Spark plugs

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Originally submitted by T:

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A range of plugs covering decades. Click to enlargen. Photographer being identified.

Spark Plugs:


Spark Plugs accept the spark from the Distributor or Coil Pack and ignite the Fuel Mixture.

Small Block Chevvy Plugs. Click to enlargen. Image by Griff One Tunner.

For many years it was thought that any spark that could ignite the mixture was all that was necessary so no big focus was made on domestic and commercial car ignitions.
Even though Holdens have always had ignitions the equal or better than their counterparts the world over, every so many years the ignition strength was increased.
After the Space Shuttle came into existence, the massive cost of each launch had to be contained somehow and so Rocket Engineers focused on reducing the fuel burn for each of them.
It was soon discovered that the amount of spark energy available made a big difference to the efficiency of the burn and consequently the amount of power produced from each ignition.
Automotive Engineers have followed this trend. The spark plugs of 20 years ago were flimsy things only designed to accept the most meagre of ignitions. Today, Spark Plugs have massive electrodes compared to their early counter parts and wide gaps.
Blue 3.3 6 Cyl CF Bedford Engine. Note the white centre electrode which is normal for HEI in a properly tuned, maintained and operated red/blue/black 6 cylinder Holden Engine. Photo by reppilF. Click to Enlargen.

Heat Range:

The Centre Electrode of a Spark Plug determines the Heat Range of the Plug.
The Heat Range of a Spark Plug describes the Spark Plug's ability to burn off the residue of combustion left on it, known as deposits.
Deposits appear on the Spark Plugs as a result of the Combustion Process and also as a result of Combustion Chamber particles that are flung onto them by the Pistons.
Deposits appear on the Spark Plugs after a start, during heavy acceleration and when other factors occur like a start after the engine was flooded.
It's important for the plugs to be able to burn these deposits off otherwise Plug Fouling will occur. Also the deposits will glow and cause Pre-Ignition and Detonation.

Heat Range Examples:

Heat travels from the hottest point to the coldest point in much the same way that pressure flows from the highest to lowest point.
The further the heat from the tip of the centre electrode has to travel to get to ground,
the hotter the range of the Spark Plug will be.
If the Centre Electrode extends a long way from the metal body of the Spark Plug that it is mounted in, then the Spark Plug is considered to have a high heat range.
If the Centre Electrode extends a medium way from the metal body of the Spark Plug that it is mounted in, then the Spark Plug is considered to have a medium heat range.
If the Centre Electrode extends a short way from the metal body of the Spark Plug that it is mounted in, then the Spark Plug is considered to have a low heat range.
Manufacturers determine the Heat Range required for a particular engine by running a special thermocouple Spark Plug in the Engine to determine the best Heat Range. One that is neither too hot nor too cold.

Spark Plug Gap:

The Spark Plug Gap is set as wide as the Ignition Manufacturer and the Engine's use can reliably sustain.

In other words the Gap is the maximum amount that the Coil's Energy is guaranteed to jump under every circumstance.

The difference between the amount of spark required when an Engine is Idling on a hot day at high altitude will be significantly less than the amount required for full power at sea-level on the coldest day of the year.

The Car Manufacturer and the Ignition Manufacturer co-operate to find the Spark Plug Gap that will deliver reliably under all these conditions.

Consequently the Gap is critical if all these requirements are to be satisfied.

For GM HEI, the Spark Plug Gap is 1.5mm.

Re-Gapping the Plugs:

Quality spark is dependent on the face of the Spark Plug Electrodes being perfectly square with one another.

If the Ground Electrode bends towards or away from the Centre Electrode the quality of Spark will diminish.

If the Spark Plugs can maintain their squareness to each other and the Electrodes are not burnt too round, then the plugs can be re-gapped. Otherwise it's best to replace the Plugs with new ones.


The length of the thread should always be matched to the depth of the hole the plug will fit into.

The length of thread can also be described as the depth of the Plug. Typical Depths are 1/2" and 3/4".

The longer the thread of a Spark Plug is the more control the manufacturer has over the Heat Range of the Plug.

Installing a 3/4" plug into a 1/2" hole can be a problem on removal. The threads will fill up with Combustion Chamber Deposits and jam the Plug in there. The thread can be stripped trying to remove the Plug.

Spark Plug Nose:

There are many types of Spark Plug Nose, or in other words the portion of the plug that extends into the cylinder.

There are wide philosophies on Spark Plug Nose. Some bury the Centre Electrode deep away from the Combustion to keep it protected, others stick it right out in the middle of the party.

The Nose type for the plug will depend on the use of the Engine and what suits that purpose best.

Spark Requirements:

Car Manufacturers tune their Spark Plug Gaps to constantly deliver peak energy so the Engine will run at peak efficiency giving optimum Performance and Economy.

Modern Spark Plugs have very thick electrodes to preserve the Spark Plug Gap longer. Since the Electrodes are thicker it takes longer for them to burn down.

The wider the Spark Plug Gap, the more energy is required to send a spark across the gap. When the gap becomes too wide, the Ignition can no longer spark the plug properly and misfiring, hard starting, poor performance and poor economy result.

Since a weaker Ignition needs a richer mixture to ignite, the Fuel Economy will suffer.
The Fuel System will automatically richen the mixture in response to the weak ignition since you will have to hold the Throttle open wider to achieve the same result.

The amount of energy required to spark a Spark Plug is also related to the Compression Pressure of the Engine. At Idle, the Compression Pressure is low. Consequently the least Spark Energy is required at Idle.

At Full Throttle the Compression Pressure is very high. Consequently the most Spark Energy is required at Full Throttle.

Because of the change in spark requirement from Idle to Power an Engine will miss at the changeover because the Compression Pressure increases and demands more spark.

This is the time when worn out plugs fail to deliver. Only after the Mixture richens up will the Engine pick up.

When to Replace Spark Plugs:

It will soon become obvious when Spark Plugs need replacing.

The accelerator will have to be pressed harder to the floor to deliver the same power and the Fuel Consumption will increase.

It's always best to change Spark Plugs at a regular interval rather than waiting until the Engine starts running poorly. Manufacturers give stated Kilometres the car will run before the plugs need replacing. There are many reasons for changing Spark Plugs at a regular service intervals.

1. The convenience of scheduled servicing of the car. Re-Gapping or changing Plugs with a particular Oil-Change can be beneficial. It gives you something to do while the Oil is draining out.

2. The wide Plug Gaps put excessive strain on the Ignition System making the leads, Cap and Coil work harder. Consequently they will deteriorate faster.

3. It's easier to service the Ignition System at home than on the side of the road on a rainy night.

Removing Spark Plug Leads:

Always twist Spark Plug Leads from Spark Plugs, never pull on them. Pulling the Leads will damage the Internal Conductor of the Spark Plug Lead.

Removing Spark Plugs:

Spark Plugs are most easily removed from Cast Iron Engines when the Engine is Hot.

Spark Plugs are most easily removed from Alloy Engines when the Engine is Cold.

You'll do yourself a real favour if you match the temperature to the task.

The Plugs Tell the Story:

After removing the Spark Plugs from the Engine, their condition will tell you a good deal
about how the Engine has been performing.

Red and Grey Holden 6 cylinder engines (which have 9 port heads) run cylinders 3 and 4 slightly richer than the others and 1 and 6 with the ideal mixture.

This is caused by the different length of inlet manifold that each pair of cylinders sees.
1 and 6 have the longest pipes, 3 and 4 have the shortest. The different length is also responsible for the staccato cranking that red 6's have. Some cylinders breathe more easily than others so they are the harder for the starter to turn.

Consequently plugs 3 and 4 will normally look darker in colour than plugs 1 and 6.

Types of Spark Plugs:

Red, Blue and Black Motor Holden Engines have always favoured what are called Projected Nose Spark Plugs.

Projected Nose Plugs:

A Projected Nose Spark Plug extends the Centre Electrode into the Mixture.
There is a twofold intention here.

1. The exposed portion experiences more of the flame front during the Combustion Process.

2. The exposed portion experiences more of the intake charge during the Induction Stroke.

The result of these 2 intentions is that the Plug runs hotter at Idle and Cooler under Power giving the Plug the best chance of staying clean and running cool enough to prevent Detonation and Pre-Ignition.

A projected nose plug provides about 2 degrees of extra spark advance compared to a non-projected nose plug.

Resistor Plugs:

These have a resistance built into the Centre Electrode.
The resistor reduces Radio Frequency Spatter with each spark and extends the life of the plug by limiting the peak current through the plug.

These plugs are desirable for EFI cars since interference can disrupt the computer.

Brands of Spark Plugs:

Nearly everyone has their favourite brand of Spark Plug and for many reasons.

Everybody is different and we all use our cars in different ways so it can be difficult to insist on exactly which brand or model of plug for a particular car and use.

Spark Plug Manufacturers try to be responsible about the products they sell and try to cater for every requirement the engine will have, cruise, city, performance.

Often it can be that one particular Manufacturer will make a very good Street Plug and lousy Performance Plugs. With yet another Manufacturer their plugs can be very good on the Track and lousy on the Street.

Every Manufacturer publishes a chart to help you identify the Spark Plug you need for your Engine.

Spark Plug Life:

Resistor plugs have a longer electrode life than non-resistor plugs because they prevent the current from reaching a peak. With some models of sprak plug the resistor can break down electrically and fail before the electrodes show signs of deterioration. 

Waste Spark:

Spark Plugs used in Waste Spark Ignition show erosion of the Centre electrode on half the Plugs and erosion of the Ground Electrode on the other half of the Plugs. This is because the Spark jumps in different directions depending on the Spark Plug's position in the Engine. The particular Coil Lead determines which direction the Spark will jump when multiple double ended Coils are used. The Red Baron on Thu, 21/08/2008 - 11:18.
I6 Ford 4ltr engines which had the waste spark coil pack. EF and AU etc. They definitely do as you mentioned. 
Some Heads have no Spark Plug Thread for the first 3/8". In this case 202 Plugs won't fit because there is no Thread for them. A 3/4" Reach, Tapered Seat, Projected Nose Plug similar to Bosch HR7DCY should be used. Photo by TUFFVB.
Some Heads have no Spark Plug Thread for the first 3/8". In this case 202 Plugs won't fit because there is no Thread for them. A 3/4" Reach, Tapered Seat, Projected Nose Plug similar to Bosch HR7DCY should be used. Photo by TUFFVB.
Bosch HR7DCY for blue and black 3.3 and 2850 Sixes. Photo by T. Click to Enlargen.
Bosch HR7DCY for blue and black 3.3 and 2850 Sixes. Photo by T. Click to Enlargen.

Early Head Spark Plug Thread Length:

The 14mm spark plug holes in the early heads are not threaded for the first 3/8". The new plugs only have a 3/8" reach so would just touch the first thread. Dr Terry


NGK Plugs Breaking Down


Early Heads with missing Thread

Bosch Spark Plug Thread



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