Spark plugs should be replaced when the engine is at ambient temperature.
This is because the head and spark plug are made of different materials, so they expand and contract at different rates. If you try to remove or install a new spark plug when the engine is hot or cold, it may be a different size than the threads, which can cause thread damage or spark plug breakage.
What happens if you replace spark plugs when the engine is cold?
If your gasoline engine has been completely shut down and cooled, it is best to wait until the engine has had a chance to warm up and reach ambient room temperature before replacing the spark plugs.
If you try to replace spark plugs when the engine is very cold, you may find that the torque setting is different than you expected. In this case, torque refers to the amount of pressure required to turn the spark plug.
Since torque is determined by the diameter of the spark plug's threads, and the spark plug and head are made of different types of metal, they expand and contract at different rates. Therefore, when the engine is cold, the torque setting will be lower than usual. This means that you may not be able to tighten the spark plug properly, causing damage.
What happens if you replace spark plugs when the engine is hot?
It is not recommended to replace spark plugs when the engine is hot for several reasons. First, as the engine and spark plug heat up, the threads expand. If you try to remove a spark plug while it's still hot, you may find that the plug is stuck in place.
Trying to remove a stuck spark plug can damage the head threads, which will then require repair. In severe cases, the entire head may need to be replaced.
If you try to install a new spark plug while the engine is hot, the plug may be too loose or too tight, depending on the application. When the head is hot, the threads may expand.
Therefore, when you install a cold spark plug, it may not engage the threads properly. This can cause a range of problems, including damaged threads, cross-threading of the spark plug, or the spark plug not fitting properly in the head.
Another reason not to change spark plugs when the engine is hot is that you could get burned. The engine, spark plug, and surrounding area can all be hot, so be careful when changing spark plugs.
