MkIndy7
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| posted on 22/4/08 at 08:09 PM |
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Causes of spark plug breakages
Following on from this thread a while back:
Whats destroying my NGK-R's
The car started sounding like a Subaru agan tonight, and after checking plugs no1 and 2 the centre cores of the plugs have broken again.
Were still in the process of tuneing the MS up so its been running overly rich at times and i've made a massive improvement tonight by advancing
the timing 4 degree's throughout the whole rev range so it looks like its been running too retarded.
I replaced the NGK's we had before (heatrange 6?) for BOSCH of the later pinto injection plugs (maybe even the twin cam?) which are heatrange 7
incase that was anything to do with it.
The plugs themsefles show no sign of impact damage, and look a good colour and condition etc.
Anybody have any suggestions? 
[Edited on 22/4/08 by MkIndy7]
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Mr Whippy
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| posted on 22/4/08 at 09:30 PM |
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I suggest you put a picture of the damage on here.
My guess is it's some kind of shock that's cracking them so I'd put it down to detonation, maybe too high a compression ratio or the
timing is out.
Fame is when your old car is plastered all over the internet
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MkIndy7
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| posted on 22/4/08 at 09:39 PM |
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I spoke to an NGK rep and they suggested it was stress on the leads but there's definately no stress.
The timing has been too retarded by the looks of it and its been getting quite hot
(I thought that was normally when the timing was too advanced but If advancing it 4deg makes the performance massively better I doubt that).
I haven't heard of felt it pinking once.
It looks like we had a set of low compression pistons in it so we've decked the block to try and get it back to spec if not slightly raised and
improved and its got an injection spec cam in it which is quite torquey, so there prob is more stress on them.
There's no phyical damage to take pics off, its the centre electrode that cracks somewhere internally, so with the nipple on the end of the plug
done up tight you can turn it through a further 5 deg or so while the tip of the electrode never moves.
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MikeRJ
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| posted on 22/4/08 at 11:14 PM |
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If it's a resistor plug the center core isn't made in one piece.
How are you tightening the nipple, finger tight or big pair of pliers...
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Hellfire
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| posted on 23/4/08 at 07:02 AM |
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Which one best describes your plugs:
EDIT I just read your other answer - I obviously misunderstood - so I've removed some of this posting
If you dont get an answer off here call 01442 281000 and ask for Technical.
Steve
[Edited on 23-4-08 by Hellfire]
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MkIndy7
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| posted on 23/4/08 at 07:31 AM |
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Thanks for the Number Hellfire. In responce to the other qn they will be resistive plugs and the tips only finger tight n a nip. When they feel ok the
car sounds fine, when there 'loose' it sounds like a Subaru!.
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RazMan
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| posted on 23/4/08 at 07:33 AM |
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4 degrees is quite a significant amount of advvance and will make everything run cooler. That might be the simple answer so keep us posted.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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DarrenW
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| posted on 23/4/08 at 03:40 PM |
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You have to be careful comparing heat ranges with other manufacturers plugs - not all follow the same rule of thumb. i had this with NGK vs Champion.
There was a thread about it ages ago.
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MkIndy7
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| posted on 23/4/08 at 05:03 PM |
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Well i've managed to get a number for BOSCH themselfes today and gave them a ring.. they didn't really have that definate an answer of
what could have caused it,
Too much pressure and heat would be likely to cause blow by between the porcelain and the big hex nut you tighten them with.
They suggested vibration, but everythings moving together and the plug leads aren't stressed, unless there's some harmonics going on.
Too tight.. but were unlikely to make that that mistake they were finger tight with a little tweak with the ratchet.
So the heat is all we've got to go on really after a run last night when I tried to check for the fault with the nipple they were a bit too hot
to try really, not sure if thats how hot they normally are?
I was rather under the impression that having the timing too advanced made to too hot, but quite rightly as mentioned having it too retarded as well
can make it hot, and the increase in performance after advancing it 4 deg appears to prove that.
BOSCH advised I try a Heatrange 6 plug (quite rightly again there scale goes the other way to some) instead of the 7's I had before so i'm
off to collect them from pages tomorow... just got to hope the weather nice to try them out!. 
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rusty nuts
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| posted on 23/4/08 at 06:58 PM |
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IIRC someone used to produce an adaptor to fit motor cycle plugs to the pinto head because bike plugs are available in a wider heat range. I may have
read about it in the old CCC mags
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