 Use of a feeler gauge with flat blades instead of round wires, as is used on distributor points or valve lash, will give erroneous results, due to the shape of spark plug electrodes.The simplest gauges are a collection of keys of various thicknesses which match the desired gaps and the gap is adjusted until the key fits snugly.With current engine technology, universally incorporating solid state ignition systems and computerized fuel injection, the gaps used are larger on average than in the era of carburetors and breaker point distributors, to the extent that spark plug gauges from that era cannot always measure the required gaps of current cars.