Why Properly Gapping Your Spark Plugs Is So Important

Spark plug gapping may sound like one of those old-timey chores, like setting breaker points in a distributor or lubricating the frame, but having the proper spark plugs and ensuring they're properly gapped is a crucial part of engine operation. Before we dive too deep, let's start with some basics about spark plugs: They provide ignition for the air-fuel mixture in most internal combustion engines. (You diesel people can sit this one out; we'll talk about glow plugs another time.) 

At the top of a spark plug, you have the terminal nut. This is where the spark plug wire, or the individual coil in a coil-on-plug system, connects to the spark plug. Current travels through the plug to the center electrode on the bottom and then jumps to the ground electrode — that curved, half-horseshoe-looking part of the plug. That current is what we need to control with the gap. Fine-tuning the gap between the ground electrode and the center electrode is what people are referring to when they say "gapping" a spark plug. Set the right gap, and you can potentially avoid issues such as misfires, fouling, quicker plug wear, bad fuel mileage, and reduced horsepower, all of which can all stem from the spark plug gap being too wide or too narrow. The trick is to be like Goldilocks and find that gap that's just right.

There are two main categories of plugs: hot and cold. "Cold" is relative, of course. If you were holding a "cold" plug in your hand as it sparked, the nearly 930 degrees Fahrenheit would feel pretty warm. Cold plugs have shorter center electrodes and a shorter porcelain insulator, while hot plugs have longer center electrodes and porcelain insulator. You fastidious, maintenance-focused folks probably already know all the signs that you need new spark plugs in modern vehicle scenarios, but if you're modifying your vehicle or working on something vintage, you might need to change which kind of spark plug you use — hot or cold.

Turbocharged and supercharged engines tend to run warmer, as do high-rpm engines, and with their hot operating temps, cold plugs will typically be the right choice. Engines running low rpms, such as those in lawnmowers and ATVs, can use nice and toasty hot plugs to help burn away deposits and keep the chambers clean. With a "hot plug," the insulator (the white porcelain part) will be longer than in a "cold plug." Since the insulator, well, insulates, the more of it there is, the better it is at retaining heat. Hot plugs push more heat to the combustion chamber and are better at burning away deposits, something cold plugs can struggle to do.

The specific gap set for your spark plugs depends on a number of factors, including compression ratio, optimal spark advance, quality of the gasoline you use, the elevation at which the car is typically driven, and any mods you may have done, like adding forced induction. 

These days, unmodified cars rarely need fussy fine-tuning of spark plug gaps. But, if you're in the habit of checking and re-checking every tolerance on your car, or you're tuning the engine in your weekend drag car after diagnosing whatever the heck was causing plug wear, gapping can be done with some low-tech tools. The type of spark plug gap tool you need depends largely on the type of plugs you have. The coin-style or ramp-gauge tools are fine for old-school copper plugs, but not for plugs made from platinum, iridium, or ruthenium. Coin tools have gradual measurement ramps and little bottle-opener-style sections for prying gaps wider, but precious metal plugs need something more accurate and less likely to damage the plug.

Feeler gauges are what you want for platinum and iridium plugs. When using feeler gauges, never attempt to use the gauges to pry the gap larger — the gauge will just bend. For the convenience of a coin tool and the precision of a feeler gauge, get a wire loop tool. It's shaped like a coin, but has various thicknesses of wire loops sticking out so you can get that exact gap you want. But, if you need to gap the plug to some odd size, that wire tool might leave you hanging because it only has room for a small number of loops. 

To set your spark plug's gap tighter, you can gently tap the ground electrode against something hard, heavy, and immovable, such as a vise. To loosen the gap, use either the prying section of your gapping tool or a pair of pliers. You'll know the gap is right when your measuring tool needs a decent tug to pull it out. Think of it like a good handshake. You don't want the loose and unpleasant "limp fish," nor do you want the tight "phalange cruncher."