Hi Ron. If the information here is correct, it is 5/8. : https://www.oreillyauto.com/detail/b/platinum-5897/ignition---tune-up-16776/spark-plug-12874/b14eb31b13d7/motorcraft-platinum-spark-plug/sp520/6231902Does anyone here know what size is the hex on a Motorcraft SP-520 spark plug ??? In other words, what size spark plug socket fits the plugs in our MKS's. Is it 9/16" or 5/8" ???