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 some things you actually should to know about plugs.

Heat range
A spark plug is 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. It's its whole world. 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 tranfer path. The colder plug's insulator is short, and thus its 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 off this relative miracle; 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 at high engine speed.

When we talk about "hotter" and "colder" we are talking only of the plug's ability to retain enough heat to keep itself clean. We are not talking about the spark. Advertising notwithstanding, there is no such thing as a "hot" spark. A spark plug has to fight off deposits and it does so by burning them off. The amount of heat necessary to do this is where heat comes into the picture. A spark plug is hotter or colder in absolute terms, not in ways the engine can sense. 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 more. In fact, as already stated, not only is more heat useless, it is harmful as it can quickly lead to plug and engine overheating.

Japanese spark plugs are heat-numbered counterintuitively, with the larger numbers being colder and the smaller hotter. This is because the Japanese focus on the spark plug's heat-removing ability. The American spark plug companies on the other hand focus on the plug's heat retaining attribute, thus a larger number is hotter in American-made plugs. Bith approaches are right, of course. In its all-important heat monitoring role, the spark plug both subtracts and adds heat.

Spark plug sizes
In modern times, we're concerned with only three spark plug diameters: 10mm, 12mm, and 14mm. This diameter is at the thread. Confusingly, there is no completely consistent correlation between spark plug diameter and wrench (hex) size; any combination of the two may be found. But 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 heat tolerance.

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) Hondas. 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 third, in a very few instances, resistive ignition parts actually promote better plug performance, though this is rare and is almost exclusive to very early CDI ignitions 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 more 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 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 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 believe it is. It works only when there is continuous high rpm and load, using ultra-consistent and specially-formulated 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. And once you have sand-blasted plugs, you can forget all about reading them anyway. In fact, plugs should never be abrasively cleaned. Not only does it make carbon adhere easier, it also changes the plug's heat range, is too aggressive on modern plugs' delicate fine wire electrodes, and the abrasive can and will stick inside the plug's cavities to come out later inside the cylinder. All around not a good practice.

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 feature. 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. Madison Avenue's "New and Improved". Read more about this here.

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 four-cylinder Honda, 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. Splitfire got spanked and were forced to stop making these claims.

Seized 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 of anti-seize compound. Pro techs have never embraced that. Keep the hole in the cylinder head clean, screw the thing in straight, and tighten it correctly, and that's all you have to worry about. Concern yourself especially with the angled plug holes in early Honda fours (the SOHC 750 is the worst) 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.

Servicing plugs
Folks have more than a little trouble with spark plug maintenance. They remove them from the engine and then check spark, putting themselves in danger as this is a prime way to burn your bike down, not to mention your garage and house! Cross-threading plugs is another quick route to anguish, not the least because even after you get the plug out, you still have to repair the threaded hole in the cylinder head, and there is no easy way to do that, though a very interesting special tool has recently become available. Some people overtighten their spark plugs, again ruining the theads in the engine or making the plugs difficult to remove. Proper plug torque is very low, about the same as for a 6mm bolt, i.e. about 90 inch-pounds max. It may seem odd to go this low on such a large diameter thread, but remember plugs are mostly hollow, and modern plugs in particular are quite thin on their walls. Many current model powersports vehicles take tiny 10mm spark plugs whose body's wall thickness is deceptively narrow. These plugs have to come back out eventually, of course, and overtightening them will cause them to snap off at the threads because a plug, like any bolt, requires more torque to remove than to install. Few powersports mechanics are fans of putting anti-seize compound on spark plugs. None of the major OEMs recommend it.