Spark Plug Socket Size Chart
Quick answer: the most common spark plug socket sizes are 5/8 inch (16 mm) and 13/16 inch (about 21 mm). Many newer engines also use 9/16 inch or 14 mm thin-wall sockets, while some motorcycles, small engines, and older vehicles use 18 mm, 3/4 inch, or 7/8 inch spark plug sockets.
Common Spark Plug Socket Size Chart
| Socket size | Metric equivalent | Typical use | Notes |
|---|---|---|---|
| 9/16 in | 14 mm | Newer automotive engines, some Ford and Asian applications | Often needs a thin-wall spark plug socket. |
| 5/8 in | 16 mm | Many modern cars, motorcycles, scooters, and small engines | One of the two most common sizes. |
| 11/16 in | 17.5 mm | Specialty/older applications | Less common; verify before buying. |
| 18 mm | 18 mm | Motorcycles and some specialty plugs | Do not confuse thread size with hex/socket size. |
| 3/4 in | 19 mm | Lawn mowers, outdoor power equipment, some small engines | Common on older or larger small-engine plugs. |
| 13/16 in | 20.6 mm | Older cars, trucks, and many small engines | Also commonly described as about 21 mm. |
| 7/8 in | 22.2 mm | Rare or older specialty applications | Check the plug manufacturer before using force. |
What Makes A Spark Plug Socket Different?
A spark plug socket is usually deeper than a normal socket and often has a rubber insert, magnet, or retaining spring to hold the plug. That matters because spark plugs sit down in a well and can be fragile. A normal deep socket may fit the hex but still make the job harder or risk dropping the plug.
Before Removing Spark Plugs
- Let the engine cool before removing plugs.
- Blow debris away from the plug well before loosening the plug.
- Use the correct socket size and extension.
- Start new plugs by hand to avoid cross-threading.
- Torque plugs to the engine or plug manufacturer’s specification.
- Use a thin-wall socket when the plug well is narrow.
Related references: socket sizes in order and American wrench size chart.
Source Notes
Common spark plug socket sizes were cross-checked against current spark plug socket set specifications and automotive parts references, including listings that include 14 mm, 16 mm, 9/16 inch, 5/8 inch, and 13/16 inch spark plug sockets. Always verify the socket size for the exact plug installed in your engine.
