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Gapping spark plugs

Discussion in 'Car Repairs / Maintenance' started by DIYspanner, 12 Nov 2018.

  1. DIYspanner

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    I know a lot of them are pre-gapped in the factory and surely all plugs work best if the gap is as the plug is designed to work best so why do different engines need a different gap from the same plug.

    Actually, the set of Bosh Super Plus I've got to in an uncle's Merc I doubt could be gapped because a gapping tool is more likely to cause damage with the plugs having several earth electrodes.

    I once took the plugs out of a Vauxhall Carlton 2.0 and the earth electrodes were melted back to the thread. The car was still running fine.
     
  2. Yafo

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    I don't think many cars use the same plug nowadays like they used to. Used to be 1 plug fitted something like 75% of the cars on the road, so was gapped to suit. Several plugs now of the same type come in various different gaps ready for the specific vehicle. And there are lots more plug types to cover the range of vehicles, compared to years ago.

    Most common cause of the earth electrode burning away used to be the coil wired the wrong way round.
     
  3. Motman

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    Yeah, used to be mainly N9Y and N5Y until Ford OHC’c came out and they were five bob (5/- or 25p in new money ) for years. As an apprentice we had a spark plug cleaning and testing machine that we used to clean the plugs by blasting them with grit.
     
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  4. Burnerman

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    Two or three earth plugs don't require gapping....the central ceramic would be damaged anyway if you tried.
    If the plugs are really burnt away then either they are ancient or the wrong heat grade.....over advanced ignition timing (thing of the past) could also cause this.
    The plug gap really depends on the output of the ignition coil, but to be honest I don't set them anyway - just a visual check in case one has been dropped.
    John :)
     
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  5. DIYspanner

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    This fella seems passionate about spark plugs!
     
  6. Burnerman

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    Ye gods - only in the States :eek:
    Sorry, could only stand the first 20sec!
    John :)
     
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  7. nauseous

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    Had a few nasty belts off spark plugs, ht leads in the past, messing about seeing if there's a spark on various mopeds lol. Glow plugs ftw ;)
     
  8. Keithmac

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    I've found the stricter the emissions control the bigger the gap, bikes used to be 0.7mm across the board but with newer Euro emission laws some are up to 1.1mm!.

    Better lighting of leaner mixtures at idle and cruise..
     
  9. Avocet

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    That's the thing. The bigger the spark, the more likely it is that you'll achieve complete combustion of what's in the cylinder. However, you also put a higher load on the coil packs (or other ignition components if you still have a good old fashioned distributor). Different manufacturers will take different decisions on where to make the compromise. If you have a lousy combustion chamber shape and "wild" valve timing, you'll probably need a meatier spark to get through the type approval emissions test (or in extreme cases, TWO sparks, like the Alfa Twinspark engines).
     
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  10. norseman

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    There's a blast (no pun intended) from the past! The numbers (9 or 5 in your example) refer to the heat range, usually the higher the number the hotter the plug for greater fouling resistance, whereas a cooler plug is better for higher speed engines & resistance to pinking. My 31 year old Rover V8 is happy on good 'ole Champions.
     
  11. Astra99

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    Hmm. Rover V8. Champion L87Y if my memory is correct! With modern multi-electrode plugs, setting the gap is virtually impossible using standard feeler gauges, so once the gap becomes too great and misfiring occurs, the time for a new set has arrived! BTW, my preference is now for NGK plugs.
     
  12. Keithmac

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    Worth noting NGK are the other way regarding heat range, the higer the number the colder the plug..
     
  13. norseman

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    I don't know how far back in time you are going but the manuf. stated Champion for the later 3.5 efi is RN9YC though mine has RN11YC's installed, fitted by a local independent. As for brands I've stuck with Champs. for decades, in various vehicles & never had a problem.
     
  14. Avocet

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    "Lodge " always used to be Alfa Romeo's plug of choice - there's a name in spark plugs from the past! Alas nowadays NGK seem to have dominated the market - with Bosch hanging on in there, and Champion only just hanging on. Nothing against NGK - excellent plugs, but I do miss the variety. Monopolies are never good for the consumer.

    As mentioned before, modern ignition systems demand more from the spark plug. The old single electrode Champions would be fine in the old Rover (and my old Alfa), but I'm not sure how long they'd live in a modern car.
     
  15. norseman

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    Too true! I've no idea what's in our daily driver, other than checking the levels/tyres anything under that plastic cover is down to the local garage :unsure:
     
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