What Spark Plug to Use for a Lawnmower?

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The spark plug is a vital component of gasoline engines where it ignites the compressed air-fuel mixture in the combustion stroke to create a blast for the release of energy. Like automobile gasoline engines, a lawn mower’s gasoline engine needs a spark plug to ignition the air-fuel mixture. All spark plugs essentially have the same basic working principle. However, it is important to note that all gasoline engines need spark plugs, but they don’t need the same kind of spark plug. A spark plug type is characterized by various features such as thread length, thread pitch, and the gap between the two electrodes, etc. So, if you are in some lawn mowing mood on a lovely sunny morning and don’t want to end up just cranking the engine again and again, choose the right kind of spark plug for your lawnmower engine. We don’t want the wrong spark plug choice to destroy your mowing mood.

What Spark Plug to Use for a Lawnmower:

Use a “small engine” Spark Plug as most car engine spark plugs have a larger thread. Ensure that it has the right thread size (usually, 10 to 12mm) and the correct thread length (usually, 3/8 inch). Also, set the proper gap that matches the engine’s gap requirement. Check your Lawn Mower manual for the correct dimensions.

What Spark Plug to Use for a Lawnmower? 1
(CC BY 2.0) by rockindave1

A detailed description of these features will help you understand their significance and how the wrong choice could harm your plug or engine, or both.

Spark Plug Components:

Interchangeability between a lawn mower spark plug and an automobile spark plug is not completely impossible, but generally, the two applications require spark plugs with different physical features. Some spark plugs are “hotter,” some are “colder,” some are “longer,” and some are “shorter.” What do all these nicknames mean, and what are they based on? Let’s have a detailed look at a spark plug’s physical features.

Spark plug components and features are not something all laymen know. But having a basic knowledge of them is not too difficult. A spark plug has four basic components:

  • A metal casing or shell that is threaded.
  • A central or live electrode: usually contains a resistor and is connected to the ignition coil’s output terminal by a heavily insulated wire. 
  • An insulator: to electrically isolate the outer shell from the central electrode. It prevents the escape of high voltages from the electrodes.
  • An outer or ground electrode: It is also sometimes referred to as the side electrode.

In addition to these four basic components, spark plugs have other components, too, such as a ring, packing washer, center shaft, glass seal and gasket, etc., that make its working smooth.

The choice of lawn mower spark plug should be based on these features’ specifications since certain specifications will best meet your lawn mower engine’s requirements while others won’t.

Spark Plug Size:

Size is an important specification that must be considered when choosing a spark plug for your lawnmower engine. Size is one of the few parameters that makes a lawn mower spark plug different from an automobile engine.

For the threads to fit together, the thread sizes must be similar. If the threads don’t have similar sizes, they just won’t fit. If you try to fit together threads of different sizes by applying force, you’ll damage the spark plug or the engine or both. It is highly inappropriate to mate threads of different sizes forcefully for your engine’s health and spark plug. After all, we all want to keep our machines in order, working, and healthy.

Lawnmower engines usually use smaller thread sizes as compared to automobile engines. Most automobile engines use spark plugs with a thread size of about 14mm, whereas most lawn mower engines use spark plugs with a thread size of about 10 to 12mm. Because the size specifications are different, the two spark plug types can’t be interchanged. They won’t fit in the same unit.

So, next time you go shopping for a spark plug for your lawnmower, keep in mind the spark plug size. You can check the spark plug size of your lawnmower engine from the user manual, or you can go on the internet to get spark plug size info from the model number of your lawnmower engine.

Spark Plug Thread Length:

The second important specification that differentiates between an automobile engine and a lawnmower engine is the thread length.

Here again, lawn mower spark plugs usually have smaller thread lengths as compared to auto engines. Threads on automobile plugs are usually between 1/2 inch to 3/4 inch long. On the other hand, lawnmower engines usually use spark plugs whose threads are only 3/8 inch long.

● What happens when you don’t use the right thread length?

Many engines use a tapered spark plug that fits with a tapered hole in the cylinder head. The mating of the two features seals the cylinder. If the right thread length is not used, the components would not fit together, and the lawnmower cylinder won’t be sealed. The use of the wrong thread length will impair the lawnmower engine’s starting or damage the engine or do both.

So, the spark plug thread length is another consideration to be kept in mind while choosing one for your lawnmower engine. Remember the golden rule: Right plug for the right application!

Spark Plug Gap:

As mentioned earlier, a spark plug has two electrodes. There is a live electrode in the center and a ground electrode on the side. Spark is generated at the live electrode, and it travels to the ground electrode across a gap between the two electrodes. The spark plug gap is something that can be changed. If your engine has a problem starting, check if the spark plug gap is right. If it’s not, you can adjust it to match the spark plug packaging or engine manual’s gap specification.

The spark plug gap can change during shipping or handling or if the plug falls on the floor or if the anode gets thinner. So, always check the gap before installing a spark plug.

● Why does the spark plug gap have to be right?

Depending on the type of engine your lawnmower uses, the gap between the two electrodes must be set to a specific width. If the gap between the live and ground electrodes is too small, there is not enough space for the spark to be generated with enough power to ignite the cylinder’s air-fuel mixture. On the other hand, if the gap is too wide, the spark generated at the live electrode won’t travel to the ground electrode, and there will be no ignition.

You can use a feeler gauge to measure the gap on your spark plug and compare it to the gap requirement. The feeler gauge has many stripes of different thicknesses. Adjust the gap width to the required specification and check if the stripe with that thickness passes through the gap without any clearance. If it does, your plug is good to go, and so are you!

Other Specifications to be Considered:

Some spark plugs have their center electrodes operating at a higher mean temperature than the other spark plugs. Such spark plugs are called “hotter spark plugs.” Similarly, there are “colder spark plugs,” too.

Furthermore, some spark plugs extend further into the combustion chamber as compared to others. Such plugs are called “longer.” Similarly, there are shorter “plugs,” too.

Such features affect the starting and working of an engine and must be optimized based on its specifications. If the spark plug you are using in your lawnmower engine does not have the right features, it will cause problems and cause some symptoms to emerge that will indicate the problem.

Using a too hot plug for your engine may cause pre-ignition, while a too cold plug might result in your engine’s poor ability to run at any speed.

If the plug is too long, the piston will touch the plug, and both will be damaged, while if the plug is too short, the spark is not created in the right location in the cylinder, resulting in poor running characteristics.

Taking care of the basic specifications can help you avoid such problems. Just make sure that the size, thread length, and gap, etc., match your lawn mower engine’s requirements based on the specifications provided by the manufacturer.

Your Guide to Buying the Right Spark Plug for your Lawn Mower:

You already know about the basic components of a spark plug. You can check their required specifications from the user manual or can get them from the internet. But where do you buy a spark plug from?

Spark plugs are a vital engine component and are therefore very popular. You can easily find them in engine spare parts stores. If you know of a store that sells lawnmowers, they will surely have the spare parts.

Or you can buy online from popular online shopping websites and have the right spark plug delivered to your doorstep. Make sure to buy from a credible website and a seller with a positive customer rating. Don’t forget to go through the customer reviews to be sure.

All you need before going shopping is the required specifications. You can either use the old spark plug as a reference for them or using the user manual.

Related Questions:

● Can I use a car spark plug in my lawnmower?

Automobile and lawnmower spark plugs are interchangeable in some cases, but not always. The two types usually have different thread sizes, thread lengths, and gaps, etc. So, use a plug for your mower whose specifications match the ones mentioned in the manual.

● Can I change the gap on my spark plug myself?

Yes. It’s quite simple. Use a feeler gauge to check the thickness and adjust it till it matches the requirement.

Final Remarks:

The right spark plug is essential for your lawnmower engine’s smooth performance and, ultimately, your lawnmower. The spark plug needs to ignite the air-fuel mixture to start the engine and needs to be capable of surviving the cylinder environment. So, always get the right spark plug for your lawnmower engine based on the features specifications that we just discussed.