What socket do I need to change spark plugs?
The socket you need is the deep-well spark plug socket that matches the plug’s hex size. In most cars, that means either 16 mm (5/8 inch) or 21 mm (13/16 inch). Verify the exact size from your vehicle’s manual or the spark plug packaging before purchase.
How to identify the correct hex size
To choose the right socket, start by identifying the hex across the flats on the spark plug itself. The size is often stamped on the plug, listed on its packaging, or can be measured with calipers. Once you know the hex size, select a deep-well socket with a rubber insert to grip the plug securely and keep it from dropping into the engine bay.
Common hex sizes you’ll encounter
The vast majority of passenger-car plugs use either 16 mm (5/8") or 21 mm (13/16") across the flats. A small subset of engines uses other sizes, so verify before buying a socket.
Common spark plug socket sizes
Use the following sizes as a practical guideline. The exact size depends on your plug, so confirm it before buying or attempting to remove the plug.
- 16 mm (5/8 inch) hex: The most widespread size for many modern and older cars, especially Japanese and many compact models.
- 21 mm (13/16 inch) hex: Used on a number of American and some European engines with larger plug shells.
In practice, matching the hex size to the plug prevents rounding and makes removal safer. If you’re unsure, start with the two closest sizes and work carefully, or consult your service manual.
Choosing the right socket features
Before you shop for a spark plug socket, understand which features will help you reach plugs in tight spaces and avoid damage.
- Deep-well design with a rubber insert to grip the plug securely.
- Appropriate drive size; 3/8" drive is the standard for most home garages, 1/2" drive for some high-torque tasks.
- Flexible or swiveling head can help access hard-to-reach plugs.
- Magnetic vs non-magnetic options; rubber inserts reduce the risk of dropping a plug into the engine.
- Six-point grip design to minimize the chance of rounding the plug corners.
With these features, you’ll be better equipped to remove and replace plugs across a range of vehicles and engine layouts.
Additional toolkit tips
Consider adding a torque wrench to ensure proper reinstall torque, a tube of anti-seize compound where recommended by manufacturer, and a small magnet or plug-grabber to ease handling in tight spaces.
Summary
The key to selecting the right spark plug socket is matching the hex size of your plugs, most commonly 16 mm (5/8") or 21 mm (13/16"). Use a deep-well socket with a rubber insert to grip the plug securely, choose an appropriate drive size, and consider features like a flexible head and a magnet or insert to prevent drops. Confirm the exact size from your vehicle manual or plug packaging, and equip yourself with a few accessories for a smooth plug-change process.
