When I was a kid, I remember my dad cleaning and gapping spark plugs to get a few more hours out of them. Frugality was his answer to a lot of problems, and it might be where my sense of thriftiness took root, but the practice of cleaning and gapping spark plugs has gone the way of points and condensers.
If you look at a spark plug used in engines in the early 1900s, you can see major differences between it and those used in the last few decades of the 20th century. The differences are striking, and most of the advances stemmed from engineering and design. If you look at today’s spark plugs, they don’t seem much different from those my dad used to clean with a wire brush — but they are markedly improved, due largely to advances in materials science.
Better nickel alloys, advanced metals such as platinum or iridium laser-welded to electrode tips, and ceramics with fewer impurities have all gone a long way toward making spark plugs better suited to their task, and more durable. They cost a bit more, too, and boat owners need to know what the new technology means to them in terms of selection and maintenance.
For starters, the use of advanced metals on the center and ground electrodes resists gap erosion, the slow process in which the firing of the plugs actually wears away the electrode’s surfaces. These fine-wire plugs—so called because the electrodes are much thinner than conventional nickel alloy plugs—are more resistant to fouling and last longer.
When installing fine-wire plugs in your boat’s engine, the differences in materials also require different techniques. For example, attempting to set the gap on a precious-metal spark plug is a big no-no. Gapping tools actually can break the precious metals off the electrode tips. In fact, NGK’s official policy is that customers should not regap platinum and iridium spark plugs, period. They’re gapped at the factory and packaged so that the user can take them out of the box and thread them into the engine.
Regapping nickel spark plugs with a “coin style” gapper is still acceptable, though, as long as proper care is taken.
Use dielectric grease on the terminal where the spark-plug boot connects to keep water and corrosion at bay. Never use a lubricant such as antiseize on the threads because it changes the torque values. And always use a torque wrench— one person’s hand-tight is not necessarily the same as another’s. NGK publishes torque figures in its catalog and online because it is that important.
As a word of caution, spark plugs should never be used to cure a problem. Whether it’s worn piston rings or seals, or fuel delivery too rich or too lean, the spark plug is a bandage fix. Cure the cause, not the symptom.
When iridium or platinum plugs look badly worn, don’t bother to regap them. Throw them in the recycling bin and get new ones. Odds are they have lasted longer than conventional plugs anyway, so you’re still probably breaking even on price. For all its merits, frugality isn’t always the answer.