In olden times, part of the ritual of changing your engine's spark plugs was making sure that the gap was set correctly for optimum performance. That is, the distance between the spark plug's center electrode and the ground, or side electrode. If that distance, or gap, is too small, the spark that leads to internal combustion will be weak. On the other hand, too large of a gap can result in no firing at all — a misfire. Neither of those scenarios are great and can lead to a loss of power, reduced fuel economy, and heightening spark plug fouling or wear.
Proper spark plug gap varies by vehicle, so you'll want to consult a service manual, the internet, or the literature that's issued by spark plug manufacturers. A typical range is approximately 0.028 inches to 0.060 inches. Put another way, that entire adjustment range is less than one millimeter, so we're dealing with a tiny but crucial adjustment. Fortunately, today's spark plugs are usually pre-gapped for their intended application, saving you the chore of doing it yourself. However, it's still not a bad idea to give new plugs a quick pass with a gapping gauge to double check.
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In general, changing spark plugs isn't required as frequently as it once was. New electrode materials like platinum and iridium on modern plugs replaced fast-wearing copper for spark plugs that can easily last 100,000 miles. Likewise, there's a much broader selection of plugs available with some designs intended for just one specific engine. That means that the gap can safely be pre-set during manufacture without need for change.
However, if you're using a spark plug for other than its intended application, the gap may need to be adjusted to suit. That also goes for older vehicles that may use a more universal spark plug, or for engines that have been modified for performance. For example, converting a classic car from points to high energy ignition (HEI) will permit a larger-than-stock spark plug gap. On the other hand, you'll want to reduce the factory-specified gap if you've added boost like a supercharger or nitrous oxide.
In summary, the majority of drivers shouldn't need to adjust the pre-set gap on an application-correct set of modern spark plugs. If you'd like to double check anyway, it certainly can't hurt and measuring tools are commonly available at auto parts stores for less than $5. If a wider gap is necessary, avoid touching the sensitive center electrode. Instead gently pry on the ground electrode. To close a gap, carefully tap the ground electrode against a hard surface.
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