I keep an old spark plug with a gap of about 1.5 mm just for testing spark. Sparking across a 0.6 mm gap - especially if the spark is red - does not tell you there is enough spark to run an engine. The old Champion spark plug tester that petrol stations used to have long ago tested plugs under continuous sparking in a chamber with a glass window - you could vary the air pressure in the chamber up to 80 psi, and watch the spark. Often at maximum pressure it started to run down the outside of the insulator instead of across the gap. If you increased the plug gap, it always ran down the insulator.
