Champion Spark Plugs for Vintage Triumph
The right spark plug...
The right spark plug for your Triumph is as important as any other part of your ignition system. As a general rule, Triumph 500 / 650 / 750 aluminum heads use a 14mm 3/4" reach plug. All Triumph iron heads use a 14mm 1/2" reach plug. Pre Unit square barrel TR5 and 500 Grand Prix also use 1/2" Finding the correct heat range is up to the individual tuner.
Your Triumph probably came from the factory Champion equipped
For most Triumph 500, 650, and 750 twins a good place to start is a set of Champion N3C plugs. These were factory recommended and are a good starting point when tuning. Champion spark plugs heat range increases as the number gets higher. N3C is a colder plug than N4C, and N5C is even hotter.
Iron head Triumph such as the 5T Speed Twin, 6T Thunderbird, and Pre Unit T110 use a 1/2" reach plug. The coldest in this range is the L78C, with the L82C being hotter, and the hottest is the L86C. We suggest trying the L78C first, and keeping in mind the gap should be .018" with magneto ignitions.
Champion (higher number=hotter plug)
Reading spark plugs can be challenging with lead free ethanol added gasoline. Please use caution with a major change or new engine and always use a fresh set of plugs when tuning for speed.
Stan Dibben wrote a great book on spark plugs in vintage motorcycles, and we are providing a link to read it for educational purposes. Although a lot of focus is on NGK plugs, the concepts are the same with all spark plugs.