Heat Ranges
Ever wonder why American spark plugs and those made elsewhere are numbered in opposite fashions? On Champion, AC and Autolite plugs a larger number indicates a hotter-running plug, while NGK, Denso and Bosch number theirs higher for cold-running plugs. Why the difference? It has to do with perspective. American engineers view the plug largely as a heat producer, that is, they focus on the plug's heat additive role. Thus a larger number is hotter. The other manufacturers see the plug as a heat remover and focus on the spark plug's role of maintaining a certain amount of heat and getting rid of the rest. To them a larger number is a plug that is more of a heat sink than is a smaller number, Both views are valid. They're just coming at the thing from different directions.

Understand that 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. The heat relationship is, 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 pretty much the same from one plug to another. What changes is how much heat is transferred from the plug--the hottest point in the engine--to the rest of the engine. Thus a hotter plug is one whose heat conductivity is slower (through a longer heat path) while a colder plug is one whose conductivity is faster (a shorter heat path). 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 not only is more heat useless, it is harmful as it can quickly lead to plug overheating. So when someone says a plug has a "hotter spark," this is colloquialism for a larger spark, or a higher voltage spark, and has nothing to do with the plug's heat range, which is a different thing altogether. Manufacturers carefully choose a plug that is hot enough to stay clean, but not so hot it makes the engine hotter. This is not as simple as it may seem, and the plug that does this best is known is the "extended range" plug, a 1970s-1980s development, actually a slightly hotter temperature plug whose tip has been stretched a bit to catch airflow and thereby cool back down. Thus it has a wider temp range. NGK’s “L” (later changed to “A”) series is such an extended range plug (both are however now discontinued). A spark plug is a lot like a thermostat. Or more accurately, a heat sink.

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. 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. It may seem odd to go this low, but remember plugs are mostly hollow, and modern plugs in particular are quite thin on their steel 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. In connection with this, few powersports techs are fans of putting anti-seize compound on spark plugs. None of the major OEMs recommend it. Just an FYI. 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. I know of no career mechanic who advocates anti-seize, and no OEM either in publication or in training material. Keep the hole in the cylinder head clean and tighten the plug correctly, and that's all you have to worry about. Occasionally when I see corrosion in the plug hole, I will add a drop of engine oil to the plug’s threads before installing it. Concern yourself most with cross-threading plugs, which many riders seem to do. To avoid this, put a two-inch length of fuel hose on 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. I like to use an old kickstart lever rubber. Of course, stripping out the plug hole from overtightening is also a concern, but in more than fifty years at this I have not witnessed this very often, not even when training future mechanics. I think most people intuit that the plug does not need to be in there very tight.

Reading plugs
Reading spark plugs is not the science many believe it is. It requires a continuous high rpm and load, using ultra-consistent racing gas, and 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. In fact, plugs should never be sand-blasted. Not only does sand-blasting 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. If the plug can't be cleaned with an aerosol cleaner or with a quick touch with a torch, it needs to be replaced.

U-Groove plugs
Mid to late 1970s Hondas came from the factory with two brands of spark plugs: NGK and ND (later renamed Denso). ND (who incidentally also once made Bosch’s plugs) went through a soft sales period in the late 1970s and to generate new interest they introduced their U-Groove design. Theoretically, the U-Groove plug is supposed to capitalize on the fact that Honda’s four-cylinder bikes fire two of their plugs from positive to negative while the other two fire from negative to positive. Of course, a few of Honda’s early twins do the same thing. And so do older Harleys. Firing from the ground electrode's blunter surface theoretically requires more voltage. The U-Groove design puts a sharp surface on the ground electrode, so that none of the plugs fire from a blunt surface, thereby potentially lowering the ignition system's overall voltage requirement, and consequently improving ignition performance. Whether or not all this works as advertised, well, U-Grooves are still good plugs. And other than the dramatic and over-the-top, surrealistic artist’s graphics in its advertising, Denso has never made sensational claims about its U-Groove plug. However, a company called Splitfire, who had their own version of this two-sharp-edges design, did make claims they had to defend in court before the Federal Trade Commission. They lost their case and were thereafter prohibited from making fuel economy and performance claims. The case is of course in the public record, thus you can find out all about it on the ‘net. Check it out, wherein Splitfire got spanked by the FTC.

What isn't well known is why the U-Groove plug was actually developed. It was simply a marketing ploy to direct attention to improved quality control. Denso's earlier plugs fouled easily. The company 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. When you buy a Denso U-Groove plug, you're actually, more than anything else, buying a more accurately numbered spark plug.

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. 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.