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Thread: Spark plugs

  1. #21
    Senior Member jiott's Avatar
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    Default Re: Spark plugs

    The NGK DCPR8E plugs are a standard NGK plug used in many motorcycles. So even though NGK has decided not to take the liability on aircraft, these plugs are still readily available from auto parts and motorcycle shops. In fact for 6 years now I and many others have been purchasing these plugs at these stores, rather than from aircraft parts suppliers like ACS, etc. They are still the same plugs; you will just have to ignore the red X thru the airplane, and of course not try to collect from NGK if you hurt yourself using these plugs. To me personally it changes nothing.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  2. #22
    Senior Member jrevens's Avatar
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    Default Re: Spark plugs

    Old thread but new question - I got several boxes of new NGK plugs (DCPR8E 4339) and have checked the resistance on 8 of them with a Fluke 76 meter. Almost all of them are measuring greater than 5K ohms, with the majority around 5.6K, a few around 6.6K, and only one at 4.6K. I've read that 5K or a little less is most desirable. Is anyone else measuring and seeing similar results? Would you find these resistances acceptable? I've got 2 more boxes of 4 that I haven't opened up yet, all having been purchased very recently. I haven't been able to find any information on acceptable limits specifically for these plugs.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  3. #23
    Senior Member Dave S's Avatar
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    Default Re: Spark plugs

    John,

    I am probably the one guilty of introducing the idea here.

    Certainly your Fluke meter can be considered to be a very accurate instrument. The device being measured (spark plug) is more certainly prone to greater variation in materials and manufacturing. Small resistors in conventional electronics are often rated in terms of +/- variation from their labeled value (such as +/-10%, +/- 15% etc). Spark plug resistors will very seldom be found to be exactly one and only one precise value.

    Quality Aircraft Accessories has a description of their take on spark plug resistance at:

    https://www.qaa.com/resource-center/...plug-resistors

    According to QAA, the value of 5,000 ohms is a reference and not an absolute value. Readings of +/- 20% on spark plugs are probably not significant. Testing spark plug resistance helps us exclude spark plugs with cracked internal resistors (like the one that got dropped on the concrete), which can often show infinite resistance at the test voltage.

    The QAA article also includes the comment "Nothing is magical about the 5,000 ohm value"

    I would be confident using the plugs you measured with the stated variations. I find new plugs I use in the same range you indicated.
    Last edited by Dave S; 05-15-2023 at 05:54 AM.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  4. #24
    Senior Member jrevens's Avatar
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    Default Re: Spark plugs

    Thanks for the reply & comments, Dave. I was mostly curious & concerned about the ones in the 6.6K range, but probably unnecessarily. That’s about 30% over 5K. As the article suggests, some problems with rough engine operation were found with plugs having higher than the 5K resistance limit. I believe that may be attributable to the probability that those issues were encountered with magneto ignition systems. A more modern, relatively higher energy solid-state system, like that used by Rotax, probably decreases the probability of any problem with that higher resistance. I was just hoping for feedback about what others who check the resistance have been finding. Thanks again!
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

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