While 5/8" (16mm) and 13/16" are the two most common plug hex sizes, plugs can have a hex as small as 8mm or larger than 1".
If you're a tuner or engine builder in search of the perfect spark plug for your current project, pay special attention to the hex size required. Remember to take into consideration not only the diameter of the cylinder head, but also the added thickness of the plug socket needed for installation.
A spark plugs reach is calculated by the length of the threads. It is measured from the seat of the plug to the end of the threads.
It should be noted that the reach does not include the electrodes, or an extended metal shell. When custom-selecting a spark plug, the electrode length and projection should be included in the calculation when taking into consideration the clearance in the combustion chamber.
WARNING: Spark plugs with different seat types are NOT interchangeable.
- Spark plugs with a flat seat must be used in engines designed for a gasket seal.
- Spark plugs with a tapered seat must be used in engines designed for a tapered seal.
Use of an improper plug in an engine will result in engine damage.
14mm, .460" Reach, 5/8" Hex