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I recently changed the plugs in my 07 Honda Goldwing and after reading lots of material and articles about which plug was considered better I went with an NGK Iridium and followed all the technique data I was able to find, including the use of a small amount of anti seize compound on the threads and torquing to 13 ft.lbs. And using a small amount of dielectric grease in the plug boot. The bike never ran better! But I was curious about the spark gap. My question is, when a gap states a range between .038 and .043 what is the optimal? Or is there a way to determine what optimum gap to use? I set mine at .040 just guessing somewhere in the middle was sufficient. But I was curious if there wasn't some method for determining what the optimum gap should be??

  • optimal gap is the one recommended by the manufacturer. – Moab Apr 6 at 18:52
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Nobody gaps plugs anymore, unless they are doing something custom for a racing vehicle or similar.

I certainly wouldn't risk damaging an electrode on an NGK Iridium at 10-15 bucks each. The correct plug will come with an optimal gap already factory set. NGK knows far more about what makes their plugs work well than I will ever know.

In general, a wider gap gives a larger spark bolus. But the available electrical energy ultimately decides how much gap is possible, and what can be maintained at high RPM.

I've increased plug gaps on my F Production '72 BMW 2002tii, but that's combined with a very heavy duty Crane/MSD electronic ignition setup, with a slot wheel and optical trigger in place of points. And if I could easily find the exact specification of spark plug I wanted off-the-shelf, I wouldn't bother.

I really wouldn't sweat 5 thousandths of an inch gap range. You already made likely the best choice with NGK Iridium or similar. Money, but well spent. I suspect they will last 50-75K miles minimum.

  • I still have my plug gapping tool - never damaged either electrode... but those who banged the plugs on a hard surface often did... – Solar Mike Apr 6 at 5:57
  • @SolarMike Agreed. It's nice at our age to still be using our tool . . . – SteveRacer Apr 6 at 8:56
  • Respect for a craftsman - language or on the tools... – Solar Mike Apr 6 at 8:57
  • Something you didn't mention is, in newer vehicles, as the gap increases on the spark plug the ignition system tries to keep up with it, forcing it to bridge the gap. As the gap becomes bigger, it creates more stress on a stock ignition system. It's this way so the system can account for wear over time and spark plugs can last for 100k miles. If you get too wide on the gap, it can start having adverse effects on the system. – Pᴀᴜʟsᴛᴇʀ2 Apr 6 at 15:18
  • @Pᴀᴜʟsᴛᴇʀ2 The stock ignition systems now fitted deliver so much more power (and longer spark duration, even multiple discharge) compared to the old simple coil and distributor with points and a capacitor... – Solar Mike Apr 7 at 5:14

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