Today’s fuel-injected marine engines run so clean that plugs need changing less often than in years past. But the day will come. For those with a mechanical jones, or with older engines, here are some helpful tips.
The Right Plug
“Close enough” won’t do. A plug that is too long or the wrong heat range can cause engine damage. Our money’s on the dealer for the accurate information; ask him if the engine manufacturer has issued any service bulletins advising of a new spark plug — or for a special application, like extended trolling.
The Right Wrench
Spark plug sockets have foam inside to protect the plug’s porcelain snout. A standard socket may crack the porcelain. In many applications, the plugs are buried deep, requiring a careful touch, a socket extension and a tight gripping socket, so as not to drop or lose the plug during removal and installation.
The Right Torque
When reinstalling spark plugs, don’t use that old mechanic’s joke specification (“goodentite”). Use a torque wrench adjusted to the proper specification according to the engine manual. Typically, 18 to 20 foot-pounds is as tight as any spark plug needs to be. Overtightening is a huge mistake; if the threads are stripped, the cylinder head will have to be removed and rethreaded. That can mean a huge repair bill, especially with a four-stroke engine. Buy a torque wrench; it's cheaper.
The Right Gap
If an engine uses surface-gap, or “gapless,” plugs you won’t need to set the gap: the distance over which the spark has to jump. If your plugs need to be gapped, invest in a set of feeler gauges or a wire-style gap tool. Coin-shaped gap tools can damage newer precious metal plugs and might break the plug’s tip. Check new plugs as a matter of good practice. The gap setting can be found on the emissions plate of the engine or in the owner’s manual.
Unless the engine is exhibiting running problems, it’s likely the old plugs will all look the same; the insulator nose and electrode should be a deep brown color, with no abnormal deposits. If you see water droplets, aluminum speckles or a whitish color, see the dealer before running the engine. Water droplets might mean a leaky head gasket or exhaust manifold. Aluminum speckles mean preignition and impending “meltdown.” A whitish electrode or insulator nose means a very lean fuel-air mixture. All of these point to an unhealthy engine.
If one or more plugs are not “colored in” (that deep brown color), they might not be firing at all; this might indicate an ignition or fuel delivery problem. As we said before, get the engine to a dealer before running it again.
An extendable “mechanic’s magnet” comes in handy for retrieving plugs dropped in the bilge.