How you can read spark plugs and selectthem - by Gordon Jennings

Stay with motorcycling long enough to swat a few gnats with your noseand you will at least begin to realize how much there is to know aboutspark plugs. Bikers like to tinker, and will replace spark plugs even ifthey don't venture anything else. And in just replacing plugs the motorcyclistbecomes acquainted with the fact that there is more than meets the eye.

Thefirst thing you have to learn is that there are some important differencesin spark plugs' threaded ends, which are made in four diameters and lengths.Most plugs' thread diameter is a nominal 14 millimeters, but Honda -forexample- uses 10mm plugs in small displacement engines and l2mm plugs sparkall the Honda Fours. There also are 18mm plugs, seen only rarely in motorcycleapplications despite the advantage they bring to two-stroke engines. Atone time you had to cope with slight differences in thread configurationon spark plugs from different countries; this worry mercifully has beenended by an international standardization of thread forms.

Because differences in thread diameters are so large, few people getinto trouble through trying to stuff a l4mm plug into a 12mm hole -or viceversa. The same isn't true of plugs' threaded lengths, or "reach."Setting aside for the moment the small variations created by the use ofan inch-based standard in a mostly-metric world, there are just four nominalreach dimensions: 3/8-inch, 1/2-inch, 7/16-inch and 3/4-inch. These dimensionsare followed by engine manufacturers in the depths they give plug holes,and the idea is that the lower end of the plug's threaded shank shouldcome up flush in the combustion chamber.

We know from personal observation that people do make plug-reach mistakes;using 3/4-inch plugs in 1/2-inch holes is the most common error, and onefraught with unpleasant consequences. One of the disasters you can havefrom using a long-reach plug in a short-reach hole is purely mechanicalin nature. In time the plug threads exposed inside the combustion chambermay become filled with hard-baked deposits. If that happens you'll findit almost impossible to remove the plug without also removing the plughole threads. Reversing this kind of mistake, using a plug reach too shortfor the hole, lets deposits fill the plug hole's exposed threads and maycause difficulties when you try to install a plug having the correct reach.

The worst and most immediate problem created by an overly-long plugin an engine is that the exposed threads absorb a terrific amount of heatfrom the combustion process. This raises the plug-nose temperatures, andmay take them up high enough to make the side electrode function as a glowplug. And when that happens you have the white-hot electrode firing themixture far too early, like an over-advanced spark timing but worse becausethe early ignition causes yet higher combustion chamber temperatures, whichcauses even earlier ignition. This condition is known as "runawaypre-ignition," and if it is allowed to proceed it will wreck yourengine.

Even a single plug thread exposed in an engine's combustion chamberwill raise electrode temperatures quite markedly. That could be a realproblem as engine makers don't hold plug-hole depths to close tolerances,and the near-universal adoption of crushable plug washers gives the usera chance to compound errors by over-tightening when installing fresh plugs.Spark plug manufacturers have solved the problem by leaving an unthreadedrelief at plugs' lower ends. The relief also serves as a pilot, guidinga plug straight into the plug hole. Finally, the relief accommodates differencesin opinion between plug makers about how nominal reach dimensions shouldtranslate into actual metal - and there are some small differences.

Matters of thread diameter and length resolved, you can still get intotrouble with a spark plug property called "heat range." All conventionalplugs, whatever the application, have to stay hot enough to burn away deposits(oil, carbon, etc.) that otherwise would short-circuit the spark, and thatplaces the lower limit for temperature at about 700 degrees F. There aremultiple upper limits for plug temperature: sulfurous fuel elements beginchemical erosion of the electrodes above 1100 F.; oxidation of nickel-alloyelectrodes begins at 1600-1800 F.; and at some point (which depends uponcompression ratio, mixture, throttle setting, etc.), the electrodes willbe hot enough to cause pre-ignition. So, to be safe, plug temperaturesmust be held between 700 F. and 1000 F. over the whole range of operatingconditions.

If all engines, and riders, were identical, the spark plug manufacturers'jobs would be easy, as a single plug would be suitable for all applications.Instead, engines vary enormously, as do specific operating conditions,and so the plugs themselves have to be given equally varied thermal characteristics.This is done by varying the length of the path taken by heat as it travelsfrom the very hot center electrode and insulator nose to the relativelycool areas around the body's threads and the plug washer. Plugs with along insulator nose, which leads heat high into the plug body before itturns back toward the cooler cylinder head, are "hot." Short-nosedplugs, with a shorter heat path, are "cold." And these termsare very misleading, as in all cases the object is to match the thermalcharacteristics of plug and engine so the electrode temperature will staybetween 700 F. and 1000 F. We must emphasize that it is the engine thatputs heat into the plug, and not the reverse. A "hot" plug doesnot make an engine run hotter; neither does a "cold" plug makeif run cooler.

The entire question of heat range is something most people find terriblyperplexing - and deal with simply by following the recommendations of theirbike's manufacturer. But this does not always yield satisfactory results,because many motorcycle engines make impossible heat range demands. Free-aircooling broadens the range of engine temperatures; so does the typicalbike engine's specific power output, which is a level encountered onlyin outright racing engines little more than a decade ago. Manufacturerstend to specify plugs with heat ranges chosen with an eye toward "worst-condition"operation, which means that bikes' original equipment spark plugs oftenare a bit cold for those who ride conservatively. Unfortunately, the conservativerider is mostly likely to also be conservative in other ways, and in mostcases will stick with whatever plug his owners manual suggests; the speedmerchants, who are the people manufacturers have in mind when they maketheir heat-range recommendations, usually assume their own bikes need colderplugs.

Knowing which plugs are hotter or colder than the ones you presentlyhave in your bike is easy if you're content to stay with the same brand.Nearly all of the world's plug makers use a number-based code to designateheat range: foreign firms follow a system in which higher numbers meancolder plugs; American companies do just the opposite, assigning hotterplugs higher numbers. Unfortunately, there is no semblance of order beyondthis point. One company, Champion, is in a state of nomenclature transitionthat makes its product line inordinately confusing. The American Rule appliesat Champion, but in an odd way, spread across three series of heat rangesthat encompass touring and racing spark plugs, old and new, with double-digitnumbers assigned to some and single digits for others.

Bosch's three-digit numbers are a holdover from the early days, whenplugs were rated for engines' "indicated mean effective pressure."But combustion chamber pressures alone soon proved inadequate, for it wasfound that the thermal load on a plug also depended upon spark timing,cylinder head cooling and even on the flow of mixture into the cylinder.These factors greatly complicate the business of assigning plugs thermalratings. Each spark plug manufacturing firm has its own test procedure,and though there are efforts being made to bring the whole thing undersome international standard no agreement exists today.

On the other hand, there is an enormous amount of mutual product testingbeing done, and this enables plug manufacturers to offer accurate cross-brandconversion charts. However, it should be understood that the equivalentsare not exact. When plug maker-A's chart shows "equivalents"from maker-B and maker-C it only means those are the nearest equivalents;they aren't necessarily identical. This creates a little confusion, andan opportunity: if you think a particular plug is just a hair too hot ortoo cold, try its equivalents in other brands. You might hit upon preciselythe thermal characteristics you want.

The last point of confusion in the area of heat range is the fact thatthe progression of numbers within a manufacturer's line of plugs may notaccurately reflect the extent of the shift toward hotter or colder thermalgrades. It appears that all the companies began with some neat, evenly-spacedarrangement of numbers and heat ranges, and then had to shuffle everythingaround to align themselves with reality. Apparently some plugs are thermallybiased, hotter or colder, to make them better suited to particular applications- as when an engine manufacturer is willing to order large volumes of plugsif they're biased to suit his needs. And if one of a plug maker's best-sellersis biased colder, while the next-warmer thermal grade is biased a bit hotter,you get a kind of heat-range gap, which can be bridged only by switchingbrands.

There is more to spark plugs than just thread diameter and reach, andheat range. Cramped installations have created plugs with stubby insulatorsand small-hex bodies; aircraft plugs often require strange provisions forshielding; aerospace work has brought us spark plugs that look like a deathray firing-pin. Most of the far-out variety have no conceivable applicationin motorcycling and can be ignored; but there are a few "special"spark plugs you definitely should know about.

Onevery useful variation of the standard spark plug has its insulator noseand electrodes extended from its metal shell. The projected-nose configurationmoves the spark gap a bit farther into the combustion chamber, which tendsto improve efficiency by shortening the distance traveled by the flamefront and also making the combustion process more regular. But there isa more important benefit: the projected-nose plug provides, in many engines,what effectively is a broader heat range than you get with the conventionalflush-nose type. The projected nose is more directly exposed to the firein the combustion chamber, and quickly comes up to a temperature high enoughto burn away fouling deposits after ignition occurs. Then during the subsequentintake phase this plug's exposed tip is cooled by the swirling air/fuelmixture. In this fashion the higher temperatures existing at full-throttleoperating conditions are to some extent compensated by the greater volumeof cooling air, and the net effect is to make the projected-nose plug betterable to cope with the conflicting demands of traffic and highway travel.

It should be evident that the projected-nose plug's effectiveness dependson the pattern of incoming mixture flow. Four-stroke engines often haveintake ports angled to promote turbulence. If the plug is positioned directlyin the path of the intake flow there will be a large amount of heat removedfrom the plug's tip by this direct air cooling, and that is just what youget in most four-cylinder motorcycle engines. Indeed, any hemi-head four-strokeengine gives its plugs' tips quite a useful blast of cold air during theintake stroke, and we think projected-nose plugs probably should be inwider use in bikes than is the case. Two-stroke engines can benefit fromprojected-nose plugs' fouling resistance which they get simply throughthe sheer length of their insulator (it's a long way from the center electrode'stip back up to the metal shell). However, the two-stroke's incoming chargedoesn't always do a good job of cooling its plug, and you have to be verycautious in using projected-nose plugs in the valveless wonders.

Some four-stroke hemi-head engines' domed pistons extend up into thecombustion chamber too far, at TDC, to leave room for plug tips that extendinward. This can prevent the use of projected-nose plugs; it's somethingyou check by covering the plug nose with modeling clay, shaping it so youhave a 360-degree electrode contour, and inspecting for signs of contactafter you've installed your "clearance" plug and cranked theengine over a couple of turns.

Limitedplug/piston clearance in certain racing engines has prompted plug makersto create the recessed, or retracted gap, configuration. Champion inadvertentlydid everyone a great disservice by labeling its retracted-gap design asan "R" plug: people thought the letter meant "racing"and used the R-series in all kinds of high-performance applications, whichwas a terrible mistake. Even if an R-plug's heat range (all are very cold)is right, its gap placement lights the fire back in a hole and the combustionprocess never is quite as regular as it should be. The retracted-gap plugexists only because some engines present a clearance problem; it neverwas intended for use where conventional or projected-nose plugs can befitted.

At one time there was a lot of excitement over another unconventionalplug-nose configuration. In the "surface-fire" plug the sparkgap was between the center electrode and the flanged-inward end of themetal shell, and the insulator material filled its interior out almostflush with the electrode's tip. Surface-fire plugs don't even have a heatrange; they run at about the same temperature as the combustion chamber'swalls and are completely immune to overheating. Neither can they causepre-ignition. These features were stressed at the time of their introduction,and everyone thought surface-fire plugs were just wonderful. They aren't,because they make their spark too close to the chamber wall, and requirean incredibly powerful, CDI ignition system.

Motorcycle ignition systems are the weak sisters of the world's sparkgenerators. Bikes therefore need all the ignition help you can give them,which brings us to yet another useful group of special spark plugs: thosewith precious-metal electrodes. Conventional plugs have thick, blunt electrodesmade of an alloy that's mostly iron, with a little nickel added to lendresistance to erosion. Special-electrode plugs have a side (ground) postmade of ordinary nickel-iron alloy, but a center electrode of somethingmuch more costly - which may be a silver alloy, or gold-palladium, or platinum,etc. Bosch still favors platinum; Champion, ND and NGK offer plugs withelectrodes in materials ranging from silver to tungsten. Gold-palladiumseems to be the alloy that offers the best price/performance advantage;we don't entirely trust silver electrodes, which if overheated will over-expandand crack the insulator nose.

Platinumand gold-palladium alloys can survive the combustion chamber environmentas very small wires, and in that rests their great advantage. Electronsleap away from the tip of a small-diameter, sharp-edged wire far more willinglythan from one that's fatter and rounded. So the fine-wire plug requiresless voltage to form a spark than one with conventional electrodes, andthe difference becomes increasingly biased in the former's favor as hoursin service accumulate and erosion blunts the iron-alloy electrodes. Thereare, of course, drawbacks with precious-metal plugs: they are more expensive,and they are very sensitive to excessive ignition advance. The overheatingyou get with too much spark lead effects plugs' center electrodes beforeit can be detected elsewhere in an engine, and when subjected to this kindof mistreatment fine-wire electrodes simply melt. In one sense this isa disadvantage, as it means the ruination of expensive spark plugs. Seenin another way it's a bonus feature: it is better to melt a plug electrodethan an engine.

A final variation on the basic spark plug theme you should know aboutis something NGK calls a "booster gap," and is known at Championas an "auxiliary gap." By any name it's an air gap built intoa plug's core, and it improves resistance to fouling. Conductor depositson a plug's insulator nose tend to bleed off the spark coil's electricalpotential as it is trying to build itself up to spark-level strength. Ifso much energy is shunted in this way that firing does not occur we saythe plug is "fouled." It is possible to clear a lightly fouledplug by holding the spark lead slightly away from the plug terminal andforcing the spark to jump across an air gap. The air gap works like a switch,keeping plug and coil disconnected until the ignition system's output voltagerises high enough and is backed by enough energy to fire the plug eventhough some of the zap is shunted by the fouling deposits. Mechanics discoveredthis trick; plug makers have incorporated it into some of the plugs theysell, and booster/auxiliary gap plugs work really well in bikes with anignition system strong enough to cope with the added resistance. Such plugsmore or less mimic the fast-voltage-rise characteristics of CDI systems- and offer no advantage used in conjunction with a capacitor-dischargeignition.

It is necessary to know all these different plug configurations if youare to be completely successful in doing your own maintenance work, andit is absolutely essential that you know how to "read" plugsif you're dealing with a high-performance bike (whether factory-built ordo-it-yourself). Sports/touring machines usually are well sorted out beforethey're sent to market, but even the best racing bikes seem to be timedand jetted a little off-the-mark for our fuels and riding conditions. Wesuspect that the laboratory-quality gasoline that some factories use intheir development work warps manufacturers' ignition advance recommendations;whatever the cause, nearly all the factory-built racing engines with whichwe have direct experience run better when their spark timings are slightlyretarded. Typically, too, their spark plugs are one heat range too coldand they're jetted a bit rich. Also typically, these same bikes are fittedwith even colder plugs, richer jetting and sometimes are given more sparkadvance by those who buy them.

The worst, most destructive, combination of mistakes we see begin withtwo widely-held assumptions: first, that a cold spark plug will help fendoff that old devil detonation; second, that more spark advance -not less-is the thing to try when reaching for power. Try to use a too-cold sparkplug and you very likely will have to jet for a lean mixture to avoid plugfouling - and as you lean an engine's air/fuel mixture down near the roughly-14.5:1chemically-correct level it becomes extremely detonation-prone.Excessive spark advance is even worse in its ability to produce detonation,and when combined with a lean mixture it's enough to quickly destroy anengine.

Most people who've had some experience with racing bikes (especiallythose with two-stroke engines) know that detonation is a piston-killer.Few really know the phenomenon for what it is: a too-sudden ending to thenormal combustion process. You may imagine that the ignition spark causesan engine's mixture to explode, but it actually burns. There's a smallbubble of flame formed at the spark gap when ignition occurs, and thisbubble expands - its surface made a bit ragged by combustion chamber turbulence- until all the mixture is burning. This process begins slowly, but quicklygathers speed because the mixture beyond the flame_ bubble is being heatedby compression and radiation to temperatures ever nearer the fuel's ignitionpoint. When the initial spark is correctly timed the spreading flame bubblewill have almost completely filled the combustion chamber as the pistonreaches top center, and all burning will have been completed by the timethe piston has moved just a millimeter or two into the power stroke. Butthe final phase of this process can be shifted from simple burning intoa violent detonation of the last fraction of the whole mixture charge.

Starting the fire too early will produce detonation, as it gives themixture out in the chamber's far corners time enough to reach explosion-leveltemperature. And a slightly lean mixture detonates at a lower temperature.It's all a function of ignition timing and mixture in any given engine,and spark plug heat range plays absolutely no part in it.

Your engine's spark plug doesn't cause detonation but it can tell youwhen and why the phenomenon has occurred. Moreover, the spark plug cantell you with remarkable precision how much spark advance and what jettingyour engine needs. Those are things you can "read" in a sparkplug, and all that is written there will be revealed very clearly whenthe heat range is right.

So how can you tell whether you've chosen the right heat range? It'seasy: a spark plug should be getting hot enough to keep its insulator nosecompletely clean, with all deposits burned away, but not so hot that itselectrodes show signs of serious overheating. These are things to lookfor on a new plug that has been subjected to a few minutes of hard running.After many miles of service insulators acquire a coating of fuel deposits,with some coloration from oil in two-stroke applications, and there willbe some erosion of the electrodes even when everything is normal. Don'ttry to read old spark plugs; even the experts find that difficult. Newplugs present unmuddled information about what's happening inside an engine,and can give you a complete picture after just minutes of hard running.At least they will if they're running hot enough, and that should be hotenough to keep the insulator clean.

It'simpossible to separate the question of ignition advance from the primaryevidence of spark plug overheating, which is most strongly shown on theplug's center electrode. If you inspect this electrode's tip with a magnifyingglass and see that its edges are being rounded by erosion, or melting,then you know there's overheating. You should also have a close look atthe tip of the ground electrode, checking for the same symptoms. Finally,inspect the condition of the insulator, which should be white but witha surface texture about like it was when new; a porous, grainy appearanceis evidence of overheating. If the signs of overheating are confined mostlyto the center electrode you can bet you're using too much ignition advance.Retard the spark timing in small (two or three degrees) increments andas you get close to the optimum advance you'll find two things happening:first, the whole plug will be running colder; second, the center electrodewill begin to acquire a film of fuel deposits extending out from the insulatornose toward its tip.

The fuel film mentioned here is what you watch when making fine adjustmentsin ignition advance. In an engine that's been given just a few degreesexcessive advance (as most have) the fuel film will only extend outwardalong part of the center electrode's exposed length, ending abruptly acouple of millimeters from the tip. The portion remaining won't be filmedover simply because it has been hot enough to burn away the fuel saltsdusted on the rest of the electrode, and you'll see that sort of localizedoverheating created by too much spark advance even on a plug that is twoor three heat ranges too cold. And you'll have the correct spark advancewhen the center electrode's fuel film continues right out to within a hairof its tip. There are a couple of caveats to be observed in this matter.An overly-retarded spark timing won't show except as an absence of anyevidence pointing to too much advance. Also, the spark itself will blastclean spots in the electrode's fuel film, and when there's enough combustionchamber turbulence to blow the spark sideways into a curved path you'llget a cleared area on one side of the electrode. This lop-sided spark blushshouldn't be mistaken for the more sharply defined ring associated withthe electrode tip overheating produced by excessive spark advance.

Once you have brought your engine's ignition timing close to optimumyou'll almost certainly have to make a further change in spark plug heatrange. Manufacturers' specifications for racing models very often adviseyou to use too much advance and a too-cold plug, and when you shorten thespark lead to suit commonly-available fuels it almost certainly will benecessary to use a warmer plug. Then, when you have found plugs of a heatrange that will keep that insulator nice and clean you can start adjustingyour engine's air/fuel mixture - a task that will be easy if you can forgeteverything you thought you knew about this aspect of plug reading.