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Spark Plugs


Glow Plugs



Spark Plugs NGK Spark Plug


By producing a high intensity electrical discharge within the engines combustion chamber the spark plug provides the heat energy required to start the air/fuel mixture burning. It has to be able to do this many thousand times per minute. NGK has the technology to produce plugs for every type of automotive engine, as well as for motorcycles, marine, horticultural and many other applications. In fact NGK produces and stocks well in excess of 1,000 different types of plug. NGK always works very closely with the engine manufacturer to ensure a perfect match between plug and machine.






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FirNGK Spark PLuging End Appearances


The appearance of the firing-end of a used spark plug graphically reflects the condition of an engine, the suitability of the spark plug heat range, and whether or not the fuelling and ignition systems are correctly set.




Normal Condition Spark Plug from NGKNormal condition

An engine's condition can be judged by the spark plugs firing end appearance. If the firing end of a spark plug is brown or light gray, the condition can be judged to be good and the spark plug is functioning correctly.



The accumulation of deposits on the firing end is influenced by oil leakage, fuel quality and the engines operating period. Deposits come from: Fuel: C (carbon), Pb (lead), Br (bromine) Lubricating oil: C, CA (calcium), S (sulphur), Ba (barium), Zn (zinc) Others: Fe (iron), Si (silicon), Al (aluminium), etc.


Dry and wet fouling

Although there are many different causes, if the insulation resistance between the center electrode and the shell is over 10M ohms, the engine can be started normally. If the insulation resistance drops to 0, the firing end is fouled by either wet or dry carbon.


Dry Fouling
Wet Fouling

Dry Fouling

Wet Fouling


Lead FoulingLead fouling

Lead fouling usually appears as yellowish brown deposits on the insulator nose and this cannot be detected by a resistance tester at room temperature. Lead compounds combine at different temperatures; those formed at 370 420 degrees Celsius having the greatest influence on the resistance.


Overheating Spark PLugOverheating

After having overheated, the insulator tip is glazed or glossy, and deposits which have accumulated on the insulator tip have melted. Sometimes these deposits have blistered on the insulators tip.


Spark Plug BreakageBreakage

Breakage is usually caused by thermal shock due to sudden heating or cooling.



Spark Plug Normal LifeNormal life

A worn spark plug not only wastes fuel but also strains the whole ignition system because the expanded gap requires higher voltages. Worn spark plugs also reduce the engines efficiency, resulting in reduced fuel economy and increased exhaust emissions. The normal rate of gap growth is about 0.01 0.02mm/1,000 Km for four stroke engines and about 0.02 0.04mm/1,000 Km for two stroke engines.


Spark Plug ErosionErosion, Corrosion, Oxidation

The material of the electrodes has oxidized, and when the oxidation is heavy it will be green on the surface. The surfaces of the electrodes are also fretted and rough.



Spark Plug Abnormal ErosionAbnormal erosion

Abnormal electrode erosion is caused by the effects of corrosion, oxidation, reaction with lead, all resulting in abnormal gap growth.



Spark Plug Lead ErosionLead erosion

Lead erosion is caused by lead compounds in the gasoline which react chemically with the material of the electrodes ( nickel alloy) at high temperatures. Crystals of nickel alloy fall off because of the lead compounds permeating and separating the grain boundary of the nickel alloy. Typical lead erosion causes the surface of the ground electrode to become thinner, and the tip of the electrode looks as it it has been chipped.


Spark Plug MeltingMelting

Melting is caused by overheating. Mostly, the electrode surface is rather lustrous and uneven. The Melting point of nickel alloy is 1,200 1,300 degrees Celsius.


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Resistor Plugs


Resistor PLugsAs well as reducing electrical noise for radio, television and mobile telephones etc, many modern ignition systems require resistor plugs to stop electrical noise from interfering with the vehicle's on-board electronic control units (ECUs). Resistor spark plugs should always be fitted where specified. NGK automotive resistor spark plugs contain a ceramic monolithic resistor. The resistor filters out excessive electrical noise generated by the ignition system. The most effective place to situate a resistor in the high tension circuit is as close to the spark plug as possible making the spark plug an ideal place to house the resistor. In nearly all cases - apart from some very old low output ignition systems - resistor spark plugs can be used in place of the non resistor versions with no detrimental effect on engine performance, power output or vehicle emissions.

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Spark Plug Gap


Spark Plug Gap Always check that the spark plug gap is compatible with the engine manufacturers specification. A gap that is too small means that the spark duration will be very quick and the spark will be thin and weak. The consequences of this may be bad starting and high exhaust emission levels. This will result in an increase in fuel consumption. If the gap is set too large, the ignition system will not be able to cope with the demands and a misfire situation will occur. Some wide gap spark plugs have a longer ground electrode to accommodate a wide gap setting. These must be used where specified, as opening up a standard plug to a wider gap setting may result in the electrodes not running parallel to each other. This could result in abnormal and premature electrode wear. The re-gapping of fine wire spark plugs that have very small diameter platinum or iridium alloy electrodes is not recommended due to the risk of damaging the electrodes. Virtually all NGK spark plugs are set to the correct gap for the catalogue applications at the point of manufacture.


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Tightening Torque


Tightening Torque fig.1Spark plugs must be secured to the correct tightening torque. A plug left too loose may overheat and suffer vibration damage. One that is too tight may suffer from shell distortion, cracking of the insulator or shell separation upon subsequent renewal.


Tightening Torque fig.2Refer to current NGK catalogue for correct spark plug selection.


  • Check condition and cleanliness of threads in cylinder head
  • Ensure plug is gapped according to vehicle manufacturers specification
  • Install new spark plug by hand until it seats. A length of rubber tubing pushed over the insulator can be a useful aid for plug installation where access is difficult
  • Tighten to specified torque setting as shown in the chart below
  • If torque wrench is unavailable fig.3 below will serve as an angular guide for tightening new spark plugs
  • Reused gasket type spark plugs require only 1/12 turn
  • Always use the correct tools for removal/installation to prevent damage to the spark plug or engine
  • Inspect spark plug cover and renew if necessary


Installation Tips

Torque Tightening Chart


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Corona Stain


Corona StainA brown stain - often mistaken for gas leakage - is sometimes observed in a ring around where the insulator joins the metal shell of the spark plug. This discolouration is generally called Corona Stain and is the result of oil particles present in the air around the engine adhering to the insulator surface.


The Corona Stain is often seen on spark plugs that are

installed deep into the engine such as on many four valves per cylinder engine applications. This is because oil particles are not blown away by air circulating around the engine compartment and thus adhere to the insulator surface. It is important to understand that Corona Stain causes no deterioration to the function of the spark plug.


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Fouling Range


As a by product of combustion, carbon will become deposited on components within the combustion chamber including the spark plug. Excessive carbon deposits on the plug can cause a misfire due to deterioration of spark quality. When the spark plug reaches a certain temperature it will start to burn off the carbon and enter a self cleaning region.


Fouling will occur if plugs are operating at lower than the self-cleaning temperature of:

400 degrees Celsius - 450 degrees Celsius (750 degrees Fahrenheit - 850 degrees Fahrenheit)


Air-fuel mixture richer than 8:1 to 10:1


This can be seen as:


Wet Fuel Fouled - Wet Black Deposit

The firing-end of the spark plug becomes saturated with fuel and its insulation ability deteriorates and misfiring occurs.


Check for rich air/fuel mixture. Check the entire ignition system. If condition recurs, engine overhaul may be necessary.


Carbon Fouled

Black Carbon Fouling

Carbon accumulates in large quantity and, while the firing-end of the plug is dry, its insulation is abnormally decreased. This, too, is regarded as a prime cause of misfiring.


Check for rich air/fuel mixture. Check the entire ignition system and cooling system (excessive cooling).


Other fouling - Oil Fouled

When the firing end of a spark plug is fouled by oil, an electrical leakage path is formed and the insulation deteriorates, consequently the available voltage from the ignition system is lowered and a spark can not jump at the spark gap.


Causes of carbon fouling

  • Fuel mixture too rich
  • Excessive use of choke
  • Blocked air filter
  • Incorrect spark plug gap setting
  • Over-retarded ignition timing
  • Compression loss due to imperfect cylinder-piston seal or valve seating
  • Prolonged low speed driving or idling
  • Too cold a spark plug fitted.


Causes of oil fouling

  • Lubricating oil entering into combustion chamber


Spark plug tip temperature

Spark Plug Tip temperature chart


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Pre-Ignition Range


Pre-ignition is a condition whereby combustion is initiated within the combustion chamber before the spark occurs at the plug. This results in uncontrolled ignition and combustion conditions.


If a spark plug operates at a temperature in excess of 1000 degrees Celsius for a prolonged period the electrodes can start to overheat. When excessively overheated, pre-ignition can occur, the electrodes and insulator tip may melt and piston damage may result.


Pre-ignition is usually caused by a temperature exceeding 850 degrees Celsius - 1000 degrees Celsius (1600 degrees Fahrenheit - 1800 degrees Fahrenheit)


Causes of overheating


  • Over-advanced ignition timing
  • Too lean fuel mixture
  • Excessive deposits accumulated in combustion chamber
  • Insufficient cooling
  • Insufficient spark plug tightening or failure to fit gasket
  • Too low octane gasoline
  • Too hot a spark plug fitted.



The plug has been subjected to a relatively heavy load and evidence of overheating can be seen in the oxidized electrodes and melted deposits which have formed on the insulator surface.


Check for over-advanced ignition timing and too lean fuel mixture. Check spark plug tightening and gasket. If conditions recur, use plug one step colder in heat range.



Overheated plug with melted electrodes and blistered ceramic insulator surface. Recommendation

Check for over-advanced ignition timing and too lean fuel mixture. Check spark plug tightening and gasket. If conditions recur, use plug one step colder in heat range.


Worn Spark Plug

A worn spark plug not only wastes fuel but also loads the whole ignition system because the expanded gap requires higher voltage. As a result, a worn spark plug may also cause engine damage and increased exhaust emissions.


Spark plugs should be replaced.



The accumulation of deposits on the firing end is influenced by oil leakage, fuel quality and engine operating period.


Check for excessive amounts of lubricating oil entering into the combustion chamber. High quality oil should be used.


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Alternative Electrode Designs


Why have spark plugs with multiple ground electrodes?

Every time the plug sparks, minute particles of material are worn away from the electrodes. This phenomenon is called spark erosion. This continuous process over time increases the spark plug gap between the centre and the ground electrode. If the gap becomes too large, misfiring will occur.


In order to extend the service interval of vehicles, the service life of the spark plug must be increased. Some manufacturers are fitting multi electrode spark plugs as original equipment to achieve this. Multi electrode spark plugs can have two, three or four ground electrodes depending on the service life requirement of the manufacturer.



Do multi ground electrode spark plugs provide multi ground electrodessimultaneous sparks to each ground electrode every time the plug sparks?

No. No matter how many ground electrodes the plug has, every time the spark plug fires, only one spark occurs between the centre electrode and the ground electrode which has the lowest required voltage or the least distance to travel between the centre and the ground electrode.



What are fine wire plugs?

The spark plug plays a vital role in the quest to improve ignition quality, engine performance, reduce emissions and reduce fuel consumption. Spark plugs that employ small diameter centre and sometimes ground electrodes can offer benefits in several areas. These fine wire plugs require less voltage to create the spark, have a more consistent spark position, better gas flow around the firing position and experience less quench effect than other designs. As the electrodes erode during use we need to compensate for the use of smaller electrodes by some means otherwise the plugs would have an unacceptable service life.


IridiumIncreasing the service life

By using small chips of special precious metals such as platinum or even iridium which are welded to the tips of the electrodes we can increase the service life significantly whilst maintaining the highest ignition performance. These metals are extremely hard and have very high melting points thus making them ideal for use in this hostile environment.



What is an NGK Hybrid plug?

HybridSome modern vehicles use a direct fuel injection system and these vehicles demand high ignition quality and extreme anti-fouling performance. NGK has developed a plug that has several special features designed to offer the required performance. Essentially a very projected fine wire spark plug with platinum electrodes is combined with a semi-surface discharge design. The resulting plug has three ground electrodes, two of which are mostly redundant unless in extreme circumstances the plug becomes very carbon fouled. At this point the spark will discharge across the insulator nose to one of the side electrodes preventing a misfire and unburned fuel reaching the catalyst. This type of plug must only be use in the specific applications as listed in the NGK catalogues.


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Glow Plugs


Diesel engines do not use a spark plug to ignite the air/fuel mixture instead they rely on the heat produced by greatly compressing the air within the cylinder and then injecting diesel fuel at the correct time which burns spontaneously. When the engine is cold it is considerably more difficult to get the air/fuel mixture to burn and achieve stable combustion therefore we require a method of adding heat energy to the mixture.


A glow plug is essentially an electrical heating element installed within the combustion space. By elevating the temperature of the air within the combustion chamber the glow plug allows easier cold starting, improved performance from cold and a significant reduction of unwanted emissions. Unlike a spark plug a glow plug usually works for a short period of time upon starting the engine only.


As diesel engines have evolved considerably over recently years so has glow plug design. This means that we now stock around 150 different part numbers with several major design differences. Although performance criteria differ greatly they are all manufactured to the same demanding standards.


Diesel Engine


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Metal Single Coil Types


In years gone by glow plugs would have an exposed heating coil. This led to a relatively short service life due to the coil being exposed to the combustion process. A far superior performance was afforded when the coil was then placed within a protective metal sheath and packed in an insulating powder. This is the basis of most contemporary glow plugs. These so called metal glow plugs are divided into several categories; the most basic are briefly described below.


Standard Types

A sheathed single coil glow plug, which heats at a fairly uniform rate to its maximum temperature with a constant current consumption.

25 sec to reach 800 degrees Celsius


Rapid Glow Types

The single heating coil installed in this design allows a higher current flow initially to promote a faster heating time. As the temperature rises, the resistance of the coil rises to restrict the current flow.

15 sec to reach 800 degrees Celsius


QGS Types (Single Coil)

The 'Quick Glow System' glow plugs have specially designed heating coils, which can allow a very high current flow as soon as it is energised. Due to the extremely quick heating of these glow plugs, they require strict regulation and are only used in conjunction with specifically designed QGS control systems.

5 sec to reach 800 degrees Celsius


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Metal Double Coil Types


A significant development to the control and durability of glow plugs came about by the addition of a second control coil connected in series within the glow plug protective sheath. These glow plugs are commonly known as double coil types. The resistance of the control or regulating coil increases as current starts to flow which then limits the current flowing into the heating coil. This provides a measure of protection against overheating and increases service life. There are three main types of double coil glow plug.


QGS Quick Glow System (Double Coil)

Similar to the single coil variety the 'Quick Glow System' glow plugs have specially designed heating coils, which can allow a very high current flow as soon as it is energised. Due to the extremely quick heating of these glow plugs, they require strict regulation and are only used in conjunction with specifically designed QGS control systems. These types are still generally used in engines requiring preheating only.

5 sec to reach 800 degrees Celsius


SRM - Self Regulating Metal

Used in the majority of passenger cars and light commercial vehicles and available in a wide range of sizes to suit a broad range of engine types. By use of an alloy specially developed by NGK the coil materials allow the SRM glow plug to provide long post glow operation with relatively simple control systems. In rush current is generally lower than QGS types and stable current and final temperature is reached relatively quickly.

4.5 sec to reach 800 degrees Celsius


AQGS - Advanced Quick Glow System

Very rapid temperature rise and long post glow operation is offered by this design. Three stage voltage application ignition on, cranking and post glow is strictly controlled to allow fast heating whilst maintain control during long post glow periods. This offers more stable control of smoke emission & reduction of hydrocarbons combined with more stable idle speed less engine speed fluctuation.

2 sec to reach 1000 degrees Celsius


Three stages of applied voltage

a) Battery voltage with ignition on

b) 6.5V when cranking

c) 5.0V after engine fires


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Ceramic Types


First used in 1985 these extremely high performance glow plugs have undergone three major changes in design development. Due to their unique design these glow plugs are afforded the greatest protection against the prolonged high temperatures involved with extended post-heating periods. They also possess the ability to withstand the high degree of thermal shock resulting from lightning-quick heating times. Silicon nitride is used for the probe material offering very high resistance to thermal shock and generally extreme durability. A further advantage of ceramic glow plugs is that the cross sectional area heating portion can be made a much smaller. This is a major advantage to modern diesel engines where space is severely restricted around the combustion chamber.


SRC Self Regulating Ceramic

Heating coil encased in silicon nitride and regulating coil to provide current control.

3 sec to reach 800 degrees Celsius


HTC High Temperature Ceramic

Improved heating element now electrically conductive ceramic offering extended high temperature operation (1100 degrees Celsius)

4 sec to reach 800 degrees Celsius


NHTC New High Temperature Ceramic

All ceramic heating element in diameters as small as 8mm. Extended post glow times of over 10 minutes at temperatures of up to 1350 degrees Celsius. 2 sec to reach 1100 degrees Celsius


Ceramic ypes

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