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  • spark plug criteria

    For the engineer in all of us...
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    '41 BC12-65

  • #2
    Re: spark plug criteria

    from: http://www.qualityaircraftaccessorie...plug-resistors


    Discussion about the “right” way to check spark plug resistance can go on until the cows come home. While what happens when 10,000 volts is forced across a resistor under laboratory conditions is interesting, it is hardly relevant to resolving the problems faced by IAs, A&Ps and AEs. The question is; “What’s causing the rough running engine and how can we fix it?” Conditions on a blazing hot or freezing cold ramp aren’t laboratory conditions. What we, as aircraft technicians, need is a useful available tool to identify a spark plug that might be causing a rough running engine.

    Military Specification MIL-S-7886 provides guidance for aviation spark plug manufacturers. It addresses checking spark plug resistance using a low voltage ohmmeter. Hundreds of millions of spark plugs later it’s safe to say that the MIL Spec’s advice is good. A typical low voltage ohm meter (or the Tempest AT-5k meter which relies on a low voltage circuit) can help identify suspect plugs. Although many volt meters use nine volts batteries, that’s close enough. The “low voltage” referred to doesn’t hinge on exactly 8 or 10 volts. It addresses the difference between a “megger” or high voltage resistance check, say 500 to 1000 or more volts, and a typical shop floor meter’s use.

    Resistance checks DO NOT replace the familiar spark plug cleaner/pressure bomb test box that actually sparks the plugs under air pressure. However, the pressure bomb doesn’t check resistance, and as we know from experience, sometimes plugs that check good on the pressure/bomb test box still don’t work properly in the engine. Resistance checks supplement normal cleaning and pressure checking.
    Why Use 5000 Ohms as the Cut-Off Point?

    Five thousand ohms of resistance is a good cut-off point because it provides a value between where spark plugs are manufactured and where problems seem to appear with high resistance. Most reports Tempest receives about rough running engines associated with high spark plug resistance involve values of 7000 ohms or more. Nothing is magical about 5000 ohms, but using it as a cut-off point for spark plugs has proven to work well; just like using 80/60 for compression checks works well.
    New Spark Plug Resistance

    Resistance values differ between manufacturers. Champion Spark Plug Company’s Engineering Manual gives the nominal value for their resistor type of aviation spark plugs as 1000 - 1500 ohms (no +/- value is shown, but minus 400 and plus 1000 probably wouldn’t be unreasonable). Tempest’s “fired in resistor” nominal value is 2500, +/- about 1000 ohms. The absolute values for new plugs aren’t too important as long as they are fairly low; say less than 4000 ohms and remain reasonably stable throughout the plug’s life. Ask your spark plug sales man for the “as manufactured” resistance of his new spark plugs. Then you can compare that to new plugs as received and older plugs during cleaning and inspection.
    Resistor Stability – Check Plugs Before You Install Them

    Resistance instability is at the heart of high resistance spark plug problems. If resistance increases significantly after the plug is manufactured, what was once a good plug becomes a bad plug. Check resistance of all new and reconditioned/cleaned plugs prior to installation. Old technology “stack type” resistor design plugs sometimes come right out of the box with resistance values several times that at which they were manufactured. This type of resistor design is also recognized as sometimes drifting high in service. You’ll save time and money by not putting high resistance plugs in an engine just to have to take them out when you find that they’re causing the engine to run roughly. Resistor-drift is not a problem with the Tempest’s “fired in” resistor design.
    What About the Low Limits for Resistance?

    The resistor’s biggest job is to help provide a sharp, clean ‘pulse’ type spark. If the spark ‘dribbles’ across the gap in the beginning, it doesn’t light the fuel/air charge properly. If it dribbles at the end of the spark, it eats away at the spark plug electrodes and other ‘arc gap’ components in the magnetos, shortening their useful life. The lowest spark plug resistance that seems to perform the task of chopping off the front and back “tails” of the spark is about 500 - 600 ohms. Fortunately, excessively low resistance is almost never a problem.
    Serviceable Spark Plug Resistance

    To help assure reliable and trouble free operations, Tempest recommends replacing spark plugs in service having a resistance value of more than 5000 ohms (5k ohms) or less than 500 (0.5k ohms). For new plugs, we recommend 4000 ohms as the maximum acceptable value.
    Additional Information

    For additional information on this Tempest Tech Tip and Tempest products, please go to www.tempestplus.comor call (800) 822-3200.
    '41 BC12-65


    • #3
      Re: spark plug criteria

      "Is It The Mag"

      The following troubleshooting guide is designed to help you, the mechanic or pilot, determine if the source of your ignition system’s “bad mag check” lies with your engine’s magnetos. This guide is, of necessity, incapable of addressing every conceivable ignition or engine fault. However, it is a good foundation in initial basic troubleshooting and will enable you to find the source of the “bad mag check” most of the time.

      Consult your pilots operating handbook for acceptable mag drop values.

      Most “bad mag checks” are spark plug related. The spark plug is fouled and shorted to ground or is open and the magneto, which is functioning normally, is unable to fire it. A typical bad plug will cause an immediate drop of 250 or more RPM, at the mag check. The key indicator is the suddenness of the drop.

      You have tried leaning and cleaning the plug to no avail. How do you find it? No fancy equipment is needed to isolate the cylinder and its defective plug, if you follow this method:

      •Shutdown the engine and remove the cowling as required.

      •Allow the engine to cool completely.

      •When the cylinders are at ambient temperature, or just slightly warm, restart and immediately turn to the “bad mag.” Adjust the RPM to allow the engine to run at its roughest.

      •Run the engine for approximately 1 to 2 minutes, reduce RPM to idle and shutdown with mixture to idle cutoff. Mags off.

      •With the palm of your hand placed on the cylinder head fins, go from cylinder to cylinder comparing the temperatures.

      •The cylinder with the bad plug will be colder, if not dramatically colder.

      •Trace the ignition harness from the “bad mag” to the cold cylinder spark plug and you will find the non-firing or misfiring spark plug.

      •The spark plug could be lead fouled, fuel fouled, oil fouled, or effectively opened through its resistor.

      •Pick out lead deposit clinkers, inspect the barrel for cracked insulator, inspect the nose core for cracks.

      •Clean and inspect the plug (correct gap for most plugs is .015” to .019” consult your plug specifications).

      •Take an OHM Meter and measure the resistance value from the connection in the bottom of the barrel to the clean center electrode at the firing end, electrode must be bare metal.

      •A new Champion plug will have a value of 800 to 1200 OHMS. New Tempest (formerly Unison-Autolite) will measure 1000 OHMS. Replace any plug above 5000 OHMS.

      •A spark plug bomb tester can test a bad plug and lead you to conclude it is serviceable. The OHM Meter check is simple, readily available, and amazingly accurate in finding misfiring plugs.

      •Reinstall the cleaned, tested, and inspected plug. Re-run the engine.

      If you get the same cold cylinder test, the spark plug lead is possibly shorted to ground. You will need a high tension lead tester to find the fault:

      •Remove the harness cap and test the lead for high voltage breakdown and continuity. (resistance values increase with lead length.)

      •Inspect the insulator boots at both ends of the lead. Leaks resemble a dark pin point on the insulator.

      •Inspect the magneto distributor block tower that goes to your cold cylinder. You are looking for evidence of carbon tracking and a resultant short to ground.

      If you cannot discern any major difference in cylinder to cylinder temperature you have a bad magneto. It is firing all the plugs intermittently and all the cylinders have been functioning.

      •Remove the P-Lead from the “bad mag” and run the engine again to eliminate the mag switch, p-lead wire, and filter capacitor if one is installed.

      Caution: Mag is hot when p-lead is removed.

      A magneto drop that exceeds the allowable limit, but is smooth, with no roughness is in most cases, late engine timing. Cam follower wear makes point opening late.

      •Check magneto to engine timing.

      The magneto designers have gone to great lengths to have the cam follower, or cam in the case of Slick, wear at the same rate as the point faces. This design feature keeps the magnetos internal timing, “e-gap”, at the correct angular opening point for long periods of time.

      •Inadequate lubrication of cam and cam follower will accelerate wear and result in late engine timing.

      •A late spark reduces the sustainable RPM at the mag check. All the fuel is not burned and returned as energy to the piston, but is burned in the exhaust system. Higher than normal EGT will result with late ignition timing.

      •On impulse coupled magnetos a broken impulse coupling spring will severely retard engine timing and result in a very large, slowly occurring RPM drop. In some cases the engine will slow down sufficiently to quit. With single impulse coupled installations hard starting will occur.

      Keep the spark plug ceramic barrel and harness insulator clean. No finger prints. Inspect the ignition harness cigarette springs where they contact the spark plug in the barrel. Look for evidence of arcing. The spring will erode and become razor sharp. This is a bad connection. Change the spring. High resistance connections dissipate energy and can cause hard starting and underperforming ignition.

      Email: Website:
      '41 BC12-65


      • #4
        More on plugs:

        Champion Aerospace: From Denial to Acceptance

        March 19, 2015 / Mike Busch / 35 Comments

        Champion Aviation Spark PlugsAccording to the model popularized by Dr. Elisabeth Kübler-Ross in her seminal 1969 book On Death & Dying, there are five stages of grief: denial, anger, bargaining, depression, and acceptance. This is apparently what Champion Aerospace LLC has been going through over the past six years with respect to the widely reported problems with the suppression resistors in its Champion-brand aviation spark plugs. I last discussed this issue in my August 2014 blog post Life on the Trailing Edge.
        I first became aware of the Champion spark plug resistor problem in 2010, although there’s evidence that it dates back to 2008. We were seeing numerous cases of Champion spark plugs that were causing bad mag drops, rough running and hard starting even though they looked fine and their electrodes weren’t worn anywhere near the retirement threshold. The thing these spark plugs had in common were that they were all Champion-brand plugs and they all measured very high resistance or even open-circuit when tested with an ohmmeter.
        We also saw a number of cases where high-resistance Champion plugs caused serious internal arc-over damage to Slick magnetos (mostly in Cirrus SR20s). If the damaged mag was replaced without replacing the spark plug, the new mag would be damaged in short order. The cause-and-effect relationship was pretty obvious.
        In researching this issue, I looked at the magneto troubleshooting guide on the Aircraft Magneto Service website, maintained by mag guru Cliff Orcutt who knows more about aircraft ignition systems than just about anyone I know. Cliff owns and operates my favorite mag specialty shop, and that’s where I send the mags on my own airplane every 500 hours for inspection and tune-up. In reading Cliff’s troubleshooting guide, I came across the following pearls of wisdom:
        • Take an OHM Meter and measure the resistance value from the connection in the bottom of the barrel to the clean center electrode at the firing end, electrode must be bare metal.
        • A new Champion plug will have a value of 800 to 1200 OHMS. New Tempest (formerly Unison-Autolite) will measure 1000 OHMS. Replace any plug above 5000 OHMS.
        • A spark plug bomb tester can test a bad plug and lead you to conclude it is serviceable. The OHM Meter check is simple, readily available, and amazingly accurate in finding misfiring plugs.
        We started asking the maintenance shops we hired to maintain our clients’ aircraft to ohm out the plugs at each 50-hour spark plug maintenance cycle. The number of plugs that measured over 5,000 ohms was eye-opening. Many plugs measured tens or hundreds of thousand ohms, and it wasn’t unusual to find plugs that measured in the megohm range or even totally open-circuit. Here, for example, is a set of 12 Champion plugs removed for cleaning and gapping from a Cirrus SR22 by a shop in South Florida:
        Champion spark plug resistance
        Notice that only two of these 12 plugs measured less than 5K ohms, and one of those had to be rejected because its nose core insulator was cracked (a separate issue affecting only Champion fine-wire spark plugs, and unrelated to the resistor issue that affected all Champion plugs).
        Why spark plugs have resistors

        Worn spark plug A worn-out spark plug.

        Early aviation spark plugs didn’t contain resistors. They didn’t last long, either. The reason was that each time the plug fired, a significant quantity of metal was eroded from the electrodes. Magnetos fire alternate spark lugs with alternate polarities, so half of the plugs suffered accelerated erosion of their center electrodes, and the other half suffered erosion of the ground electrodes. Eventually, the ground electrodes became so thin or the center electrode became so elliptical that the plug had to be retired from service.
        Spark plug manufacturers found that they could extend the useful life of their plugs by adding an internal resistor to limit the current of the spark that jumps across the electrodes. The higher the resistance, the lower the current. And the lower the current, the less metal eroded from the electrodes and the longer the plug would last before the electrodes got so worn that the plug had to be retired.
        Adding a resistor to the plug also raised the minimum firing voltage for a given electrode gap. The result is a hotter, more well-defined spark that improves ignition consistency and reduces cycle-to-cycle variation.
        The value of the resistor was fairly critical. If the resistance was too high, the plug would fire weakly, resulting in engine roughness, hard starting, excessive mag drops, and (if the resistance was high enough) arc-over damage to the magneto and/or harness. If the resistance was too low, the plug electrodes would erode at an excessive rate and its useful life would be short. Experimentation showed that a resistance between 1K and 4K ohms turned out to be a good compromise between ignition performance and electrode longevity. Brand new Champion-brand aviation spark plugs typically measure around 1,200 ohms fresh out of the box. New Tempest-brand plugs typically measure about 2,500 ohms. Both of these represent good resistance values right in the sweet spot.

        As word of these erratic and wildly out-of-spec resistance values began reaching aircraft owners and mechanics (primarily via the Internet), Champion went on the defensive. At numerous aviation events and IA renewal seminars, Champion reps dismissed the significance of resistance measurements. They explained that the silicon carbide resistor in Champion-brand plugs is made to show the proper resistance whenever a high-voltage pulse is present, and can’t necessarily be measured properly with an ohmmeter. Further, they stated that the proper way to test a spark plug is on a spark plug testing machine (so-called “bomb tester”), and claimed that if a plug functions well during a bomb test, it should function well in the airplane.
        Champion old insulator assembly Champion old insulator assembly.

        Of course, this “company line” from Champion didn’t agree with our experience. We’d seen numerous instances of high-resistance Champion plugs that tested fine on the bomb tester but functioned erratically in service. Nor did it agree with the Mil Spec for aviation spark plugs (MIL-S-7886B) which states clearly:
        4.7.2 Resistor. Each spark plug shall be checked for stability of internal resistance and contact by measurement of the center wire resistance by the use of a low voltage ohmmeter (8 volts or less). Center wire resistance values of any resistor type spark plug shall be as specified in the manufacturer’s drawings or specifications.
        One enterprising Cessna 421 owner named Max Nerheim performed high-voltage testing of Champion spark plugs, and found that plugs that measure high-resistance or open-circuit with a conventional ohmmeter also had excessive voltage drop when fired with high voltage, and required a higher minimum voltage to produce any spark. Max Nerheim wasn’t just an aircraft owner, mind you, he was also Vice President of Research for TASER International, Inc. and was exceptionally qualified to perform high-voltage testing of Champion spark plugs. Nerheim’s findings flatly contradicted Champion’s company line, and agreed with what we were seeing in the field. Nerheim also disassembled the resistor assemblies of a number of high-resistance Champion plugs and found that the internal resistor “slugs” were failing.

        What's your resistance?The spit really hit the fan when Champion’s primary competitor in the aviation spark plug space, Aero Accessories, Inc., launched a marketing campaign to promote sales of its Tempest-brand aviation spark plugs by highlighting the resistance issue. (Aero Accessories acquired the Autolite line of aviation spark plugs from Unison Industries in 2010, an re-branded them under its Tempest brand.) In February 2013, they issued a Tempest Tech Tip titled “The Right Way to Check Spark Plug Resistors,” started selling a fancy spark plug resistance tester, and launched a big “What’s Your Resistance” advertising campaign in the general aviation print media.
        Predictably, this provoked a rather hostile response from Champion. Their field reps ratcheted up their public relations campaign claiming that the ohmeter check was meaningless, and insisting that Champion spark plugs didn’t have a resistance problem that affected the performance of their plugs.

        In the face of both overwhelming technical evidence from the field that their spark plugs had a resistor problem, and a virtual blitzkrieg from their principal competitor that was starting to erode their dominant market share, Champion began having some self-doubts. Max Nerheim discussed his high-voltage test findings with Kevin Gallagher, Manger of Piston and Airframe at Champion Aerospace, and Gallagher acknowledged that Champion was looking into the issue with the resistor increasing in impedance, but did not have it resolved yet. Meanwhile, the Champion field reps continued to insist to anyone who would listen that claims of resistor problems in Champion spark plugs were false and that the ohmmeter test was meaningless.

        Sometime in late 2014, it appears that Champion very quietly changed the internal design of their spark plugs to use a sealed, fired-in resistor element that appears to be quite similar to the design of the Tempest/Autolite plug. They didn’t change any part numbers. So far as I have been able to tell, they didn’t even issue a press release. The Champion Aerospace website makes no mention of any recent design changes or product improvements. But the cutaway diagram of the Champion spark plug now on the website shows the new fired-in resistor. Here are the old and new cutaway diagrams. Compare them and you’l clearly see the difference.
        Champion spark plug cutaway (old) Champion spark plug cutaway (old)
        Champion spark plug cutaway (new) Champion spark plug cutaway (new)
        I checked with a number of A&P mechanics and they verified that the latest Champion spark plugs they ordered do indeed have the new design. It’s easy to tell whether a given Champion spark plug is of the old or new variety. Simply look at the metal contact located at the bottom of the “cigarette well” on the harness end of the plug. The older-design plugs have a straight screwdriver slot machined into the metal contact, while the newer-design plugs do not.
        As I write this, it’s still too early to tell whether Champion’s quiet resistor redesign will cure the drifting resistance problem, but my best guess is that it will. If I’m right, this is very good news indeed for users of Champion aviation spark plugs. I applaud Champion Aerospace for improving its product.
        Still, I can’t help but wonder why it took six years for the company to work through its grief from denial to acceptance. I suppose grief is a very personal thing, and everyone deals with it differently.

        Mike Busch
        Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).

        '41 BC12-65


        • #5
          Great article !


          • #6
            Continental service bulletin on plugs, gives testing for pressures and gap
            Attached Files
            '41 BC12-65


            • #7
              Lycoming service bulletin, requires to use copper based anti-seize and to replace the copper gaskets every time...also special cylinder color coding for long reach plug cylinders.
              Attached Files
              '41 BC12-65