Spark Plug Socket Size Chart
Quick answer: 5/8-inch and 13/16-inch sockets are common spark-plug sizes, but the correct size is the width across the specific plug hex. A close metric or inch size is not automatically an acceptable substitute.
Common Spark Plug Socket Sizes
Tool makers list spark-plug sockets in both inch and metric sizes. The metric values below convert the nominal inch sizes to the nearest 0.1 mm; they do not mean that a nearby metric socket will fit. For example, 11/16 inch converts to about 17.5 mm, not 18 mm. Use the size marked on the plug or check the vehicle service information before turning it.
| 9/16 in | 14.3 mm | Common spark-plug socket size; confirm plug hex. |
|---|---|---|
| 5/8 in | 15.9 mm | Common spark-plug socket size; confirm plug hex. |
| 11/16 in | 17.5 mm | Not the same size as an 18 mm socket. |
| 18 mm | 18.0 mm | A distinct metric size; check the plug marking or service data. |
| 3/4 in | 19.1 mm | Available in spark-plug socket sets; verify the plug hex. |
| 13/16 in | 20.6 mm | Common spark-plug socket size; confirm plug hex. |
| 7/8 in | 22.2 mm | Less common; measure or verify before use. |
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What Makes a Spark Plug Socket Different?
A spark-plug socket is usually deep enough to reach a recessed plug and may include a rubber insert, magnet, or retaining spring. Match the socket to the plug hex and well clearance. If the socket rocks, binds, or does not seat fully, stop and verify the size rather than forcing it.
Before Removing or Installing a Plug
- Let the engine cool and clear debris from the plug well.
- Use a socket that fits the plug hex fully and clears the plug well.
- Start a replacement plug by hand to avoid cross-threading.
- Follow the engine or spark-plug manufacturer torque specification.
Related references: socket sizes in order and American wrench sizes.
Sources
Socket sizes were checked against ARES spark-plug socket listings and Motion Pro ignition-tool guidance. Metric equivalents use the exact inch conversion published by NIST.
