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Are All Lawn Mower Spark Plugs The Same Size? Are They Same As Car Plugs?

Written by William
Last Updated on July 20, 2021

If you're finding your spark plugs to be playing up recently, and are spending far too much time re-cranking them, then it's about time you changed those plugs. You may be wondering which plugs to use; what are the right electric spark plugs/spark plug options for me to use for my lawn mower engines?

Since lawnmowers are designed to last the owner many years of usage, it's quite self-explanatory that you'd need to change certain components up to increase the longevity and improve performance rather than stick with poor engine performance. Spark plugs need to be changed up too- after damage incurred via the improper use of your mower, the types of engine work that have been done, the quality of the motor engine, the quality of the engine components (engine components should be of a high quality to increase the lifespan of the mower as a whole), general daily use, and the previous spark plug types used.


How Do I Know When to Change the Spark Plugs?

There are three giveaway signs that indicate the necessity to change your spark plugs:

  • The spark plug doesn't look so good: Inspect your lawn mower plugs carefully. A bad spark plug can usually be told apart by physical differences, if you've seen what a normal spark plug should look like. If you find any bending or distortion on or around the outer electrode, the spark plug gap (the spark plug gap is usually the most visible distortion), the flat top, or the metal cap- then you will need to look into changing them and get a spark plug wrench.
  • The engine is performing as well as it used to: You know your spark plugs need replacing when the engine (flathead engines, auto engines, automobile gasoline engines, combustion engines, for example), isn't working how it used to. You'll need a new engine spare unless you want to spend time re-cranking your mower all the time.  If something is wrong with the engine’s spark plug, it won’t run smoothly like it ought to. It will frequently require re-cranking to start again.
  • There's excess fuel consumption: If you feel as though your mower is drinking up oil like it's been parched for days, then you need to look into replacing the standard resistor spark plug that has been damaged and purchase a new one with the same replacement spark plug number.

Mower vs Automobile Plugs

There are some key differences and information to note when comparing auto plugs and mower plugs:

  • Both automobile and lawn mower gasoline engines use spark plugs to help ignite and mix the fuel in the gas-packed chamber
  • They cannot be interchanged because they differ in physicality, in terms of their shape and appearance (as we discuss below)
  • Both engines work the same way; a voltage with high pulse is shot through an electrical spark, to quickly jump past the gap to the other electrode (outer)

They Differ in Size

One main difference between automobile and lawn mower spark plugs is the size; lawnmower spark plugs' thread size is marginally smaller than automobile spark plugs- but the difference is significant enough to prevent successful substitution of the two plugs. 

Many automobile spark plugs' thread sizes range between 14-15 millimetres, while lawnmower spark plugs' thread sizes range between 9-12 millimetres. Hence, their spark plugs simply won't fit in with eachother. 


Thread Length

As we touched on thread length earlier, this has a whole explanation for itself- lawnmower spark plugs are generally shorter than automobile ones- why? What does this mean? The difference in shape will essentially result in the spark plugs being unable to fit into the tapered holes for eachother- so always buy the right one for your specific device and model- and ensure you have the equipment necessary at hand. 

About the author
Written by William
I have always had a passion for gardening and that with a background in selling lawn mowers for the past 10 years, I have become very knowledgeable in all types of gardening tools. The site TheBestMowers.co.uk was created as a hub where I can review and write about all of the tips around gardening.
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