America's Definitive Marine Engine Magazine

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Marine Spark Plugs
for Boat Motors

marine spark plug cutaway

A detailed call-out of the many components of a sparkplug

What Exactly Makes
Marine Spark Plugs
Marine Grade?

Cold-rolled threads prevent spark plugs from seizing, even when installed in an aluminum cylinder heads especially in a salt water environments. Similarly, the double-dipped zinc chromate-coated metal shell is particularly corrosion-resistant.

Ever wonder exactly what makes a marine spark plug a marine spark plug? There is a vast difference between spark plugs and it's most often measured in heat range. Per se, other than corrosion resistance, there really isn't any big differences between spark plugs intended for inboard or outboard engines and car engines. But it might be helpful to cover a few of the essentials.

The term heat range refers to a marine spark plug's ability to dissipate combustion heat from its firing end into the cylinder head and cooling system. A spark plug must run cold enough at wide open throttle to avoid pre-ignition and hot enough at idle and low rpm to burn off the conductive deposits that would otherwise short-circuit the ignition pulse and misfire the engine. So you can see it's important to have the correct heat range plug installed in your marine engine.

If you want to know how well your spark plugs are matched-up, remove and examine the plugs. In general, if an engine logs lots of hours trolling, and the plugs are black, it's a safe bet the plug isn't hot enough. On the other end of the spectrum, if the engine logs most of its hours running at wide-open throttle, and the insulators are blistered, try a colder plug.

Further, because so often the wrong plugs end up on a marine engine, it's a good idea to check the its numbers to see if the correct type is installed. For example, consulting the Champion Master Application guide, we see that the RBL8 is recommended for some 250 different engines, but you need to reference the serial number to be sure. Another plug, the J2J, also finds a home with some 250 marine engines. As an interesting aside, through the years the venerable J series Champion plug, has seen service in outboard motors, inboards and even Chevy small block full race engines.

Undoubtedly, due to the media blitz, you're probably aware of the recent arrival of the copper core spark plug to the marine world. Although a relative newcomer to automotive and marine power plants, they've been standard fare in aviation engines since the 1930's. Copper Plus spark plugs derive their name from the copper nucleus encapsulated inside the center electrode. And because copper dissipates heat faster than the normally nickel-alloy electrode, the heat range is broadened.

marine spark plug cutaway marine spark plug cutaway an ngk iridium sparkplug a ngk laserplatinum-sparkplug an NGK Iridium sparkplug

LEFT: Laser-Platinum electrode.
RIGHT: Iridium electrode.

Copper core spark plugs feature a longer than normal insulator nose. Thanks to the longer nose, and the inherently longer fouling path, it takes longer to carbon foul a still cold engine. In fact, this breed of spark plug brags twice the carbon-fighting ability of its everyday cousins. With copper, low-speed-fouling, the bane of the trolling fisherman, is reduced. Also, that maddening engine bog on full throttle, after extended idling at the dock, often disappears. A bonus for parts room managers is that with the extended heat range of Copper Core Spark Plugs, fewer models of spark plugs are needed, which in turn means the parts place is less likely to be out of stock when you come a calling.

Platinum spark plugs are renowned for their long life due to a high melting point. A thin wafer of platinum is bonded at the firing point of the center electrode so it doesn't wear as quickly as a traditional spark plug. These are the plugs you typically found in low-maintenance engines.

Iridium Spark Plugs: Iridium is a precious metal six times harder and eight times stronger than platinum and boasting a 1,200 degrees (F) higher melting point. It also conducts electricity better. So the center electrode can be a smaller diameter without sacrificing service life. The ignition spark is more concentrated, improving ignition for better fuel economy, stronger acceleration and sharper throttle response.

Regardless of the manufacturer, or type, spark plugs are easier to remove and replace dockside rather than 20 miles from port with a heavy swell rolling your boat. It's times like these you're liable to discover you don't even have a spark plug wrench on board, let alone the spare set of plugs that probably would have gotten you home. The finale of this sermon is that spark plugs replaced at spring commissioning are cheap while get home under power insurance isn't. While there are some do-it-yourselfers who would disagree with this sage advice, few will deny that in spite of recent technological advances, spark plugs are still a weak link in the ignition system. Remember, even though a spark plug looks good, it might be as dead as door nail. (By the way, what is a door nail?).

Without a doubt, pro-mechanics don't have to play the guess and by golly game do-it-yourselfers do. That's because they've rely on the shop's electronic engine analyzer. When test running an engine, the oscilloscope pinpoints which plugs are firing and which ones are dead. It also shows how well the ignition system is sparking, and how well the cables are shooting the juice to the spark plugs.

With such an tester, you don't suffer the expense of blind remove and replacement. You should also know there's more to changing spark plugs than you might think. For instance, consider the handy-dandy cross reference charts that tell you an Autolite XYZ is the same as a Champion QRS. The problem is the manufacturers don't always agree on plug design specifications and a chart might not recommend the correct plug for your power plant. Don't use cross reference charts, instead use a Champion chart to recommend a Champion brand plug, an an AC chart for AC plugs, and Autolite for Autolite and NGK for NGK.

Reading Spark Plug Deposits

Taking the time to remove a marine engine's spark plugs and examining the firing surface and color of the deposit lends valuable clues as to the general state of affairs inside the combustion chamber. When the firing end of a spark plug is brown or light gray, its condition is good and the spark plug is functioning optimally. Perform this exam as routine maintenance, even when the engine is running fine. But in the real world, folks procrastinate and it's most often done as a last result when if something is amiss, like rough running, stumbling or poor acceleration, degraded fuel economy or a cloud of smoke out the exhaust.

a normal, unfouled, marine spark plug

A healthy marine spark plug insulator presents as a light tan to gray with few deposits.

What a fouled sparkplug looks like

A Burned Spark plug looks like this

a marine spark plug wiht burned marine spark plug showing overheating

When a sparkplug overheats, deposits accumulate on the insulator tip, then melt and look glazed or glossy. Deposits caused by overheating with the electrode appearing lustrous and uneven. If the plug insulator is blistered white or gray, the spark plug heat range is likely to be too hot. But also check for over advanced ignition timing to rule out that malady. Other possible culprits might be the wrong pitch propeller or a defective cooling system running the engine over temperature. Finally, the fuel octane might be too low for the engine, or the gas stale. But aluminum deposits adhering to the electrodes means pre-ignition is melting aluminum alloy off the pistons. Find this and immediately consult a mechanic to find the problem.Blisters on the insulator tip, melted electrodes, or white deposits are signs of a burned spark plug that is running too hot. Causes can include the engine overheating, incorrect spark plug heat range, a loose spark plug, incorrect ignition timing or too lean of an air/fuel mixture. The spark plug should be replaced.

marine sparkplug with melting electrode

A carbon-fouled Spark plug looks like this

marine sparkplug


Black, dry soot on the electrodes and insulator tip indicates a carbon-fouled plug. This can be caused by a dirty air filter, excessive driving at low speeds, too rich of a fuel/air mixture or idling your vehicle for too long. Your mechanic can offer you advice on what type of spark plug to buy to replace the damaged plug, but you may want to consider switching to a "hotter" spark plug (the higher the spark plug number, the hotter the plug).

There are many possible causes for dry carbon or wet carbon fouling. Damp, or wet black carbon fouling is caused by a too cold of a heat range plug, or by prolonged trolling, hour after hour after hour. Other possibilities are the carburetor is adjusted too rich, or on a two-stroke outboard, when pouring too much oil in with the gasoline. Weak spark could also be the culprit. Black, oily deposits on the electrodes and insulator tip points to an oil-fouled plug. Oil could be leaking into the cylinders, getting past pistons that are worn or valve guides that are worn. Finding the source of the leak is very important. Consult a mechanic for guidance. Once the problem is addressed, you can replace the spark plug.


A wet spark plug can be the result of the engine flooding. Flooding is what happens when you try to start the engine several times without it firing up. You can clean the spark plugs or you can just wait for them to dry out. Worn and eroded electrodes are symptoms of a spark plug that is past its prime. The plug has been in the engine too long and needs to be replaced. If the electrodes are broken off or flattened, it is likely that the wrong spark plugs are installed. A spark plug that is too long can cause extensive damage to your engine while a short spark plug can cause poor gas mileage and spark plug fouling. Check your owner’s manual to ensure that you have the correct spark plug for your vehicle.

Tips for re-installing sparkplugs

sparkplug gapping tool

A Lisle spark plug gapping tool.

Some say spark plugs are gapped and ready to go out of the box. Personally, I don't trust that axiom and gauge the clearance to make sure. Technically, gauging is the process of measuring spark plug gap while gapping is the process of bending the electrode wider or narrower to meet specification. Tip: Savvy mechanics set spark plug gap tight side of its specified range. For example, if the gap is supposed to be 30- to 40-thousandths of an inch, as close as possible to 30-thousandths. By this method as the powerplant gains engine hours the gap widens into its spec, as opposed to an initially wide gap that begins to goes out of speck with the first turn of the key. Also know that a feeler gauge won't accurately measure gap on spark plug with a lot of miles under the keel. That's because the ignition arc cuts a furrow in the electrode, a valley that a feelers gauge's flat blade can't dip into and measure. Which is precisely why gapping tools sport an array of different thickness wire loops that can dip into those valleys. Also important to note: Gapping tools for standard and electronic ignitions look identical but have critical differences beetween them. For example, those intended for electronic ignition have loops stepped from .040 to 0.80 thousandths of an inch. While the ones intended for standard ignition are stepped from .025- to 0.45-thousandths of an inch.

When the time comes to install new plugs, screw them in finger tight. If there's a gasket, tighten an additional one-quarter turn; you'll feel the gasket squish as the plug snugs up tight. Another variation, the tapered seat plugs, the ones without a gasket, are tightened one sixteenth of a turn past hand tightening. Regardless of the plug, don't over tighten. If the threads strip, it means an expensive repair bill and a rig tied up in the shop instead of out on the water.

Spark plug sockets are typically lined with a rubber shell that grips the insulator, holding it firmly, while lifting away from the cylinder head. If for whatever reason the liner is missing, an alternative is either a six- or 12-inch length of common 1/4-inch fuel hose. Loosen the spark plug until it's free of its threads, slip the fuel hose snugly over the spark plug's ribbed terminal then yank it out.

For Standard Ignition For Electronic Ignition (wider spark plug gaps)
Inches Metric (mm) Inches Metric (mm)
.025 .64 .040 1.02
.030 .76 .044 1.12
.034 .86 .045 1.14
.035 .89 .050 1.27
.040 1.02 .060 1.52
.045 1.14 .080 2.03