® Spark Plugs

The lowly spark plug. How much it is taken for granted! If you can't sleep some night, Google spark plugs and you will get all the esoteric engineering information you can stand -- and then some. However, here are six things you actually need to know about plugs: heat range, sizes, resistor plugs, fine wire plugs, reading spark plugs, and finally, real-world suggestions for working around spark plugs.

Heat range. A spark plug is a lot like a thermostat. Or more accurately, a heat sink. It can't help but be; its firing tip is the single hottest part of the engine. This is a big thing, this temperature deal. Any old plug can throw a spark; the real technology in a spark plug is its contribution to the engine's heat management. One of the spark plug's most important roles is to absorb engine heat, adding to the cooling the engine gets through its fins and circulating liquid. As the plug draws away heat, it uses some of this heat to stay free of carbon deposits. It's meticulously calibrated to do that. If the plug didn't borrow enough of the engine's heat, then it would accumulate too much of combustion's carbon and this would affect the plug's function, because carbon is electrically conductive. On the other hand, if the plug absorbed too much heat, then while there would be less carbon, damage might result to both the plug and the engine, as the plug wouldn't be removing enough heat from the engine; it would be overwhelmed with heat problems of its own. Manufacturers therefore tread a fine line when they determine the correct plug heat rating to use in their newly-marketed bike's engine: hot enough to stay clean, but not so hot the engine ends up itself being hotter. And every engine design is different. Interestingly, the physical difference between a hot and a cold plug is simply the relative length of the insulator above the firing tip. A hotter plug's insulator is long, resulting in a long heat path. The colder plug's insulator is short, and thus thermal conductivity faster. The spark plug is properly matched to the engine when it retains just the amount of heat it needs to ward off carbon deposits, and no hotter.

All kinds of innovations have happened that help the poor spark plug pull that off; that is, stay clean, yet not so clean that it overheats. The most interesting of these are what are called "extended range" plugs, common now for decades. The extended range plug (L or A in NGK coding) is a slightly hotter running plug whose tip has been stretched a bit to catch airflow and thereby cool back down.

Speaking of NGKs, Japanese spark plugs are numbered counterintuitively, with the larger numbers being colder and the smaller hotter. This is because the Japanese focus on the spark plug's role of maintaining a certain amount of heat and getting rid of the rest. To them the plug is a heat remover. Fair enough. The American companies on the other hand view the plug as a heat producer. They're looking at the heat additive role in the thermal equation. Thus a larger number is hotter in American-made plugs. They're both right, of course. In its all-important heat monitoring role, the spark plug both subtracts and adds heat.

Spark plug sizes. Considering just powersports' recent decades, we're concerned with only three spark plug diameters: 10mm, 12mm, and 14mm. This diameter is at the thread. Confusingly, there is no correlation between spark plug diameter and wrench (hex) size; any combination of the two may be found. No matter. The important thing size-wise is thread diameter. But just as important is reach, that is, how long the threaded part of the spark plug is. Beware that installing a plug with the wrong reach can ruin the engine. In NGK spark plugs, thread diameter and reach are coded in the plug's identification number. For example, the common vintage Honda D8EA means D = 12mm thread diameter, 8 = the heat range, E = a reach of 3/4", and the A = NGK's 1980s and later designation for extended range.

Resistor spark plugs. Resistive technology may be found in the spark plug itself, or in the spark plug wire, or in the plug's connecting cap. In some cases you'll find more than one resistive element on the same bike: for example, both resistive plugs and caps are typical in later (post-1980) Japanese bikes. Resistors are added to the secondary side of an ignition system for a few different reasons. The most common purpose on street bikes to eliminate RFI (radio frequency interference) that would disrupt emergency vehicle communications. Second, resistance is sometimes relied on to protect the vehicle's onboard electronics such as computers and LCD instrumentation from the ignition's influence. This is almost as common as the RFI effort, and on very late models is critical. And lastly, in a very few instances, resistive ignition parts actually promote better plug performance, though this is rare and mostly relates to very early CDI ignition found on offroad bikes.

Fine wire plugs. These have been around a lot longer than many realize, originating first as gold alloy, then platinum, and now iridium. Fine wire plugs were originally developed for turbo engines whose higher cylinder pressures made the spark plug's job more difficult. A highly conductive material throws a spark easier, and in a very small diameter easier still, and find wire plugs have both. The exotic metal also resists electrical and chemical erosion despite its firing tip being so tiny. Today's non-turboed but still very high output sport bikes having very high cylinder compression can benefit similarly. However, use of fine wire plugs in vintage, standard compression engines is probably not the best investment and offers questionable benefit other than slightly longer wear life.

Iridium plugs. This is true even with today's iridium plugs, the latest iteration of fine wire. Though now standard equipment in a lot of newer vehicles, the surprising present Internet forum advocacy of iridium in vintage Hondas is strange, misguided and ill-informed. Again, the only benefit is longer plug life. But even that is negated in vintage. The iridium plug's longer plug life was designed to cooincide with the much longer service intervals of modern bikes, many of which are up to 20,000 miles. Vintage Hondas with 3,000-4,000 mile service intervals conflict with this. You are not going to do a complete maintenance service every 3,000 miles and each time ignore the spark plugs. That would be idiocy. Thus it makes no sense to run iridium in forty to fifty year old Hondas. Just one more way forums totally miss the point.

Reading spark plugs. "Reading" spark plugs is not the science many think it to be, at least not in anything other than very specific, highly controlled racing conditions. That is, continuous high rpm and load, using ultra-consistent racing gas, and with almost no trailing throttle. In these very special circumstances it is possible to tell a great deal from the appearance of the plug, even something as subtle as ignition timing. However, outside these very unique conditions, you can instead expect on a plug to "read" only the most general, gross implications, things in fact that are just as easily observed in other ways. Like whether your air filter is clogged, the carburetor jetting way off, or a valve guide seal is ruptured.

Working around spark plugs. Many folks get in a sweat over the spark plug threading into aluminum in powersports vehicles. All kinds of tactics are proposed, including the use an anti-seize compound. Pro techs have never embraced that. Keep the hole in the cylinder head clean and tighten the plug correctly, and that's all you have to worry about. Concern yourself instead with the angled plug holes in early Honda fours which can easily be cross-threaded. To avoid this, put a two-inch length of fuel hose at the top of the plug, and thread the plug into the spark plug hole at least two full turns by hand, before picking up a wrench. Don't use the wrench until the plug threads easily.

A heavily-fouled spark plug in most cases is permanently damaged. It is not a good idea to abrasive-clean spark plugs. Abrasively cleaning a plug reduces its ability to shed carbon; the pebbled texture that results attracting more carbon, effectively lowering the plug's heat range, and making it more susceptible to electrical misfire. Additionally, many modern plugs such as fine wire type will not withstand abrasive cleaning, resulting in their firing electrodes being damaged. And finally and most importantly, abrasives shot onto a spark plug tend to jam themselves into the tight crevice between the steel shell and porcelain and then vigorously resist coming back out again until heated by the engine. Not so good. If you can't clean a plug by aerosol or gentle torch flame cleaning, then it cannot safely be cleaned.

U-Groove and Splitfire. First, Denso's U-Groove design. Its history is interesting. 1970s Hondas came from the factory with two brands of spark plugs: NGK and ND (later renamed Denso). The Nippon Denso plugs were almost always replaced during the pre-sales service because they came already fouled. In fact they fouled easily. ND ultimately responded by revising its numbering, essentially renumbering all their plugs with the next colder number; i.e. the old X22 became the new X24, and the old X24 became the new X27. Then, to draw the public's attention to the improved product, they used the common marketing ploy of doing something visible to capitalize on the invisible--they introduced the U-Groove technology. So it was just an attention-getter, for the most part. The market thought they were buying the new plug for its U-Groove, but were actually buying a more accurately numbered spark plug.

But does it work? Well, the U-Groove design does in fact have a scientific basis. It has to do with the very common "waste spark" ignition system. With just two ignition coils firing the vintage Japanese bike's four cylinders, each coil must fire two plugs, and this at the same time. That is, in series. This means that two of the bike's plugs fire from the center electrode to the ground electrode, and two from the ground electrode to the center electrode. If you're skeptical, look closely at plugs that have a lot of miles on them. Two of the plugs will be worn mostly on their center electrodes, while the other two will be worn most on the ground electrode. Theoretically, the ones firing from the ground electrode's blunter surface should require more voltage, and this might result in a higher required firing voltage throughout the system overall (because they're wired in series). The U-Groove design effectively makes all four plugs fire from a sharp surface, and none from a blunt one, thereby lowering the system's theoretical voltage requirement and resulting in improved combustion as the ignition becomes a more efficient user of electricity. So the U-Groove specifically targets the waste spark ignition system. But is it really an advantage? No one knows. Properly maintained, vintage Kettering type ignition systems work extremely well without needing any tricks.

The more recent and nearly identical Splitfire spark plug operates on exactly the same principle as the ND U-Groove. The difference is purely marketing: the Splitfire's target market is the Harley-Davidson crowd. Carbureted Harleys also use waste-spark type dual-output ignition coils, just as the Japanese inline fours of the 70s through the early 90s did. Incidentally, Denso never made sensational claims about its U-Groove plug. Splitfire however did, and even found itself in court to defend improved horsepower and fuel economy claims to the Federal Trade Commission. Check this out, wherein Splitfire got spanked by the FTC and were forced to stop making these claims.

Related reading
Resistor Plugs, Wires, and Related Issues
U-Gap and Split-Fire Spark Plugs
Powersports Ignition Evolution
High Performance Ignition Coils
GL1000 Ignition Tech
Reading Spark Plugs
Optimizing Early SOHC Four Point Ignition
Servicing the CBX Ignition System
Honda CBX and DOHC Ignition System Troubleshooting
The CBX Ignition Trilogy
Tuning Up the Honda SOHC Four
Plug Wire Tech
Spark Plug Facts

Last updated April 2023
Email me
© 1996-2023 Mike Nixon