Technical Information

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Spark Plug Anatomy / Construction
Although the spark plug is a familiar engine component, spark plug terminology often varies. The diagram below identifies the major components of a spark plug and their correct terminology.

Spark Plug Anatomy

Resistor Spark Plugs

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Resistor type spark plug’s incorporate an internal resistor to suppress ignition noise generated during sparking. The ignition noise which is often known as either electromagnetic interference (EMI) or radio frequency interference (RFI) can disrupt electronic communication signals, effecting the performance of sound systems, cellular phones, two-way radios and even engine management systems.

As EMI or RFI can also cause premature failure to other electronic components in a modern vehicle, it is important that resistor spark plugs are used to prevent this possibility.

Put simply, when a spark is formed it creates a mini-earthquake in the electronic world; the internal resistor in a spark plug filters out the noise created that may affect electrical components.

Precious Metal Spark Plugs
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In recent years NGK developed a range of precious metal spark plugs that provide two major benefits over conventional spark plugs:

  • High Ignitability – is only achieved by utilising an exposed fine tip centre electrode allowing for superior flame growth.
  • Long Service Life – is achieved by utilising a high percentage of precious metals patented by NGK.

As emission laws become more stringent, the performance expectations of spark plugs also increase. NGK have developed a range of Iridium and Platinum spark plugs to meet and surpass the modern requirements of vehicle manufacturers.

Precious metals such as Iridium and Platinum have much higher melting points over traditional metals such as Nickel.

  • Iridium – 2410°C
  • Platinum – 1772°C
  • Nickel – 1453°C

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This allows centre electrode diameters to reduce from 2.5mm using Nickel, to 0.6mm using Iridium or Platinum. The finer point means that once a spark forms, the flame kernel does not have a large mass of centre electrode that reduces absorption of heat and the flame shadow impeding the flame progress.
This equates to a more complete burn that translates to:

  • Improved acceleration
  • Improved fuel consumption
  • Smoother idling

Spark Plug Installation

Spark plugs are designed to operate at an optimal operating temperature and correct tightening is critical to the operation of a spark plug. The following two major problems commonly occur if the incorrect torque is applied:

Excessive Tightening > Thread breakage
Insufficient Tightening > Overheating

Installation Instructions for new Spark Plug:

  1. Check gap on every plug prior to installation. Set gap size to engine manufacturer’s recommendation.
  2.  Confirm that the thread reach of the spark plug is the right one for the engine.
  3. Remove any dirt at the gasket seat of the cylinder head.
  4.  Tighten the spark plug finger tight first until the gasket reaches the cylinder head, then screw about ½ – ¾ turn more with a plug wrench. (Conical seat; about 1/16 turn more). Refer to diagrams below.
  5. If using a torque wrench, please refer to the table below.
  6.  Always remove and install spark plugs when the engine is cool.

Spark Plug Type
(thread diameter)

Cast Iron Cylinder Head Aluminium Cylinder Head

Flat Seat Type (with gasket)

18mmØ

3.5 ~ 4.5 kg-m 3.5 ~ 4.0 kg-m
(25.3 ~ 32.5 lb-ft) (25.3 ~ 32.5 lb-ft)

14mmØ

2.5 ~ 3.5 kg-m 2.5 ~ 3.0 kg-m
(18.0 ~ 25.3 lb-ft) (18.0 ~ 21.6 lb-ft)

12mmØ

1.5 ~ 2.5 kg-m 1.5 ~ 2.0 kg-m
(10.8 ~ 18.0 lb-ft) (10.8 ~ 14.5 lb-ft)

10mmØ

1.0 ~ 1.5 kg-m 1.0 ~ 1.2 kg-m
(7.2 ~ 10.8 lb-ft) (7.2 ~ 8.7 lb-ft)

Conical Seat Type (without gasket)

18mmØ

2.0 ~ 3.0 kg-m 2.0 ~ 3.0 kg-m
(14.5 ~ 21.6 lb-ft) (14.5 ~ 21.6 lb-ft)

14mmØ

1.5 ~ 2.5 kg-m 1.0 ~ 2.0 kg-m
(10.8 ~ 18.0 lb-ft) (7.2 ~ 14.5 lb-ft)

It is essential to select a spark plug specified by the vehicle manufacturer from the NGK Recommendation Chart.

hread damage Metal shell damage Insulator breakage
Details of problem
Cause Spark plug inserted at an angle. Excessive tightening. Spark plug wrench used at an angle or it slipped.
Corrective actions Tightening the plug by hand first instead of using a wrench from the start. Tighten to the specified tightening torque Use a hexagonal socket wrench that does not slip easily.

Torque is one of the most critical aspects of spark plug installation. Torque directly affects the spark plugs’ ability to transfer heat out of the combustion chamber. A spark plug that is under-torqued will not be fully seated on the cylinder head, hence heat transfer will be slowed. This will tend to elevate combustion chamber temperatures to unsafe levels, and pre-ignition and detonation will usually follow. Serious engine damage is not far behind.

An over-torqued spark plug can suffer from severe stress to the Metal Shell which in turn can distort the spark plug’s inner gas seals or even cause a hairline fracture to the spark plug’s insulator, in either case, heat transfer can again be slowed and the above mentioned conditions can occur.

The spark plug holes must always be cleaned prior to installation, otherwise you may be torquing against dirt or debris and the spark plug may actually end up under-torqued, even though your torque wrench says otherwise. Of course, you should only install spark plugs in a cool engine, because metal expands when its hot and installation may prove difficult. Proper torque specs for both aluminium and cast iron cylinder heads are listed below.

LPG Compatibility
There are two critical factors that need to be understood when converting a vehicle from petrol to LPG:

  • LPG burns hotter
  • LPG increases the voltage requirement at the electrode gap

In an attempt to correct these two factors, as a general rule we suggest to:

Use a spark plug that is one heat range colder than standard to compensate for the increased heat from LPG:
The temperature profile of an engine may vary between vehicles. As such, the effect of LPG on the operating temperature of one vehicle may not be the same for another vehicle.
For example, lets say that LPG increases the average operating temperature by 200°C. If the average operating temperature of a Holden Commodore (which uses a 6 heat range) was 700°C the addition of LPG would exceed the operating range of 870°C. As such a colder plug may be required. However, a Ford Falcon for example (which also uses a 6 heat range) may have an average operating temperature of 500°C, the addition of LPG is still within the 450 – 870°C operating range and a colder plug may not be required. Please note that these values have only been used for illustration purposes.

Reduce the electrode gap by 0.1mm to compensate for the increased voltage:
It is difficult to gauge exactly how much the voltage requirement has changed as other factors such as engine wear, loss in compression and ignition system weakness for example may provide an influence. As such the electrode gap may need to be adjusted several times until the correct gap setting is achieved for that unique vehicle.

Unless the vehicle is a production vehicle designed for LPG, the installation of LPG results in a modified engine. As such, it is the end users responsibility to ensure that the engine components have been considered and altered if necessary to allow for the modifications.

Operating Temperature
The operating temperature of a spark plug varies between 450 – 870°C. At 450°C the spark plug reaches its self cleaning temperature; this means that carbon deposits which are produced during combustion are actively burnt off the insulator nose. When too many carbon deposits accumulate along the insulator nose carbon fouling occurs and engine misfire may occur. If the temperature of a spark plug exceeds 870°C overheating may occur leading to spark plug and possible engine damage.

Heat Range Explanation
Typically the heat range for NGK Spark Plugs varies from 2 – 11. This number indicates the thermal characteristics of a spark plug, or how ‘hot’ or ‘cold’ a spark plug is. The term hot/cold is commonly used to describe whether a spark plug heats up easily (hot) or whether it provides resistance to heating up (cold). Generally, low power engines such as lawn mowers don’t produce a large amount of heat, hence use a low heat range (or hot) spark plug such as a 4 heat range. This means the spark plug will heat up easily and reach its optimal operating temperature. High performance engines on the other hand produce a large amount of heat, hence a high heat range (or cold) spark plug such as a 10 heat range needs to be used to resist the heat developed by the engine.

Several factors influence the heat range of a spark plug, although typically the insulator nose design provides an indication of the heat range of a spark plug.

When a spark plug absorbs heat produced from combustion, the heat is transferred through the centre electrode and insulator nose to the metal shell, which then transfers the heat into the engine casing and circulating coolant.

A low heat range (or hot) spark plug typically has a long thin insulator nose which will heat up easily however will not dissipate readily to the metal shell (above left). Conversely, a high heat range (or cold) spark plug has a short thick insulator nose which will dissipate heat much easier (above right).

When the heat rating is too high:
The spark plug temperature remains too low and causes deposits to build up on the firing end; the deposits offer an electrical leakage path that gives rise to loss of sparks.

When the heat rating is too low:
The spark plug temperature rises too high and induces abnormal combustion (pre-ignition): this leads to melting of the spark plug electrodes as well as piston seizure and erosion.

NGK Spark Plugs pioneered the use of a copper cored electrode in 1958, which enables a spark plug to heat up quickly and also dissipate heat quickly giving an ultra wide heat range. It is essential to use a spark plug that fits a specific engine and its conditions of use.

As spark plugs are positioned in the head of an engine, their analysis can give a good indication of how your engine is operating.

http://www.ngk.com.au/images/preignition_table.jpg

Projection
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The projection of a spark plug is measured as the distance from the end of the metal shell to the tip of the centre electrode as shown here.

A projected spark plug protrudes into the combustion chamber further and provides higher ignitability and improved performance. Simply this can be understood by representing the combustion chamber as a circle. A projected spark plug in effect produces a spark in the middle of the circle (or combustion chamber) allowing for an even flame spread. Conversely, a non-projected spark plug means the flame spread is not even.

Electrode Gap

The electrode gap of a spark plug dictates the spark magnitude. An incorrect electrode gap may affect engine performance as the spark magnitude may be insufficient to ensure complete combustion of the air-fuel mixture.

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The electrode gap is meticulously controlled when a spark plug is manufactured, and due to the very large selection of plugs, in most cases the electrode gap is set to suit the vehicle as recommended in our catalogue. In some rare cases however, a specific electrode gap may be required for some vehicles which is not commonly manufactured. As such the electrode gap may need adjustment, and it is always best to quickly consult you owner’s manual (Please refer to Electrode Gap Adjustment).

Modified engines with higher compression or forced induction will typically require smaller electrode gap settings (to ensure ignitability in these denser air/fuel mixtures) as the voltage requirement at the electrode gap is increased due to higher combustion pressures. As a rule, the more power you are making, the smaller the gap you will need.

Electrode Gap Adjustment

NGK do not recommend gap adjustment for precious metal spark plugs.

A variety of techniques are used to adjust electrode gaps. An appropriate method and tool should be used for gap adjustment although it has come to our attention that a variety of improper methods are used that lead to implications.

When adjusting the electrode gap, the gap may commonly be made too small, as such, the gap may need to be opened slightly. Typically some customers use a lever such as a flat screwdriver to pry open the gap. Traditional Nickel plugs have a solid piece Ø2.5mm centre electrode that may withstand such force. Precious metal spark plugs however have Ø0.6mm fine tips that are laser welded to the centre electrode. These tips are very fragile and not designed to withstand any force. As such NGK will not provide warranty for the breakage of precious metal centre electrode tips.

It is possible to adjust the electrode gap on precious metal spark plugs using an appropriate method that will not place any force on the centre electrode tip.

Modified / Performance Engines

NB: We highly suggest having read and thoroughly understood the information in the prior sections Operating Temperature, Heat Range, Projection and Electrode Gap in relation to Modified / Performance Engines.

It is critical to note that once an ENGINE IS MODIFIED, THE RECOMMENDED SPARK PLUG listed in the NGK Spark Plug catalogue for that application MAY NO LONGER BE SUITABLE.

When an engine is designed by a vehicle manufacturer, thousands of hours of testing and design are dedicated to ensure that each engine component will operate to satisfaction across a variety of engine and environmental conditions. Any engine modification in effect alters the engine characteristics and as such, it is the end users responsibility to ensure that every engine component that may be affected, is considered and redesigned if necessary.

(Once an engine is modified the engine’s characteristics are changed and as such each engine component affected by the modification needs to be considered and appropriately re-engineered.)

Example If a V8 engine was fitted to a 4 Cyl. application, the transmission would most likely fail due to the higher power output of the V8. In this instance it would be unreasonable to blame the transmission manufacturer claiming the transmission was at fault. Similarly, if an engine is modified, the spark plug needs to be appropriately changed as the standard spark plug may no longer be suitable due to the different characteristics of the engine.

The two major factors that need to be considered when an engine is modified:

    • Heat range
    • Electrode gap

HEAT RANGE

he most critical factor for a spark plug that needs to be considered when an engine is modified is HEAT RANGE. Spark plugs are designed to operate in a temperature range of 450 – 870°C, when an engine is modified (generally to produce more power which produces more heat) in turn the engine and subsequent spark plug temperatures increase. Depending on the modification will depend on the change in temperature profile.

It is not possible for NGK to provide a recommendation for a modified engine. For NGK to make a recommendation, the spark plug needs to be tested and certified for that engine. The list for modifications and variations is endless and as such making a recommendation is not possible. NGK will happily suggest a spark plug however due to the infinite options for modification, the onus is on the customer to test and ensure that each engine component is suitable based on the specific modifications performed.

Example Take two identical vehicles for example a Ford 4.0L I6 Falcon. One customer may modify the engine with a turbocharger and exhaust, whilst a second customer may use a bigger turbocharger and different exhaust, on paper the engines have similar modification however the engine characteristics may differ greatly. Quantifying the extent is very difficult without performing extensive testing. As such it is the end users responsibility to perform some testing.

CHOOSING A PLUG:

When selecting a spark plug for a modified engine it may be difficult to initially gauge the heat range. A very expensive heat range test can be performed however a more viable option is to start with a colder heat range spark plug and perform some testing. As mentioned a spark plug operates in the range temperature 450 – 870°C, at 450°C carbon deposits being to burn off and if a plug may have a very black insulator nose at a lower temperature. At 870°C a spark plug will have a very white insulator nose which will blister if this temperature is exceeded and the ground electrode will melt.

Example Take a Holden engine which as standard uses a BPR 5ES spark plug. (The heat range for this spark plug is 5, please refer to our part number identification for further information) If a customer were to install a turbocharger, intercooler and exhaust system to this vehicle, it would be suggested to start with a spark plug with a 7 heat range (BPR 7ES) to be on the safe side. The customer would then need to perform some road testing,

        • firstly applying half throttle
        • then inspecting the plugs,
        • ¾ throttle then again inspecting the plugs
        • and eventually full throttle.

At any stage of inspection if the plugs upon removal are very clean and white along the insulator nose this would indicate that the spark plugs are reaching their upper temperature threshold and as such obviously may not withstanding the temperature at full throttle. Fouling a set of spark plugs is very cheap compared to the cost of the engine modifications performed and insignificant compared to the amount of research and design that goes into producing the engine by a vehicle manufacturer.

Using “racing” spark plugs
Be cautious! In reality, most “racing” spark plugs are just colder heat ranges of the street versions of the spark plug. They don’t provide any more voltage to the spark plug tip! Their internal construction is no different (in NGK’s case, as all of our spark plugs must conform to the same level of quality controls) than most standard spark plugs.

NGK as a company tries to stay clear of saying that a racing spark plug (or ANY spark plug) will give you large gains in horsepower. While certain spark plugs are better suited to certain applications (and we’re happy to counsel you in the right direction) we try to tell people that are looking to “screw in” some cheap horsepower that, in most cases, spark plugs are not the answer.

To be blunt, when experienced tuners build race motors, they select their spark plugs for different reasons: to remove heat more efficiently, provide sufficient spark to completely light all the air/fuel mixture, to survive the added stresses placed upon a high performance engine’s spark plugs, and to achieve optimum piston-to-plug clearance.

Some of these “specialised racing plugs” are made with precious metal alloy centre/ground electrodes or fine wire tips or retracted-nose insulators. Again, these features do not necessarily mean that the spark plug will allow the engine to make more power, but these features are what allow the spark plug to survive in these tortuous conditions. Most racers know screwing in a new set of spark plugs will not magically “unlock” hidden horsepower.

Many of the more popular aftermarket ignition systems are of the capacitive discharge type. They store voltage, or accumulate it, until a point at which a trigger signal allows release of this more powerful spark. Companies like Mallory, MSD, Crane and Accel, to name a few, offer such systems.

They affect spark plugs in that they allow the gaps to be opened up to take advantage of the increased capacity. The theory is that the larger and the more intense the spark you are able to present to the air/fuel mixture, the more likely you will be to burn more fuel, and hence the more power you will make.
We encourage the use of such systems, but only on modified or older non-computer controlled vehicles.

In reality, computer controlled vehicles do such a good job of lighting off the air/fuel mixture (as evidenced by the ultra-low emissions), added ignition capacity would do little to burn more fuel since the stock configuration is doing such a good job. Older non-computer controlled vehicles or those that have been modified with higher compression or boosted (nitrous, turbo, supercharged) engines can certainly take advantage of a more powerful ignition system.

With modified engines (those engines that have increased their compression) more heat is a by-product of the added power that normally comes with increased compression. In short, select one heat range colder for every 75-100 hp you add, or when you significantly raise compression. Also remember to retard the timing a little and to increase fuel enrichment and octane. These tips are critical when adding forced induction (turbos, superchargers or nitrous kits), and failure to address ALL of these areas will virtually guarantee engine damage.

An engine that has poor oil control can sometimes mask the symptom temporarily by running a slightly hotter spark plug. While this is a “band-aid” approach, it is one of the only examples of when and why one would select a hotter spark plug.

Spark Plug Analysis

Appearance of a used spark plug tells a lot about the operating conditions of the engine and the plug. Thus the analysis of a plug plays an important role in the maintenance of a car.
Spark Plug Nomenclature

1 : Carbon Fouling
Carbon fouling is the most common spark plug related failure, but is not a spark plug fault. Carbon deposits are conductive, and as they accumulate along the insulator nose they reduce the insulation resistance of the spark plug. As electricity always takes the path of least resistance a misfire may occur if a significant amount of carbon deposits accumulate. A spark will not form as electricity can track along the conductive carbon deposits to the metal shell (as shown in red below) rather than forming a spark across the electrode gap which has a very high resistance.

Fouling mis-fireFouling

As mentioned the optimal operating temperature range for a spark plug is 450 – 870°C, 450°C is the spark plug self cleaning temperature at which point carbon deposits will burn off. However, if too cold a spark plug is used and this temperature is not achieved carbon fouling will occur. This is the most common reason for carbon fouling.

View Carbon Fouling Animation

Other causes for carbon fouling include:

Causes Corrective actions
Air/fuel mixture (A/F) too rich
Incorrect adjustment of carburetor It is necessary to service the carburetor, the auto choke system or the fuel injection system.
Faulty auto choke system
Faulty fuel injection system
Faulty electrical system
Poor connection of the high tension cables It is necessary to service the electric system.
Prolonged idling It is necessary to run at higher speeds (about 80 km/h) from time to time.
Continuous low speed driving
Too cold a spark plug
Air cleaner contaminated Use a hotter spark plug
(Example: BK R6E-11 –> BK R5E-11)

As carbon builds up, the insulation resistance of the spark plug drops and the voltage generated by the ignition coil is reduced. When the generated voltage becomes lower than the required voltage of a spark plug (the voltage needed to cause sparks at the spark gap), sparking is suppressed and mis-firing occurs.

2 : Terminal Nut Wear
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Excessive vibration of the engine may led to abnormal wear of the terminal nut. As a result, the cover may come off the plug. For vehicles whose engines vibrate more than others, such as watercraft and snowmobiles, solid post terminal plugs with excellent vibration resistance and wear resistance are recommended.

Solid post terminal plugs are currently available with five part numbers: B7ES, B8ES, and BR9ES. A special package has been made for solid post terminal plugs.

3 : Flash-Over – Spark leakage from terminal to metal shell

Flash Over Flash-Over When the spark gap has widened due to wear of the electrodes, a higher voltage is required. The flash-over occurs when the required voltage between the plug electrodes is higher than the voltage flying between the terminal and metal shell.
The plug cable material hardens as time elapses, which in turn reduces the tightness of the cover and insulator, lowering the preventive power for flash-over.As a higher voltage is required for a turbo charged engine, flash-over is more likely to occur. It is important to recognise that a plug cable is a consumable part which needs to be replaced periodically. When there is no spark after washing the car or the engine room, check whether water has entered the plug cover or not.
Flash over Flash over
Corona Stain Corona Stain A removed spark plug sometimes has discolouration around the insulator surface which looks like gases have leaked between the insulator and the shell. This discolouration is generally called ” Corona stain” and is caused by the fact that oil particles in the air around the insulator surface are attracted by the corona discharge, are charged, and then adhere to the insulators surface. The corona stain causes no deterioration of the function of the spark plug.

4 : Metal shell – Rust, breakage at caulked portion
When water has entered the plug hole due to water resistance of the plug cover or, in the case of a motorcycle, when water has accumulated due to the inadequate draining through the plug hole, the metal shell may rust.

The rusting of the metal shell causes no deterioration of the function of the spark plug. Note, however, that water inside the cover may prevent sparks from being generated.

If the plug is forced to remove when the returning torque is abnormally high due to some causes such as plug thread seizure, the plug may break at the caulked portion of the metal shell.

When returning torque is high, the engine should be first warmed up. Then, by spraying penetrating liquid around the plug thread and leaving it for a while, the plug can sometimes be removed more easily.

5 : Firing End – Broken ceramics, melted electrodes, deposits
When the firing end of the plug has overheated, the ceramics may break or the electrodes may melt.
Under usual engine condition, the plug does not overheat. Note, however, that it gets extremely hot in the case of abnormal combustion (ex. high – speed knocking, pre – ignition).

When the A – F setting is lean due to a faulty fuel system, the combustion temperature may rise, resulting in abnormal combustion. The engines cooling system may be faulty. When the spark timing is too early, the combustion temperature may rise, resulting in abnormal combustion. When deposits (generating from combustion) are accumulated in the combustion chamber, the combustion temperature may rise, resulting in abnormal combustion.

When deposits have accumulated on the firing end of the plug, deposits may overheat, causing abnormal combustion. Especially in a two – cycle engine, oil gets burned and remains in the combustion chamber as deposits, accumulating on the plug as well. It is necessary to remove these deposits periodically.

In engines that consume larger amounts of oil, oil may enter the combustion chamber. It is necessary to check the amount of deposits during inspection of the plug. Burning of oil can also be detected by visible white exhaust gas emitted from the tail pipe.

6 : Dry and wet fouling

Wet fouling is fundamentally similar to carbon fouling. Although the root cause may vary due to a number of reasons, in essence the insulation resistance is reduced allowing a spark to track along the insulator nose and earth to the metal shell rather than forming a spark across the electrode gap as desired. Please see 1. Carbon Fouling

7 : Insulator – Breakage at the corrugation and caulked portion
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When removing or installing the plug, the plug wrench may slip or be tilted and may hit the corrugation of the plug against the inside, breaking the insulator.
Depending on the type of plug wrench, the plug may break at the caulked portion of the metal shell. This may not be noticeable from the outside appearance.

Slipped wrench leaves a mark on the hexagonal portion of the metal shell. A plug wrench with a definite hexagonal shape should be used. A rounded or loose wrench should be either avoided or used with special care.

8 : Gasket too loose, too tight
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When the plug has not been tightened enough, the combustion gas leaks out the thread portion. This reduces the radiation of the plug, causing the metal shell to be heated, resulting in discolouration of the metal shell plating. If the metal shell continues to overheat the plug temperature may rise and abnormal combustion may result.
When the plug has been tightened too much the thread neck portion of the metal shell will be lengthened. When the threaded neck portion is lengthened, the insulator and metal will not seal tight enough, causing the combustion gas to leak. When the gas continues to leak, the plug will be overheated and abnormal combustion may result.
The plug should be tightened to the following recommended torques and turning angles.

Thread damage Metal shell damage Insulator breakage
Details of problem
Cause Spark plug inserted at an angle. Excessive tightening. Spark plug wrench used at an angle or it slipped.
Corrective actions Tightening the plug by hand first instead of using a wrench from the start. Tighten to the specified tightening torque. Use a hexagonal socket wrench that does not slip easily.

9 : Breakage at the thread neck
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When the plug is tightened too much the metal shell thread neck may be broken at around the first to second thread.

The plugs with smaller diameter portions, the D (12 mm) or C (10 mm) types thread neck portion is not very strong. Observe the recommended torque and turning angle and tighten with special care.
Examples of problems caused by excessively long periods of use:

Worn-out electrodes will have difficulty in sparking.

Deposits accumulated on the firing end may induce abnormal combustion (pre-ignition), causing problems that include melting of the electrodes.

Good
Worn Electrodes
If the edges of electrodes are worn and rounded, sparks will not easily occur, which leads to engine starting problems and mis-firing during running: maximum power cannot be obtained from the engine.
Fouled
Power leaks through the carbon accumulated on the firing end, causing mis-firing as well as hindering engine starting.
Deposits
The deposits accumulated on the spark plug overheat and cause abnormal combustion (pre-ignition) that may lead to melting of the electrodes of the spark plug.

Why is the firing end appearance of a spark plug important?
Because the Firing end appearance reflects the suitability of the spark plug as well as the condition of the engine. There are three basic criteria: good, fouled, and overheated. The firing end appearance also depends on the spark plug tip temperature.

The boarder-line between the fouling and optimum operating regions (450 degrees Celsius) is called the spark plug self cleaning temperature. It is at this temperature that the deposits accumulated are burnt off.

What is spark plug fouling?
It is a phenomenon by which carbon attached to the firing end causes electrical leakage that leads to mis-firing.

As the high voltage generated by the ignition coil leaks away through the carbon, mis-firing may occur and cause running and starting difficulties.

What is spark plug overheating?
Prolonged overheating may induce abnormal combustion (pre-ignition), resulting in melting of the spark plug electrodes.
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Overheated spark plugs have a white insulator surface at the firing end speckled deposits. Electrode melting represents excessive overheating. When the spark plug temperature exceeds 870°C, the firing end serves as a heat source before sparking and induces abnormal combustion (pre-ignition), possibly damaging the piston.

Causes of overheating and corrective actions

Cause Corrective actions
Ignition timing too far advanced Adjustment of ignition timing is required.
Air/fuel mixture(A/F) too lean Adjustment of air fuel ratio (A/f) is required.
Insufficient cooling water and lubricants Top up cooling water and lubricants
Applied turbo boost pressure too high in the case of a turbo engine Adjustment of turbo boost pressure control is required.
Insufficient tightening of spark plug Tighten to specified torque
Use of too hot a spark plug Use colder spark plug
(Example: BK R5E-11 –> BK R6E-11)

What is lead fouling?
Lead deposits accumulated on the spark plug firing end help electricity to leak away and result in mis-firing.

Lead Fouling When the lead contained in gasoline as an octane enhancer, adheres as the firing end of the spark plug, the high voltage generated by the ignition leaks through the lead deposits and causes mis-firing during acceleration. This problem often takes 2,000 ~ 3,000 km (1,200 ~ 1,800 miles) to become apparent.
Insulation resistance of a spark plug vs. vehicle speed.
Lead fouling does not appear during starting or low speed running. On the other hand, mis-firing occurs when accelerating from the middle speed range since the insulation resistance of the spark plug drops quickly.

Better selection of spark plugs.
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Use hotter type
(Example: BP6ES – BP5ES)

Use projected insulator nose type
(Example: B6ES – BP6ES)

Is a stain between the insulator and metal shell caused by gas leakage?
It is not a stain caused by gas leakage but by corona discharge (carona stain).


Mechanism of corona discharge

The high voltage applied to the spark gap is also applied to the area between the centre electrode and the metal shell, causing an insulation breakdown of the air at the gap (a) between the insulator and the metal shell. The phenomena is called a corona discharge. The generated carona discharge develops toward the terminal nut. This last process is the pale blue carona discharge that is observed at night.

*Information in this page, has been sourced from NGK Spark Plugs, www.ngkspark.com.au.Dalby Mower Supplies, is not liable for this information, or what you choose to use it for.

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