How to replace a chainsaw spark plug

Sunday, March 21, 2021
If your chainsaw is having a tough time starting, it could be because the air filter is blocked, you flooded it, or perhaps you just need to replace the spark plug.

The good news it's easy to replace the spark plug in your chainsaw.

Here's a general guide to removing and replacing a chainsaw's spark plug

chainsaw spark plug - how to change

It's best if you change the plug when the machine is cold, a machine that has recently been put through its paces will be hot, meaning you have to be mindful of that when changing. 

Of course, if your machine has been running you may not need to replace the plug...necessarily

How to change a spark plug on a chainsaw

  1. This is perhaps kind of obvious but your machine needs to be turned off. Release the trigger on your chainsaw and slide the ignition switch in the rear of the engine down to the "Off" position. Simple. 
  2. There will probably be some screws on the top cover of the saw which you may need to remove. Insert a flathead screwdriver (or apt shape) into each of the screws on the chainsaw's top cover housing. Turn the screws counterclockwise to loosen them and pull each one out of the cover. Pull the cover straight up and off the engine and put the screws in a place you will not lose them. Your machine cover may have a twist-off system, if so engage that. 
  3. There may be a plastic shield, if so remove it. 
  4. There is most likely a rubber spark plug boot on the front of the engine (it's black) - pull it directly from the spark plug. A simple 'wiggle off' should suffice.
  5. Place a 3/4-inch socket wrench on top of the spark plug, turning it firmly counterclockwise will pull the spark plug out of the cylinder. It doesn't need the force of Thor, however! Be firm but gentle. If the plug is a bit sticky then a little CRC may help loosen it. When you bought your chainsaw, it most likely came with a chainsaw wrench, if so use that or it would be the correct size. 
  6. Have a fresh plug at the ready. Insert the new spark plug into the cylinder and turn it clockwise. Place a 3/4-inch socket on the spark plug and tighten it clockwise but do not overtighten. 
  7. Replace the plug boot, any shielding, or air filters (make sure the filter is on correctly or it could suck dirt into the carburettor). 
  8. Push the spark plug boot firmly onto the spark plug, replace the cover and screws, and tighten them clockwise with a flat-head screwdriver.

Extra for experts: you should test that you have the right plug and that it is 'gapped' appropriately - the electrode spark needs a sufficient 'gap' to do its thing. If you buy the correct replacement, it should be properly gapped out of the box. 

For those who like to take a more visual approach to how you learn, this tutorial video captures the basics of changing the spark plug quite well!


How can I tell if I need to replace the chainsaw's spark plug?

There are plenty of ways to determine when you need to replace your spark plug

Checking it visually will give great insight. If the engine is running as expected, the outside electrode will probably be covered with light brown deposits. 

If the engine isn't running normally, the condition of the plug will be poor. You may observe, thick black deposits that indicate that the fuel/air mixture is too rich. This means there isn't enough air in the mixture. A wide gap or corroded electrodes, on the other hand, are indications of a lean mixture. 

Oil on the plug indicates an oil leak which means your chainsaw needs maintenance or adjustment.

If the plug is coated with light brown deposits, and the 'gap' is correct, you don't need to do anything to the plug. 

If it's coated with black deposits or oil, these need to be removed. You can remove them with an emery board or a small piece of 120-grit wet/dry sandpaper or similar. Even a scratch with a screwdriver can work...

You can adjust the gap, if need be, with pliers. Do not exert too much pressure or you could snap it.  

Using a spark plug tester to check for ignition issues

A spark plug tester is a tool that allows you to check if your spark plug is working properly. It can also save you time by allowing you to check if the problem is actually in your ignition.

In simple use terms, you just need to connect the metal end of the tester to the spark plug coil and start the engine. If the ignition wire is intact, the indicator lights and you can be sure your engine is connecting to the plug coil, implying the starting fault is likely with the plug itself (assuming there's no engine failure or fuel issues of course).

What is the best kind of replacement chainsaw spark plug?

If you buy the correct replacement, it should be properly gapped 'out of the box'. 

Your starting point for guidance is in your instruction manual. This will tell you what sized spark and gapped plug you need. Look in the 'Technical data' chapter of your manual. If do not have one to hand, google your machine type and model for a PDF version. 

Remember, while your machine may be a certain brand, you do not need to use an official replacement part. Most 'after market' parts are fine - as long as they are quality and not some cheap end 'knock offs'.

Case in point, Stihl recommends owners use their NGK plugs in Stihl chainsaws.  

Local specialist chainsaw shops are also well placed to give advice on the correct plug. What I did when needing a replacement a few years back, was take the plug out and get them to swap it out, like for like. Did the same for my lawnmower spark plug too. 

NGK is a well known brand and suits most smaller chainsaws.
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