Motorcycle Spark Plugs – Understanding Plug Gap, Materials, & More

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A spark plug is tiny, but an important part of your motorcycle, and a little knowledge about it will help you to keep your bike and engine in good condition.

What Is A Spark plug?

The spark plug in your motorcycle is a simple item, yet highly important in its function.

It delivers the spark for igniting the compressed gasoline and air mixture at every power stroke.

You can see two metal electrodes (center and side (ground)) at the tip of the spark plug and the electrodes and the gap between them facilitates the generation of the spark under high voltage.

Every spark erodes a very minute portion of the electrode, and over time either the gap needs to be adjusted or the spark plug replaced.

A spark plug with a higher gap (more than the recommended) will not work efficiently and may lead to misfires, engine running rough, and reduced fuel economy.

The life of the spark plug depends on the material of the electrodes and the condition of the engine.

Spark plugs are made with electrodes (both center and ground) of materials that can withstand high engine temperatures, vibration, and wear. The main types of spark plug material include:

  • Copper.
  • Iridium.
  • Platinum.
  • Silver.

Copper and silver spark plugs have high thermal conductivity, but generally need to be replaced sooner. Platinum and iridium spark plugs last longer but have lower thermal conductivity.

Iridium is harder and stronger than platinum, and also has a comparatively higher melting point than platinum.

Due to these characteristics, spark plugs with iridium material may last up to 25% longer than platinum electrodes.

However, iridium spark plugs cost you more than platinum.

It is wise to check the recommended spark plug material in your owner’s manual and stick to it.

If you want to go for other material, discuss with an authorized mechanic or in a users forum before making a decision.

In older motorcycles, spark plugs used to wear out more quickly and the rider always used to keep a spare spark plug.

However, advanced technology used in modern motorcycles makes the spark plug more reliable and longer-lasting.

It is still important to choose a good spark plug recommended and compatible with your motorcycle.

Replace the spark plugs as recommended in your owner’s manual to keep your bike healthy and running at peak performance. Do not wait for the spark plug to fail.

When you inspect the spark plug, you may get an idea if it is good for use or to be replaced, by noticing its color.

Color of light gray or tan indicates the spark plug is still good to use, dark black color with deposits indicate the spark plug is very cold for the engine, and a bright white color may indicate overheating.

Setting the Plug Gap

The plug gap describes the distance between the electrodes on the spark plug.

You need maintaining the correct gap between the center and the ground electrodes of the spark plug.

The correct gap is provided in your owner’s manual.

The electrodes of the spark plug undergo wear due to the regular spark between them and you can reset the gap once or twice, after which the spark plug should be replaced.

Read and understand your owner’s manual (including the specified spark gap) and take out the spark plugs using the correct size socket spanner and wrench.

Before taking out the spark plug, blow the dust away using compressed air.

You need a thickness gauge (wire gauge is preferred), a gapping tool for correcting the spark gap, and the torque wrench set to the torque recommended in your owner’s manual.

Clean the spark plug (with cleaning spray) and check the spark plug gap using a wire gauge and if necessary, correct the gap using the gapping tool (using a screwdriver to do this may spoil your spark plug).

Inspect the spark plug, and if everything looks fine screw it back into the engine first by hand, followed by the socket and torque wrench. Put the plug cap connection back in place.

When the spark plug gap is too wide, the spark may not happen consistently leading to either a rough running engine or an engine not starting at all.

Heat Ranges

The heat range of a motorcycle spark plug describes how quickly it can transfer the heat from the sparking tip to the cooling system.

The heat range is mentioned by a number and its interpretation may differ from manufacturer to manufacturer.

Spark plugs are normally known as ‘hot plugs’ or ‘cold plugs’.

The hot plugs have higher insulator nose lengths (on the spark end) and hence the time required for the transfer of heat from the sparking tip to the cooling system is slow.

On the other hand, cold plugs have less insulator nose lengths with a faster transfer of heat from the sparking tip to the cooling system.

Hot plugs are suitable for low rpm engines, and cold plugs are suitable for high rpm engines.

Study your owner’s manual, and follow the recommendation when selecting new spark plugs.

If your owner’s manual does not have the information, you may have to do a physical inspection of the spark plug to know whether it is a ‘hot’ or ‘cold’ type.

Or, you can consult your mechanic.

Spark Plug Buying Tips

Find the information regarding the spark plug and plug gap in your motorcycle service manual.

If you are planning to upgrade the spark plug, study the implications, and if necessary discuss with your mechanic.

The thread size of the spark plug, hexagonal size, reach of the spark plug, spark gap, and heat range are all important.

If you are swapping for a different type, it is a good idea to place your old plugs and the new plugs side by side and physically compare them before putting them in your bike.

Spark plugs are designated with codes.

You can search for the standard designation codes for the spark plug on the internet for more details.

Each spark plug manufacturers uses codes to indicate the thread size, insulator type, resistor, heat rating, thread length, material, and the plug gap.

Going for a reputed and reliable brand is important. Some of the brands in the market are –ACDelco is an American company owned by General Motors, Bosch is a multinational company having its headquarters in Germany, NGK is a public company in Japan, Champion is a US brand, and Denso is a Japanese brand.

How Often to Replace Your Motorcycle Spark Plugs

Replace the spark plugs as recommended in your owner’s manual.

You should also inspect the spark plug for correct gap and carbon deposition after about every 5,000 miles.

If your engine is running rough, or losing power, or not starting at all, inspect the spark plugs, and replace them if necessary.

What is the shelf life of an unboxed spark plug?

A spark plug of a reputable brand, unopened, and stored in a cool dry place should be good for use even after 5 or more years.

As long as the spark plug is not exposed to the atmosphere, it will not get corroded or deteriorated.

We tend to ignore the spark plugs since they seldom trouble us. However, they have an important function and your bike won’t run without them.

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