What size socket do I need to remove spark plugs?
The correct socket size depends on the spark plug’s hex size. In practice, you’ll most often encounter 5/8 inch (16 mm) or 13/16 inch (about 21 mm) sockets, but some engines use metric sizes such as 14 mm or 18 mm. Check your vehicle’s manual or inspect the plug to determine the exact size.
Common socket sizes you may encounter
Below are the most frequently used spark plug socket sizes and when they commonly appear, so you can start with the most likely options and avoid guesswork.
- 5/8 inch (16 mm) hex — the most common size for many modern passenger cars and numerous engines from American and Japanese brands.
- 13/16 inch (about 21 mm) hex — used on some larger engines and certain European models; may require a 13/16" spark plug socket.
- 14 mm hex — widely used on many Japanese-made vehicles; a 14 mm spark plug socket is often labeled accordingly.
- 16 mm hex — appears on a subset of modern engines; a 16 mm spark plug socket fits these plugs.
- 18 mm hex — found on some heavy-duty or specialty plugs; use an 18 mm spark plug socket if required by your engine.
Because plug hex sizes vary by make and model, it’s prudent to verify using your owner’s manual, by checking the plug part number, or by measuring the plug’s hex. If you’re unsure, a spark plug socket set that includes multiple sizes is a reliable way to cover most vehicles.
How to identify the correct size for your vehicle
Use these steps to determine the exact socket size needed for your spark plugs, so you avoid stripping the plug or damaging the head.
- Locate the spark plug and remove the boot to expose the hex nut on the plug itself.
- Measure the distance across the plug’s hex flats with calipers or a precise ruler to determine the hex size in millimeters.
- Compare the measurement to common sizes (14 mm, 16 mm, 18 mm, or 20–21 mm / 13/16") and test-fit the corresponding spark plug socket to confirm a snug fit without wobble.
- If in doubt, consult the vehicle’s service manual or look up the plug part number online to confirm the correct socket size.
Having confirmed the size, store a small set of spark plug sockets for future maintenance, since different engines use different hex sizes.
Tools and best practices for removing spark plugs
Equipping yourself with the right tools and following best practices helps prevent damage to plugs and heads, and makes the job safer and quicker.
- Spark plug socket with a rubber insert or a foam insert to grip the plug securely without dropping it.
- Appropriate ratchet size (usually 3/8" drive for most cars; 1/2" drive may be used for heavier applications).
- Extensions and a universal joint as needed to reach plugs in tight spaces.
- Torque wrench to tighten to the manufacturer’s specification after installation; never rely on feel alone.
- Penetrating oil for stubborn, corroded threads, applied carefully and allowed to work before attempting removal.
- Optional: a small amount of dielectric grease inside the ignition boot to help future removal, though apply sparingly and only as needed.
Note: Do not assume a single standard torque for all plugs. Always torque to the specification stated in your service manual or by the plug manufacturer. On many modern engines, anti-seize compounds are not recommended unless the manufacturer specifies them, as they can alter the torque and risk over-tightening.
Summary
In short, the socket size for spark plugs depends on the plug hex size. The most common options are 5/8" (16 mm) and 13/16" (about 21 mm), with several metric options such as 14 mm, 16 mm, and 18 mm also in use across different makes and models. To be sure, consult your vehicle’s manual, identify the plug’s hex size, or use a comprehensive spark plug socket set that includes multiple sizes. Always clean the area, use the correct socket with a rubber insert, and torque to the manufacturer’s specification when reinstalling.
