⚡A Homeowner's Guide to Replacing a Chainsaw Spark Plug
If your chainsaw is giving you a hard time starting, there are a few usual suspects.
It could be a blocked air filter, you might have flooded it with too much fuel, or perhaps it's simply time to replace the spark plug.
The good news is that changing a spark plug is one of the easiest and most effective pieces of maintenance you can perform on your saw.
I recently ran into this exact problem at my mom's house over Christmas. The old chainsaw wouldn't start, and upon inspection, the spark plug was heavily rusted.
I managed to get it out with some old pliers (definitely the wrong tool for the job!), but the experience got me thinking.
What's the right way to do it, especially if it's really stuck?
This guide will walk you through everything you need to know.
๐งHow to Change a Spark Plug on a Chainsaw: A Step-by-Step Guide
It's always best to work on a cold engine. If your saw has been running, let it cool down completely to avoid burns.
Remember, if your machine was just running fine, you probably don't need to replace the plug just yet!
- Safety First: Make sure the chainsaw is turned off. Slide the ignition switch to the "Off" position.
- Access the Plug: Remove the top cover of the saw. This is usually held on by a few screws or a simple twist-lock system. Place the screws somewhere safe so you don't lose them!
- Remove the Boot: You'll see a black rubber boot sitting on top of the spark plug. Firmly grip it and pull it straight up and off the plug. A gentle wiggling motion can help.
- Unscrew the Old Plug: Place a 3/4-inch socket wrench or a dedicated chainsaw wrench over the spark plug. Turn it counterclockwise to loosen and remove it. Be firm but gentle; you don't need the force of Thor! If the plug is sticky, a little CRC or penetrating oil can help.
- Install the New Plug: Take your fresh, new spark plug and screw it into the cylinder by hand first. This prevents "cross-threading," which can damage the engine head. Once it's hand-tight, use your wrench to give it a final snug turn. Do not overtighten it!
- Reassemble: Push the spark plug boot firmly back onto the new plug until it clicks. Replace any shielding and the top cover, securing the screws.
Finally, test your work by starting the chainsaw. It should fire up with renewed vigor!
The Science of the Spark Plug Gap
That little metal hook at the end of your spark plug is more important than it looks. The distance between that hook (the ground electrode) and the central electrode is called the "gap."
When the engine's ignition coil sends a high-voltage pulse, it has to be strong enough to jump across this gap, creating a powerful spark that ignites the fuel-air mixture. If the gap is too wide, the spark may be too weak to ignite the fuel.
If it's too narrow, it may not create a large enough spark to start the combustion process efficiently.
Thankfully, if you buy the correct replacement plug for your model, it should be properly gapped right out of the box.
๐Reading the Signs: How to Tell if Your Spark Plug Needs Replacing
Your old spark plug can tell you a story about your engine's health. When you pull it out, take a close look:
- Light Brown Deposits: This is the sign of a healthy, well-running engine. If the gap is correct, you're good to go.
- Thick, Black, Sooty Deposits: This indicates the fuel/air mixture is too rich (too much fuel, not enough air). This could be due to a clogged air filter.
- White, Glazed, or Corroded Electrodes: This suggests a lean mixture (too much air, not enough fuel) or that the engine is running too hot.
- Oily Deposits: This points to an oil leak in the engine, which requires further maintenance.
If your plug is just dirty, you can often clean the deposits off with a wire brush or a small piece of fine-grit sandpaper.
You can also gently adjust a widened gap with pliers, but be careful not to snap the electrode.
Use a Spark Plug Tester for a Quick Diagnosis
If you're not sure whether the problem is the plug or the ignition system itself, a spark plug tester is a fantastic tool. You simply connect it between the spark plug and the boot.
When you pull the starter cord, a light on the tester will flash if the ignition system is sending a current. If it lights up but the saw won't start, the problem is very likely your spark plug (assuming fuel isn't the issue).
๐What's the Best Replacement Spark Plug for My Chainsaw?
The best place to start is always your chainsaw's instruction manual. It will tell you the exact model and gapped plug you need. If you don't have the manual, a quick Google search will usually find a PDF version.
While your chainsaw might be a certain brand, you don't necessarily need to use an official replacement part. Most aftermarket parts are perfectly fine, as long as they are from a quality brand and not a cheap knock-off. For example, Stihl often recommends and uses NGK plugs in their chainsaws.
Brands like NGK are well-known and produce reliable plugs that suit most small engines. When in doubt, take your old plug to a specialist chainsaw shop, and they can help you find a like-for-like replacement.