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Discussion Starter · #1 · (Edited)
This might seem pretty like a pretty simple task to some, but for those who have never done a tune up on their mower, and might be a little in the dark about what to do, this is a simplified look at changing a spark plug.

First thing is to gather together the tools and parts you'll need. The best tool to use is a spark plug socket. Deep well sockets work well too because they clear the porcelain tip of the plug and get down to the "nut" part of the plug. If your spark plug is out in the open, and you can get a regular closed end wrench on it, that'll work too. It's not a good idea to try to do it with a pair of pliers, and don't try twisting the plug out by grabbing onto the porcelain part. You'll most likely break something, and only make more work for yourself, or a mechanic. Pull the spark plug wire from the end of your spark plug and take the old plug out. Remember, righty tighty, lefty loosey. Turn it counter clockwise to remove, clockwise to install.

Most spark plugs use either a 13/16" or 3/4" sized socket. Some others might be different, so get the right sized wrench.

Get the right plug for your mower. One of the most commonly used spark plugs is the Champion RJ19LM, but check your owner's manual to be sure to get the correct plug for your mower. They'll most likely be in a package like this, and you can find them in the lawn and garden section of many big box stores, an auto parts store, or mower repair shop.

Once you've taken it out of the package, take time to check the gap on the plug. That's the space between the tip and electrode thing on the end of the spark plug. Most of them come out of the box set correctly, but it is best to check before installing. An improperly gapped plug can cause problems for your engine. Most call for a setting of something like .030", but again, check your owner's manual for the proper setting for your mower. A simple little tool like this one on the left is all you need to check the gap. It's a little bigger than a quarter, and has a ramped edge along a row of numbers that indicate the measurement of the gap. The picture on the right shows it in place on the plug measuring the gap.

Now it's time to install the new spark plug. Take care to line up the threads on the end of the spark plug so that you don't "cross thread" the plug. It should turn in easily using your fingers. Turn it in until it is "finger tight".

Now place the socket or wrench onto the plug to tighten it up. You don't have to be Magilla Gorilla here, it doesn't take much to snug it up. It only takes a part of a turn to tighten the spark plug. It's not even a quarter of one full turn to get the job done. Compare the position of the wrench handle in these two pictures. The one on the left was the "finger tight", the one on the right is tightened up and ready to go. If you over tighten, you run the risk of stripping the threads on your lawn mower. Not a good thing. Finally, put the plug wire back on and you should be good to go.
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