What size spanner for spark plugs?
The most common spark plug hex size is 16 mm (5/8 inch) across flats. Some engines require 14 mm or 18 mm. Always verify the exact size by checking the plug itself or the vehicle’s service manual.
Spark plugs are removed and installed with a wrench that fits the plug’s hex across the flats. Using the wrong size can strip the plug or damage the seat in the cylinder head. Size varies by engine, so confirm with the manual or compare to the old plug. Below we outline typical sizes and how to identify yours.
Common spark plug hex sizes
These are the hex sizes you’re most likely to encounter when replacing spark plugs. Knowing these helps you choose the right socket or spanner quickly.
- 14 mm (about 9/16 in) across flats — common on many small engines and some older European cars
- 16 mm (5/8 in) across flats — by far the most common on modern cars and light trucks
- 18 mm (11/16 in) across flats — used on some diesel engines and certain large-displacement engines
- Rare cases: around 20–21 mm hex on some heavy-duty or specialized engines; always verify with the plug or service manual
In practice, start with 16 mm for most vehicles; if it doesn’t fit, try 14 mm or 18 mm. Do not force a plug onto a wrong-size wrench, as that can damage the plug or cylinder head. When in doubt, consult the manufacturer’s guide or compare to the old plug to confirm the size.
How to identify the correct size for your engine
To confirm the exact size for a specific engine, use these verification steps.
- Check the vehicle’s service manual or the spark plug box for the exact hex size and torque specification.
- Inspect the old spark plug and measure the hex across the flats with a caliper or plug gauge, then match to common sizes (14, 16, 18 mm).
- If you’re still unsure, visit a parts store or dealer with your vehicle’s make/model; staff can identify the correct size by plug number or engine configuration.
- Once identified, use a spark plug socket designed for seating and removal, with a rubber insert to hold the plug securely.
Having the correct size helps prevent cross-threading or damaging the plug seat. Always torque the plug to the manufacturer’s specification after installation.
Tools and safety tips
Choosing the right tools and following basic safety practices will protect both you and the engine during plug replacement.
Choosing the right socket and drive
Use a proper spark plug socket with a rubber insert to grip the plug without damaging the ceramic insulator. Pick a drive size (3/8" or 1/2" depending on space) that fits your workshop area, and have a torque wrench to apply the correct torque.
- Never force a plug with a mismatched wrench; forcing can strip the plug or damage the head.
- Follow the vehicle’s torque specification for tightening, typically provided in the service manual.
- Some plugs come with anti-seize coating; if the manufacturer advises against additional anti-seize, do not apply it.
- Let the engine cool before changing plugs to avoid burns and handle heat-sensitive components safely.
Using the proper socket, drive, and follow-up torque ensures a proper seal and reliable ignition performance.
Summary
In short, the steady rule of thumb is that most modern vehicles use a 16 mm (5/8 inch) spark plug hex. Some engines require 14 mm or 18 mm, with a few rare cases needing even larger sizes. Always confirm by checking the plug, the vehicle’s manual, or the replacement part’s packaging. Use a spark plug socket with a rubber insert and a torque wrench to install correctly, and avoid forcing components to prevent damage.
