Why is one of the spark plugs a different size in my 2023 Great Wall X240?

That’s an odd one, to be sure. I’ve never heard of an engine that used different sized spark plugs in different cylinders, so you’d have to assume it’s the result of somebody only doing half a job when the car was serviced.

There’s at least one possible explanation for this, though. You might find that one of the spark plugs is difficult to reach or access. In that case, a lot of amateur mechanics will leave that one plug and replace the others with new plugs that have a different sized insulator (the white, ceramic bit you can see poking out of the cylinder head). It’s always best practice to replace spark plugs as a complete set, but lazy mechanics will cut sometimes corners.

There’s also a very slim chance that a previous mechanic has used a different sized spark plug during a routine replacement because they didn’t have four new ones that matched.

Spark plug size is critical in many respects. As well as having the correct threaded section to fit the cylinder head, a spark plug’s internal dimensions are crucial to ensure it doesn’t poke too far into the combustion chamber and make contact with the piston. It also needs to be located correctly in the combustion chamber to ensure proper ignition of the fuel.