In most cars, you pull a spark plug with a socket that matches the plug’s hex size—most commonly 14 mm or 5/8 inch across the flats—and you’ll typically use a 3/8-inch drive ratchet. Some engines require different sizes or drives, so always verify with the vehicle’s service manual or by inspecting the plug itself.
Common spark plug hex sizes
Knowing the plug's hex size is essential to selecting the right socket. The size is measured across the flats on the plug head and must match the socket precisely to avoid rounding off the corners.
- 14 mm spark plug hex (commonly used on European and many Asian engines).
- 5/8 inch spark plug hex (a very common SAE size for many American engines).
Note that some engines use other hex sizes due to design variations. If in doubt, compare the plug you’re removing to a spare socket or check the service manual for the exact size.
Drive sizes and accessories
Beyond the hex size, the drive size and handy extras affect ease of removal, especially in tight engine bays.
- 3/8-inch drive spark plug socket with a magnetic or rubber insert to hold the plug securely.
- 1/2-inch drive spark plug socket can be advantageous for extra leverage or particularly stubborn plugs.
- Extensions, wobble joints, or flex-head adapters help reach plugs tucked behind components without straining or damaging the ceramic insulator.
Always pair the socket with the appropriate extensions and, when reinstalling, follow the manufacturer’s torque specifications. Using the wrong size or forcing a fit can damage the plug or cylinder head.
How to confirm the correct size
If you’re unsure, confirm by inspecting the plug’s hex across the flats, checking the service manual, or bringing the old plug to an auto parts store to compare with the correct socket size. A properly sized socket reduces the risk of rounding the plug head during removal.
Safety tips and best practices
Before starting, disconnect the battery to prevent sparks, especially in a fuel-air environment. Let the engine cool, and install the new plugs with the torque spec recommended by the manufacturer. Many modern plugs are designed to be installed dry or with minimal anti-seize—follow the vehicle’s guidance, as applying anti-seize can affect torque. Use a torque wrench to avoid over-tightening, and keep debris away from the threads and cylinder head during removal and installation.
Summary
To pull a spark plug, the most common socket sizes are 14 mm or 5/8 inch across the flats, typically used with a 3/8-inch drive. Some engines may require different sizes, so always verify with the service manual or by comparing the plug itself. Equip a magnetic or rubber-insert socket, appropriate extensions or wobble joints, and torque the plug to specification for a safe, proper reinstall.