo said:
...
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'd expect to see some evidence of heat (discolouring of the material surrounding the plug). Not sure whether you'd expect to see any sooty deposits on an engine that was otherwise running well (this is a tell tale sign of a poorly sealed diesel injector but obviously diesels are a far sootier burn than petrol). I'd expect it to make a blowing noise!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?...
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