Motorcycle, Dirt Bike, ATV Spark Plugs 101


In the post, we are going to look into everything you need to know about spark plug and this is our spark plugs 101 guide. Spark plugs are one of the most important parts on your motorcycle, ATV and side by side when it comes to creating combustion.

The spark plug is a device that, takes the electricity that’s provided from the ignition coil to create the spark. Now the spark is created from a gap in the plug that the electricity must travel across in order to find its path to ground and when it does this creates the spark and turn igniting your air fuel mixture which creates the combustion.

Spark plugs come in all different shapes and sizes and there’s a lot more to them than you’d think. So in today’s post we’re going to talk to you about the parts of a spark plug the heat ranges setting the spark plug gap and what to look for when it comes to inspecting the coloration of your spark plug.

There are eight main parts to a spark plug we’ve got the terminal, the insulator, the hex body or seal we’ve got the gasket, the threads, the insulator tip, the center electrode and the ground electrode. The spark plug terminal is what connects the spark plug to the ignition system. Now there are two different types of spark plug terminal connectors we’ve got the threaded type and the stud type.

Now there are three different configurations of the spark plug’s terminals themselves we’ve got the solid terminal stud and we’ve got a threaded terminal stud and a removable terminal stud. This is just where the spark plug cap connects to the spark plug. The spark plugs insulators made of ceramic and has several purposes it helps to stabilize the center electrode it extends the connecting point above the cylinder head for ease of access and also helps to prevent what’s called flashover.

Now flashover is when the spark jumps from the terminal across the insulation to the hex body or shell without the spark actually entering into the combustion chamber. Now the ceramic also helps to control some but not all of the plugs ability to dissipate heat as it does protrude slightly into the cylinder heads combustion chamber.

The hex allows for the spark plug to be easily installed and removed. Now the most common sizes of the spark plugs hex is 5/8 inch and 13 sixteenths however there are other sizes of the spark plugs hex which range anywhere from 8 millimeters all the way up to one inch or more. The gasket on the sparkplug ensures a tight seal between the plug and the combustion chamber.

Now the gasket on the spark plug will crush during installation which will ensure the seal so before you install your spark plug or spark plugs make sure that the gasket and the mating surfaces are clean to ensure that you get a good seal. The spark plug’s threads are measured in three dimensions, how far the threads reach, the diameter of the spark plug’s threads as well as the pitch of the spark plug’s threads.

The insulator tip is ceramic and can vary in length as to how far protrude from the spark plug. So basically how much of the insulator tip is being exposed to the combustion will determine a portion of the spark plugs heat range. Now, this is also the area that we will inspect to see how our bikes has been running and we’ll talk a little bit more about that when we get to inspection.

The center electrode is what emits the spark and turns igniting our air fuel ratio which gives us combustion. Now the center electrode can consist of several different types of precious metals including iridium, platinum and rhodium all of which are able to withstand extremely high combustion chamber temperatures without melting and they help to prolong the plug’s life.

Now some of the tips are designed to resist fouling or creating more effective spark. The ground electrode is what provides the path to ground for the electrical current that’s coming from the ignition coil so it travels through the center electrode jumps the spark plugs gap and finds its ground through the ground electrode.

Now, this ground electrode can be coated with various types of precious metals including iridium and platinum now this is to increase the sparkplug’s lifespan as well as to help it operate efficiently while it’s being subjected to those extremely high temperatures inside of the combustion chamber.

Each letter and number on the sparkplug identifies the features and functions of the plug. The letters and numbers will vary in definition from manufacturer to manufacturer each with their own meanings. NGK sparkplugs, for example, their letters and numbers can range anywhere from 5 letters and numbers to approximately 8 or more.

Now the letters and numbers can identify such things like thread diameter, thread reach, construction, firing and construction, heat range, plug type, and gap, now not necessarily in that order. For more information on decoding the letters and numbers reference the sparkplug manufacturer’s website or give them a call for more information.

The sparkplug’s gap is the distance measured from the center electrode to the ground electrode and is either measured in millimeters or inches and sometimes both. The gap size is usually set to a specific distance determined from the sparkplug’s manufacturer that can range anywhere from 20,000 of an inch to 80,000 of an inch but can be adjusted for specific applications.

Now on some sparkplugs, the gap size can be determined by referencing the letters and numbers on the sparkplug with the manufacturer. You can always reference your service manual to find this information. Now the service manual for this YZ250 right here states that the sparkplug’s gap must be between 0.5 millimeters to 0.6 millimeters or 20 to 24 thousands of an inch.

The sparkplug gap is important because it can affect everything from firing temperatures to the efficiency of the combustion. Now if the gap is too small it can cause pre-ignition, detonation and complete combustion of the air fuel ratio and even engine damage. Now if the gap is too large the ignition system may not be powerful enough or efficient in order for the spark to jump the plug’s gap in turn it can create a misfire, a loss of power, poor fuel economy and/or plug fouling.

Now it’s always best to follow your vehicle manufacturer recommended settings when it comes to setting your sparkplug gap however if you have made modifications to your engine changing such things as your compression ratio, changing the fuel and/or the air fuel ratio you may want to look into adjusting your sparkplug’s gap in order to compensate for these new changes.

The color of your spark plugs tip can tell you a lot about what’s going on inside of the combustion chamber. By inspecting the sparkplug’s tip you can determine quite a bit but for starters, you’ll want to check the tip to see if the plug has been running hot from a possible lean condition or if it is beginning to foul or is fouled from a possible rich condition.

Now a normal looking sparkplug’s tip will typically show a brown to grayish tan color with little to no carbon deposits. A foul plug can be dark in color if not black sort of city looking usually from too rich of an air to fuel ratio. Now what else can cause this is poor compression, poor combustion and/or oil getting up past the piston rings and into the combustion chamber.

When you’re inspecting the tip of the plug it’ll typically be wet and black from oil fouling or it can be dry and black from carbon fouling either way if your plug is beginning to foul we recommend that you just go ahead and replace it with a new one. Now an overheated plug will be white in color and will show excessive wear especially to the center electrode as well as the ground electrode.

Now, these previous descriptions are only but a few observational signs of what could be taking place inside of your combustion chamber. Now there are several more that can show you signs of pre-ignition, gap bridging as well as detonation. Now keep in mind that there are a lot more than that those are just to name a few. Now it’s always a good idea to be swapping out your sparkplug regularly and this will not only help you to get to know your bike and how it’s running but also ensure that you get the most out of your machine’s ignition system.

While you’re changing out your sparkplug it’s not a bad idea to pick yourself up a gapping tool and this will make your life a lot easier when it comes to setting the gap before you install your plug. Now for those of you running a two-stroke picking up a sparkplug wrench will make your life a lot easier as well as picking up a sparkplug holder.

Sparkplug holders will not only help you to protect your sparkplug but will also help to protect the sparkplug gap you’ve just set especially while you’re transporting it either in your backpack or your toolbox. Now for those of you riding a four-stroke we all know that the sparkplug is a little more difficult to access than compared to a two-stroke so having a sparkplug socket will definitely make your life a lot easier.

The heat range of a sparkplug refers to the speed or rate at which it can transfer heat from the combustion chamber to the cylinder’s head. Now each plug’s manufacturer is going to have a different rating system of letters and numbers each of which will indicate that plug’s heat range. Now NGK for example, the smaller the number will indicate a hotter heat range and the higher the number will indicate a colder heat range.

Now the sparkplug that is chosen from your vehicle’s manufacturer is going to be the best one for optimal engine performance. Now the only time that you’d ever want to be making adjustments to your sparkplug’s heat range is if you’re modifying such things as your compression ratio, ignition timing or your running alternate fuels. Now even when making changes to the heat range you’re want to air on the colder side of things as opposed to the hotter side as it’s a lot cheaper to replace a fouled plug than it is to replace a damaged engine.

The gasoline engine’s optimum combustion chamber temperature range of between 500 and 800 degrees Celsius or 932 degrees Fahrenheit to 1472 degrees Fahrenheit. Now once the combustion chamber is able to reach this optimum temperature it can burn off what is called combustion deposits. Now if the chamber never reaches those optimal temperatures those combustion deposits will adhere to the sparkplug and turn to foul it.

Now if the sparkplug’s tip is to exceed that temperature range the sparkplug’s tip will overheat and can cause such things as pre-ignition and also has the potential to lead to further damage to your engine.

And that’s it that’s our sparkplug 101 post. Now if you have any questions as to what we’ve discussed here today feel free to leave us a comment below.

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