A typical walk-behind lawn mower uses a single spark plug in its engine. Every mower is different, but on average the spark plug should be replaced (or at the very least, cleaned and regapped) once a year or so, depending on use. This article will describe the procedure for replacing the spark plug on a typical walk-behind (or push) lawn mower.
If you haven't done so already, you'll need to purchase a new spark plug of the correct size and type. Your mower's parts manual or engine parts manual will have the part number for the spark plug you need. (You can download mower parts manuals using the links on the left sidebar, and engine parts manuals using the links on the right sidebar.)
In general, the spark plug is located on the front side of the mower, sticking out from the engine. It will have a thick wire "boot" leading from it (probably black). Once you have located your mower's spark plug, removing it is a fairly simple process.
To remove the spark plug, you'll need either a spark plug wrench of the correct size, or a standard socket wrench with the correct spark plug socket. You can use a regular socket of the same size, but spark plug sockets are specially designed to grab the plug better and not to damage it. Spark plug sockets are also typically deeper than normal sockets. Standard spark plug socket sizes include 3/4", 5/8", and 13/16".
Before removing the spark plug, make sure the mower is off, and wait at least 10 minutes or so for the engine to cool down. (If the engine still feels hot to the touch after 10 minutes, wait a little longer so you don't burn yourself when removing the spark plug.) Once the engine has cooled, remove the spark plug boot from the spark plug by pulling it off.
The spark plug itself is now exposed. To remove it, place the spark plug socket over the plug so it fits snugly, then attach the wrench to the socket. If you are using a spark plug wrench, fit the correct end of the wrench over the plug, then attach the handle to the other end. Turn the wrench counter-clockwise to loosen the plug, and keep turning until the plug is completely free.
You can now remove the spark plug from the mower. While it is out, you may want to take a clean rag and wipe the area around the spark plug hole to clean it off. If the plug was hard to remove, you can use a little bit of an anti-seize compound on the new spark plug threads so that next time it will be easier to remove. Then insert the new spark plug into the hole that you removed the old one from, thread it a little bit by hand, and then attach the spark plug wrench (or socket wrench with spark plug socket) and tighten the plug. You want it to be tight, but not overly tight.
Once the plug is in securely, reattach the spark plug boot. Now is a good time to start the mower and make sure it runs. If the old spark plug was still working, you may want to hold onto it in case you ever need it in a pinch.
(Originally published on Helium.com, May 2012)