How to adjust spark plug gap

Spark plugs: the forgotten Corvette service item


Today, spark plugs are pretty easy to forget. In new Corvettes, theyseem to last forever. That wasn't the case years ago and, frankly, sparkplugs are often critical when it comes to performance and economy. Theyshouldn't be neglected in your Corvette, and you should think of them asa window into the world of your engine. Certainly, proper selection isimportant, but once you understand what makes a spark plug tick and whatto look for, you might be able to pick up performance, economy, anddriveability--in some cases, lots of it.

Spark Plug Basics

Before looking at spark plugs as a Corvette tuning tool, let's discusswhat makes a spark plug function. They aren't as simple as you think--infact, they're precision pieces of equipment. Keep in mind, the vastmajority of spark plugs offer similar (although not necessarilyidentical) construction to the ones shown here.

Ribs. Insulator ribs provide added protection against secondaryvoltage or spark flashover and also help improve the grip of the rubberspark-plug boot against the plug body.

Insulator. The insulator body is molded from aluminum oxideceramic. In order to manufacture this part of the spark plug, ahigh-pressure dry molding system is utilized. After the insulator ismolded, it's kiln-fired to a temperature that exceeds the melting pointof steel. This results in a component featuring exceptional dielectricstrength, high thermal conductivity, and excellent resistance to shock.

Hex. The hexagon provides the contact point for a socket wrench.The hex size is basically uniform in the industry and is related to thespark-plug thread size.

Shell. The steel shell is fabricated to exact tolerances using aspecial cold extrusion process. Certain types of spark plugs make use ofa steel billet (bar stock) for shell construction.

Plating. The shell is almost always plated. This enhancesdurability and provides rust and corrosion resistance.

Gasket. Certain spark plugs use gaskets while others are"gasketless." The gasket used on spark plugs is a folded steel designthat provides a smooth surface for sealing purposes. Gasketless sparkplugs use a tapered seat shell that seals via a close toleranceincorporated into the spark plug.

Threads. Spark plug threads are normally rolled, not cut. Thismeets specifications set forward by the SAE along with the InternationalStandards Association.

Ground Electrode. There are a number of different groundelectrode shapes and configurations, but for the most part they'remanufactured from nickel alloy steel. The ground electrode must beresistant to spark and chemical erosion, both under extremetemperatures.

Insulator Nose. There are many insulator nose shapes and sizesavailable, but all must be capable of shedding carbon, oil, and fueldeposits at low speeds. At higher engine speeds, the insulator nose isgenerally cooled so that temperatures and electrode corrosion arereduced.

Center Electrode. Center electrodes must be manufactured from aspecial alloy that's resistant to spark and chemical corrosion. Keep inmind that combustion-chamber temperatures vary (sometimes radically);the center electrode must live under these parameters. The area betweenthe ground electrode and center electrode is called the gap.

Basic Spark-Plug Types


Fine-wire spark plugs are intended for high-performance applications andare particularly useful when the engine in your Corvette requires a wideheat-range latitude. This type of spark plug is well suited toconditions where the mixture is rich and there is a wide differencebetween cylinder-to-cylinder cooling.

There are at least a dozen different spark-plug configurations availablein today's marketplace. Some are unconventional and extrinsic. Thefollowing is a rundown of some basic high-performance configurationsyou'll come across.

Regular Gap. The regular-gap spark plug is most often associatedwith production vehicles. The side electrode extends midway to thecenter electrode, requiring less voltage at high engine speeds andpreventing particles of carbon (or other foreign material) from blockingthe gap and shorting out the plug. This spark-plug design offers goodprotection against fouling and provides good service in street-stripautomobiles.

Projected-Nose Gap. Projected-nose spark plugs can be used in anyoverhead-valve engine provided there is physical clearance in thecombustion chamber. The heat-range characteristics of this spark plugdiffer from other spark plugs in that the incoming air/fuel charge coolsthe insulator tip at high engine speeds, preventing fouling that can becaused by excessively rich mixtures.


This spark plug is useless in almost any application except wherenitromethane is the fuel of choice. The retracted-gap spark plug is anall-out race spark plug designed for applications where there is limitedphysical clearance between the pistons and/or valves. It's primarilyintended for use on supercharged, highly modified and/or nitro-poweredapplications. The protection against fouling is substantially limited. Bottom line? Don't use one in your Corvette. It won't help.

Fine-Wire Gap. The fine-wire spark plugs are intended for raceapplications and are particularly useful when the engine requires wideheat range latitude. Additionally, this type of spark plug is wellsuited to conditions where the mixture is rich and there is a widedifference between cylinder-to-cylinder cooling.

Retracted Gap. The retracted-gap spark plug is an all-out racespark plug designed for applications where there is limited physicalclearance between the pistons and/or valves. It's primarily intended foruse on supercharged, highly modified and/or nitro-powered applications.The protection against fouling is substantially limited and, as aresult, it should not be used for street applications.

Selecting A Plug

Different types of spark plugs are designed to do different things, butthey all have one common function: ignite the air/fuel mixture in theCorvette combustion chamber and, at the same time, ignite the air/fuelcharge with some manner of efficiency. But there's a caveat: There is noperfect spark plug for a particular application. Some race engines runextremely hot (i.e. nitro-burning Top Fuel Dragsters). Because of thetemperature extremes experienced in something like a Top Fuel Dragsterengine, a cold spark plug is required. On the flipside, a daily drivenCorvette might have a cold-running engine. The type of spark plugutilized is just the opposite: It must be relatively hot. Hot enginesequal cold plugs; cold engines equal hot plugs. These are the basics ofthe spark-plug heat range.


Some Corvette engine combinations prefer more spark-plug gap thanothers. A gap of approximately 0.025 inch is commonly used inconjunction with early Corvette point-triggered ignitions. On the otherhand, contemporary electronics can easily fire gaps in the range of0.045 to 0.060 inch (or more).

The thermal characteristics or "heat rating" of a given spark plugreflect the component's ability to transfer combustion-chamber heat fromthe firing end of the spark plug into the cylinder head. The range oftemperatures from idle to maximum rpm determines the heat range of thespark plug. From a design and construction perspective, the length ofthe insulator nose determines the heat range of the spark plug.

Cold spark plugs normally have a short heat flow path, which results ina quick rate of heat transfer. Additionally, the short insulator nosefound on cold spark plugs has a small surface area, which does not allowfor a massive amount of heat absorption. On the other hand, hot sparkplugs feature a longer insulator nose as well as a longer heat transferpath. This results in a much slower rate of heat transfer to thesurrounding cylinder head (and, consequently, the water jacket).

So what's wrong with using spark plugs that are too cold for yourengine? Simple. The engine loads up, carbon forms, and the spark plugsneed constant attention. If the spark plugs are too hot, they begin toglow. The result is detonation that can easily ruin a cylinder (alongwith the piston). Of course, these are extreme cases, but even a sparkplug that's slightly too hot inevitably fails over a period of time. Inessence, the combustion-chamber temperature of your engine combinationdictates the heat range of the spark plug.

But there's more to the heat range selection process than just fuel typeand hot versus cold parameters. For example, in a performanceapplication, a Corvette with an automatic transmission is generallyoperated in a wider rpm

range than a stick-shift counterpart. It's alsomore difficult to clean out a car with an automatic transmission(cleaning out means blipping the throttle to obtain crisp throttleresponse). Because of this, the spark plug must be capable of burningoff deposits that would otherwise cause fouling problems. At the sametime, the automatic-transmission car is then accelerated to the higherreaches of the rpm range. Conversely, a stick-shift car is commonly run"on the cam" (typically in the higher reaches of the rpm band). Bydisengaging the clutch and working the throttle, a stick-shift carseldom, if ever, sees the lower speed ranges. As a result, the Corvetteengine does not normally experience the fouling problems associated withautomatic-equipped cars.

In order to select the right heat range for your engine, use a safeselection process. Start with a set of spark plugs that are too cold andwork your way up until fouling stops. Use a spark plug that's severalheat ranges colder than specified in a factory shop manual for yourvehicle. According to the folks at Autotronic Controls Corporation(manufacturers of MSD ignition products), the tip temperature of thespark plug must stay lower than the normal pre-ignition temperature ofapproximately 1,400 degrees F. In addition, the tip temperature mustremain hot enough to prevent oil or carbon fouling. This requires atemperature of approximately 800 degrees F. MSD feels you should try touse the hottest tip temperature that will "live" in the engine withoutcreating any spark plug-induced problems (such as detonation). Aprojected-nose spark plug is preferred if it clears the piston dome(supercharged powerplants will likely require the use of a side-gapplug).

The Nose's Job


The aftermarket is filled with good-quality plug magnifiers. Here's acheap alternative: It's a photographer's loupe. Basically, it's a simpleplastic magnifier that's available in a number of differentmagnification powers. The example shown is an eight-times version.They're downright cheap and available at any camera store.

Projected-nose spark plugs are preferred for several reasons, butperhaps the most important is the plug's ability to act like a hot sparkplug at low rpm levels, then cool off and act like a cold plug at highengine rpm. The projected-nose spark plug is cooled by the incomingair/fuel charge that effectively blows by the extended spark-plug tip.At low engine speeds, the increased insulator length improves thespark-plug temperature characteristics, allowing the plug to burncleanly without fouling. Further, the projected nose helps place thespark plug in a well-suited position to initiate the ignition process.

The High-Power IgnitionSystem Difference

The use of modern day high-powered ignition systems such as computerignitions, multiple-spark ignitions, and many of today's digital systemsalso creates problems in spark-plug selection, but not in a way youmight first imagine. High-quality, high-power ignition systems producerelatively large amounts of coil current. Because of this, thespark-plug condition is not as critical as it once was (in the days ofsingle- and dual-point distributors). While the spark-plug condition isless critical, it doesn't mean the spark plugs are "set and forget"components. Reading of the plugs with these ignition systems iscertainly more difficult, but it isn't impossible.


Indexing is quite common in racing. What is it? Here are the basics: Inorder to index spark plugs, the insulator body must be marked with afelt marker (don't use pencil because it can create a carbon track forsparks to follow). The first step is to place a long mark on the side ofthe plug where the ground electrode attaches to the spark-plug body.

Many Corvette enthusiasts make the switch from a conventional ignitionsystem to a high-powered system of some sort, then claim the spark plugsare reading lean. Think about this: The air/fuel mixture hasn't beenchanged; how can the ignition system affect the air/fuel mixture? Itcan't, but because of the better burning properties of a modern ignitionsetup, changes may be required in the air/fuel mixture.

So why are the spark plugs so clean? Because the ignition system isefficient. In order to read spark plugs used in conjunction with acontemporary ignition system, MSD offers the following suggestions.

1. Pick up a copy of a spark-plug tuning guide (all of the majorspark-plug manufacturers offer some form of a high-performance pockettuning guide). Examine the correct color and appearance of your sparkplugs and compare them to the guide. This should give you some insightinto things like detonation, overheating, and coloration.


Instead of rummaging through boxes and boxes of spark plugs in an effortto locate the perfect (and sometimes unobtainable) combination of plugthreads that match the head threads, use aftermarket indexing washerssuch as these models from Moroso and B&B Performance. The washer kitsare typically supplied in 0.060-, 0.080-, and 0.100-inch thicknesses.

2. There are several methods to prepare for reading spark plugs, but inthe case of a high-performance Corvette, there is only one right way toaccomplish it. The engine must be shut off "clean." This means youshould run through a quarter-mile and click off the engine, allowing thecar to coast to a stop at a safe location. Don't drive the car anyfurther. The idea is to stop the engine clean, then remove the plugs andinspect them. Obviously, this type of clean shutoff will be easier withmanual-transmission Corvettes than it is with automatics. Nonetheless,it's necessary because the trip back down the return road can wipe outany valuable reading information provided by the spark plugs.

3. The point of effective heat transfer generates a ring on thespark-plug insulator nose. The ring almost looks like a shadow and ismore easily observed with a spark-plug inspection light or loupe. Thering forms quickly and is an indication of burning in the combustionchamber. The closer to the tip the ring forms, the richer the fuelmixture. The closer to the spark plug shell, the leaner the mixture. Ifthere is no ring and the spark plug insulator tip is bone white, thefuel mixture is too lean (this information applies only togasoline-fueled race cars). The total range from rich to lean (ring nearthe tip to ring near the shell) may only take a change of 7-10 percentin the air/fuel mixture.

When making the switch to a high-powered ignition system, you may findthat the actual fuel curve will have to be "fattened" by 5-10 percent,because the ignition system is now burning more efficiently. Inaddition, the total spark timing might have to be reduced byapproximately 3 degrees.

As you might have guessed, reading the spark plugs isn't a black art,but it isn't an exact science either. You have a number of variablesopen to you: Air/fuel mixture, spark-plug heat range, ignition timing,fuel type, transmission type, and a host of other things that aren't inyour control (ambient temperature, weather conditions, altitude, and soon). The idea is to make the most of your situation and tune from there.Let your elapsed times be the final judge.

Bridging The Gap

Some Corvette engine combinations like a bunch of gap and others don'trun well until the gap is tightened up. A gap of approximately 0.025inch is commonly used, but some combinations (particularly those withhigh-powered contemporary ignition systems) can fire a gap ranging from0.045 to 0.060 inch. In all situations, the gap, air/fuel mixture, andheat range must be coordinated to produce the best possible results fromthe engine. In simple terms, it's a case of trial and error coupled withthe correct reading of the spark plugs.

Once you have the correct heat range dialed in, the gap sorted out, thespark plug type selected, the jetting squared away, and all of the othervariables inside your Corvette handled, there is no need to continuemessing with the plugs. The idea is to get your combination set upcorrectly, and to use the spark plugs as a tool in the setup. Onceeverything is right, no magic power will be found in the spark plugs. Remember, they're the window inside your Corvette engine. Don't forgetthem.


Category: Forex

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