How are Spark Plug Threads Measured?

The firing end of a spark plug threads into the cylinder head. A spark plugs threads are comprised of 3 dimensions: Reach, Diameter and Pitch.

Thread pitch is the distance between threads expressed in millimetres. For example, a thread pitch of 1.25 means the distance between each thread is 1.25mm.

When a thread size is expressed in a set of two numbers, for example, 14x1.25 or M14x1.25 the first number is the thread diameter and the second number is the thread pitch.

Search for spark plugs by specifications and dimensions