The most common spark plug socket sizes are 5/8 inches and 13/16 inches. For the majority of plugs, you need a 5/8” or 16mm socket. Some Fords use 9/16″ plugs, some newer European and Asian vehicles use 14mm plugs, and there are a few other sizes available. Here are some typical spark plug socket sizes:

• 5/8 inches (16mm)
• 13/16 inches
• 14mm
• 9/16 inches
• 18mm
• 3/4 inches
• 11/16 inches

Its important to use the correct socket size to avoid damaging the spark plug or the threads in the cylinder head. Spark plug sockets usually have a rubber insert that holds the plug snugly in place.