Spark Plugs for Vintage bikes with Magnetos

By Mark Scott


Our older bikes seem to often have carbs and ignition systems that are worn, or a bit of ‘lubrication’ in the combustion chambers, resulting is the risk of fouling plugs and poor performance. Heck, my old 2 strokes came with a holder for spare plugs under the seat!


Plug technology and materials science have improved the quality of plugs we can get today. New electrode types promise to last much longer, reduce the load on the ignition system and minimize the risk of fouling.


Besides ensuring that you are using the correct thread, temperature range and electrode projection, there are 2 additional factors affecting plug performance in our bikes:

  1. Center electrode size and material
  2. Resistor/suppressor


Center Electrode Size and Material

(NGK examples, but other manufacturers use similar materials)
  • Standard, larger diameter 2.5mm nickel electrodes, like the NGK B6ES or B7ES used in our bikes
  • Thin platinum electrodes at 0.6mm like the NGK B7EVX
  • Thin Iridium electrodes at 0.6mm like the NGK BR7EIX


The science here is that a thinner, harder center electrode will run hotter on the very tip, thus avoiding fouling. The fine wire should require less energy to create a spark. The old school nickel electrodes could not be made too thin as the heat would cause early wear and failure. The larger surface area and cooler tip with the softer nickel material would have a shorter life and potentially require more energy to generate a spark.


Per NGK, their range of precious metal spark plugs provides two major benefits over conventional spark plugs:

  • High Ignitability – is only achieved by utilizing an exposed fine tip center electrode allowing for superior flame growth.
  • Long Service Life – is achieved by utilizing a high percentage of precious metals patented by NGK.
Precious metals such as Iridium and Platinum have much higher melting points over traditional metals such as Nickel.
  • Iridium – 2410°C
  • Platinum – 1772°C
  • Nickel – 1453°C
This allows center electrode diameters to reduce from 2.5mm using Nickel, to 0.6mm using Iridium or Platinum. The finer point means that once a spark forms, the flame kernel does not have a large mass of center electrode that reduces absorption of heat and the flame shadow impeding the flame progress. This equates to a more complete burn that translates to:
  • Improved acceleration
  • Improved fuel consumption
  • Smoother idling


Resistor / Suppressor Plugs

NGK "R" or resistor spark plugs use a 5k ohm ceramic resistor in the spark plug to suppress ignition noise generated during sparking. NGK strongly recommends using resistor spark plugs in any vehicle that uses onboard computer systems to monitor or control engine performance. This is because resistor spark plugs reduce electromagnetic interference with on-board electronics. Many plugs designed for modern vehicles come only with the resistor included. In the case of NGK, there are no iridium tipped plugs that come without a resistor.





The issue in our old bikes is that resistor plugs may cause extra strain on our magnetos in order to get a spark. Brighspark Magnetos conducted research to measure the magneto current and voltage using plain and resistor plugs and determined that resistor plugs require the mag to do more work to achieve a poorer spark. Their results show about a 25% higher voltage and current requirement at the magneto for resistor plugs.



(For me, at least) - in a well-tuned engine, the nickel tipped plugs are just fine. If the old girl has a suspect carb or ignition, then I carry spares, or use non-resistor fine wire electrode plugs for a bit more protection. These can be harder to come by in all heat ranges, but the NGK B7EVX on Ebay for $15 for a pack of 4. I have not been able to find a B6EVX. There is no NGK iridium plug that does not have a resistor.

Yeah, but does it really make a difference?

A couple years back, I was out in Leakey on my Norton Commando when the retaining clip on one of the carb needles broke. The needle slid all the way up, resulting in a super rich condition anywhere other than idle or wide open throttle. I was 200 miles from home but was running iridium NGK BR7EIX plugs. It took 3 stops for gas to make it home, but the bike still idled without stalling, despite the massive abuse inflicted on the plug. The photos below show the 7EIX out of the offending cylinder, next to a regular B7ES. The closeup shows the clean electrode tip, and the small clean area on the other electrode. I don’t think a regular B7ES would have gotten me home without several replacements.


DISCLAIMER: I am not an ignition specialist. The information here is based on my experience and research, and may, in fact, be wrong. Please feel free to comment or point out any errors. I reference NGK for convenience, Other plug manufacturers make great plugs too!













© Copyright 2019 by Mark Scott