Spark Plug Socket Sizes by Vehicle
Spark plug sockets come in a small number of standard sizes, and picking the wrong one is how insulators get cracked and plugs get dropped down the well. This guide covers the standard sizes, how they map to engine families, and the tools worth owning.
The two standard sizes
The overwhelming majority of spark plugs take either a 5/8" (16mm) socket or a 13/16" (21mm) socket. 5/8" is the modern standard, used on almost all current passenger vehicles and most small engines. 13/16" is the older, larger size, still found on many older engines, small engines and some trucks. If you own both, you can service nearly anything.
Use a proper spark plug socket
A spark plug socket is deeper than a standard socket and has a rubber insert or magnet that grips the plug body. That retention is the point: it stops the plug dropping into a deep well on the way out and stops the ceramic insulator cracking as you handle it. A plain deep socket will do the job right up until it does not, and a cracked insulator causes a misfire that is hard to diagnose because the plug looks fine.
Torque, not feel
Whatever the socket size, spark plugs thread into aluminium heads and are easy to over-torque. Most passenger-car plugs land in the 10-15 ft-lbs range on aluminium heads and should be torqued cold, never hot. Look up the exact figure for your engine rather than going by feel — over-torquing strips the head, which is a far more expensive repair than the plug.
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