In most lawn mowers, you’ll use a spark plug socket that fits the plug’s hex across the flats. The two most common sizes are 13/16 inch and 14 mm (roughly 9/16 inch), with 5/8 inch appearing on some older or smaller engines. If you’re unsure, measure the hex on the plug or test fit the closest sizes with a rubber-backed spark plug socket to hold the plug securely.
Common socket sizes for lawn mower spark plugs
The following sizes cover the majority of lawn mower engines. Choosing the right one prevents damaged plugs or stripped hexes.
- 13/16 inch (20.6 mm) hex — the most widely used size on many Briggs & Stratton and generic small-engine spark plugs.
- 5/8 inch (16 mm) hex — found on some older or smaller engines; less common on newer models.
- 14 mm (about 9/16 inch) hex — common on many Honda and some Tecumseh/other-brand engines.
If the plug you’re removing doesn’t fit neatly in any of these sizes, double-check the plug’s hex across the flats or consult the mower’s manual or the plug packaging for the exact specification.
How to identify the correct size for your mower
Follow these steps to confirm the proper socket size before forcefully turning the plug.
- Examine the spark plug head and measure the distance across the flat sides of the hex with calipers or a precise ruler to identify the size.
- Try the closest standard sizes (start with 13/16", then 14 mm, or 5/8") using a spark plug socket with a rubber insert to hold the plug securely.
- When the socket seats fully on the plug head and turns smoothly, you’ve found the correct size. Do not force a socket that doesn’t fit snugly.
- If in doubt, remove the plug and check its model or part number, then verify with the mower’s manual or the plug manufacturer’s site for the exact hex size.
In practice, it’s a quick check-and-fit process. Using a proper spark plug socket with a rubber insert helps prevent dropping or cross-threading the plug.
Brand-specific notes
Briggs & Stratton and generic small engines
Most Briggs & Stratton and many other common lawn mower engines use a 13/16–inch hex spark plug. A standard spark plug socket with a rubber insert is the easiest choice here.
Honda and other modern engines
Many Honda and some Tecumseh engines use a 14 mm hex spark plug. If you encounter a plug that won’t fit 13/16", try a 14 mm socket first, then a 5/8" if needed.
Other brands and older equipment
Older or less common models may require 5/8 inches or a metric size around 14 mm. When in doubt, verify the plug’s hex size from the manual or by matching the plug head in a hardware store tool section.
Safety tips and best practices
Prioritize safety and proper technique to avoid injury or damage to the engine while removing or reinstalling a spark plug.
- Ensure the lawn mower engine is off and cool before touching the spark plug area.
- Disconnect the spark plug boot or wire to prevent accidental ignition.
- Use a spark plug socket with a rubber insert to hold the plug securely and prevent dropping it into the engine bay.
- Agent: avoid over-tightening the plug when reinstalling; snug-to-till-torque per the manufacturer’s specification, or give it a firm final turn by hand after finger-tight.
- Replace worn or damaged plugs with the correct type and gap setting specified for your mower.
Following these tips helps maintain engine reliability and reduces the risk of thread damage or misfires.
Summary
For removing a lawn mower spark plug, the most common socket sizes are 13/16 inch, 14 mm, or 5/8 inch, depending on the engine. Identify the plug’s hex size by measuring across the flats or consulting the mower’s manual, then use a spark plug socket with a rubber insert to hold the plug while turning. If you’re unsure, start with 13/16" and move to 14 mm or 5/8" as needed, and always follow the manufacturer’s torque recommendations when reinstalling.