What size socket is needed for spark plugs?

To remove or install a spark plug, use a spark plug socket that matches the plug’s hex size; the most common sizes are 16 mm (5/8 inch) and 21 mm (13/16 inch), with some engines using smaller or larger hexes. Always verify against your vehicle’s manual or the plug specification for accuracy.

Understanding the right size

The size of the socket you need is dictated by the spark plug’s hex across flats. Using the wrong size can strip the hex or damage the plug and sometimes the cylinder head. When in doubt, check the owner’s manual, the service manual, or the packaging that came with the plugs. Spark plug sockets are designed to fit the exact hex size so you should identify the correct measurement before attempting removal or installation.

Common spark plug socket sizes

Below is a quick guide to sizes you’re most likely to encounter in everyday automotive work. The list highlights sizes that cover a large portion of passenger vehicles, race cars, and light trucks. The exact size for a given engine should come from the vehicle manual or the plug manufacturer’s spec.

• 16 mm (5/8 inch) hex — The most widely used size for modern gasoline engines. It fits a large number of mainstream passenger cars and light trucks.


• 14 mm (9/16 inch) hex — Found on some older or European-era plugs and certain smaller engines. Some sets include this size for compatibility with those plugs.


• 21 mm (13/16 inch) hex — Used on some larger or high-performance engines with bigger plug heads. This size appears in a fraction of vehicles but is essential for those applications.

In practice, many repair shops rely on a spark plug socket set that includes multiple sizes or a universal, adjustable socket to cover these common hexes. Always confirm with the specific plug you’re using to avoid damage.

How to confirm the right size for your vehicle

Use these steps to ensure you pick the correct socket for your spark plugs before attempting removal:

1. Check the vehicle’s owner’s manual or service manual for the spark plug specification, including the hex size.


2. Look at the existing spark plugs themselves; measure the hex across the flats with a caliper or ruler if the packaging is unavailable.


3. Inspect the plug packaging or the manufacturer’s website for the recommended socket size and any special notes (e.g., heat range, porcelain type).


4. Use a dedicated spark plug socket with a rubber insert or magnetic grip to hold the plug securely during removal and installation.


5. Fit the socket to the plug’s hex snugly; if it doesn’t fit squarely, stop and verify the size to prevent rounding off the flats.

Following these steps reduces the risk of damaging the plug or the cylinder head and helps ensure you can apply the correct torque during reinstallation.

Tips for using the right socket effectively

Beyond matching the hex size, a few practical tips can make spark plug work safer and easier:

• Use a deep-well spark plug socket designed to reach plugs in recessed areas.


• Always use a ratchet with a long enough handle and a wobble or universal joint for difficult access.


• Apply a small amount of anti-seize compound or thread lubricant if recommended by the plug manufacturer, but not on the threads of the cylinder head.


• Do not force a plug; if it binds, stop and re-check the size and reach to avoid stripping.

With the correct socket and careful technique, spark plug replacement becomes a straightforward maintenance task rather than a risky struggle.

Summary

In short, choose the spark plug socket to match the plug’s hex size. The most common automotive sizes are 16 mm (5/8 in) and 21 mm (13/16 in), with 14 mm (9/16 in) appearing on some engines. Always verify with the manual or plug packaging, and use a proper deep-well, rubber-insert socket or magnetic holder to reduce the chance of damage during removal and installation. A correctly sized socket paired with careful technique ensures a reliable plug replacement and proper engine performance.

What tools do I need to remove a spark plug?

Gather needed tools and parts - a set of spark plugs, a ratchet, a spark plug socket (make sure you have the right size), extensions, a gapping tool, and dielectric grease. Other hand tools may be needed on some vehicles.

What size ratchet do you need for a spark plug?

The size of the spark plug ratchet depends on the vehicle, but the most common sizes are 5/8 inch (16mm) and 13/16 inch (21mm). Other frequently used sizes include 9/16 inch (14mm), 3/4 inch (19mm), and 18mm, which are often used in specific applications like small engines, lawnmowers, or certain Ford and European vehicles. 
This video demonstrates the different spark plug socket sizes and their uses: 59sDanny Johnson’s GarageYouTube · Aug 3, 2025
Common spark plug ratchet sizes

• 5/8 inch (16mm): The most common size for many newer vehicles, small engines, and motorcycles. 
• 13/16 inch (21mm): Another very common size, especially for older and large-engine vehicles. 
• 9/16 inch (14mm): Often used in Ford vehicles, as well as some newer European and Asian cars. 
• 3/4 inch (19mm): Frequently found in lawnmowers and small-engine vehicles. 
• 18mm: Sometimes used in small engines and motorcycles. 

This video explains how to choose the right socket for different spark plugs, including those from NGK: 46sMotor MatchupsYouTube · Jan 19, 2025
Tips for choosing the right socket

• Check your vehicle's manual: This is the most reliable way to find the exact size you need. 
• Use a spark plug socket: These are specifically designed for the job. They are longer than standard sockets and have a rubber insert to grip the spark plug, preventing damage to the porcelain insulator and holding the plug securely for installation. 
• Consider getting a set: A set often includes a variety of common sizes and the necessary extensions to reach the spark plugs in hard-to-reach areas. 

What size socket do I need to pull a spark plug?

The most common socket sizes for spark plugs are 5/8 inch (16mm) and 13/16 inch (20mm), but other sizes like 9/16 inch (14mm) and 3/4 inch (18mm) are also used depending on the vehicle. The exact size depends on the make, model, and age of your vehicle. You may need to check your vehicle's manual or measure the spark plug to find the correct size. 
Common spark plug socket sizes

• 5/8 inch (16mm): The most common size, used in many modern and small-engine vehicles, as well as motorcycles. 
• 13/16 inch (20mm): Often used in older and larger-engine vehicles. 
• 9/16 inch (14mm): Frequently found in Ford vehicles, newer European and Asian cars, and some motorcycles. 
• 3/4 inch (18mm): Used in some motorcycles, small engines, and older GM vehicles. 
• 11/16 inch (18mm): Used in some motorcycles, small engines, and older BMW vehicles. 

You can watch this video to learn about different socket sizes and how to use them: 1mDanny Johnson’s GarageYouTube · Aug 3, 2025
Tips for choosing the right socket

• Consult your vehicle's manual: This is the most reliable way to find the correct size. 
• Measure the spark plug: You can measure the hexagonal part of the spark plug to find its size. 
• Look for a "spark plug socket": These are specifically designed for spark plugs and often have a rubber insert to protect the porcelain insulator and hold the plug in place. 

This video demonstrates how to use spark plug sockets: 51sbeachtrollYouTube · Sep 7, 2018

Do all spark plugs have the same socket size?

Spark plug sockets come in various sizes, ranging from 9/16 inches to 7/8 inches. The most common spark plug socket sizes are 5/8 inches and 13/16 inches.