Spark Plug Socket Sizes Explained (14, 16 and 21 mm)
Most car spark plugs take a 16 mm (5/8 inch) socket. Many newer engines, including a lot of BMW and MINI turbo engines, use a smaller 14 mm plug hex, while older engines and many lawn mowers and other small engines use 21 mm (13/16 inch). To be sure, look up your plug's part number or measure the hex on the old plug before you buy a socket.
What you need
- A spark plug socket in your engine's size, for example a 16 mm magnetic spark plug socket with swivel for the most common plugs
- A 3/8-inch drive ratchet and an extension long enough to reach down the plug well
- Your owner's manual, or the part number printed on the plug or its box
- A caliper or a ruler with millimeter marks, if you need to measure an old plug
Socket size is not the same as thread size
This is where most confusion starts. A spark plug has two different sizes:
- Thread diameter: the width of the threaded part that screws into the engine. Common sizes are 10, 12, 14 and 18 mm, and most car plugs are 12 or 14 mm.
- Hex size: the distance across the flats of the six-sided (or twelve-sided) nut on the plug body. This is what decides your socket.
The two don't line up neatly. Plenty of plugs with a 14 mm thread have a 16 mm hex, and other plugs with the very same 14 mm thread have a 20.8 mm hex. So when a forum post or a parts listing says "a 14 mm plug", check whether it means the thread or the hex before you buy anything.
The three common spark plug socket sizes
| Socket | Inch equivalent | Typically found on |
|---|---|---|
| 14 mm | Close to 9/16 in | Many newer engines with slim plugs, including many BMW and MINI turbo engines and some small turbocharged Ford EcoBoost engines. Often a 12-point (bi-hex) plug in a deep, narrow well. |
| 16 mm | 5/8 in | The most common size on modern car engines, and on many motorcycles |
| 21 mm | 13/16 in (plug hex is usually listed as 20.8 mm) | Older cars, many classic and large engines, and many lawn mowers and other small engines |
You'll also come across 18 mm and 3/4 inch on some motorcycles and small engines. Engine families also change over the years, so two cars with the same badge can use different plugs. Always check by engine, not just by model name.
16 mm (5/8 inch): the most common
If you own a typical modern four-cylinder or V6 car, there's a good chance its plugs take a 16 mm socket. It's the most likely size, but still worth checking before you buy.
14 mm: newer, slimmer plugs
Smaller plugs take up less room in a crowded modern cylinder head, and a smaller plug means a smaller hex. These plugs often sit at the bottom of long, narrow tubes, so you need a thin-wall 14 mm socket. The factory plugs in many BMW engines have a 12-point (bi-hex) head, which needs a 12-point socket: a standard 6-point socket won't fit a 12-point hex, though a 12-point socket fits both.
21 mm (13/16 inch): older engines and small engines
The big 20.8 mm hex is common on older car engines and on many lawn mower and other small engines. A 21 mm socket and a 13/16-inch socket both fit these plugs.
5/8 vs 16 mm: are they the same?
For spark plugs, yes in practice. 5/8 inch is 15.875 mm, and 16 mm is 16.0 mm, a difference of about 0.1 mm, roughly the thickness of a sheet of paper. Plug makers list the same hex both ways (NGK, for example, gives its 5/8-inch hex as 15.88 mm), and a 16 mm spark plug socket fits plugs sold as 5/8 inch.
14 mm vs 9/16 inch
These are close but not identical: 9/16 inch is about 14.29 mm, so a 9/16 socket is slightly looser on a 14 mm hex. Some charts list 9/16 for the same plugs. A proper 14 mm socket is the snugger, safer fit, and on a 12-point plug you need the 12-point version.
How to find your spark plug socket size
- Check the plug part number. Your owner's manual, a parts catalog or the old plug itself gives the part number. Look it up on the plug maker's website: the specs list the hex size.
- Measure the old plug. With a caliper, measure across two opposite flats of the hex. About 14 mm, about 15.9 mm or about 20.8 mm tells you which socket you need.
- Test-fit on a loose plug. The socket should slide over the hex with almost no rocking. Any noticeable play means it's the wrong size.
- Look down the well. If the plug sits at the bottom of a narrow tube, make sure your socket's outside diameter will fit too. That's what thin-wall sockets are for.
What makes a good spark plug socket
- Thin wall. Modern plug wells are deep and narrow. A thin wall lets the socket reach plugs that a thick standard socket can't get down to.
- A way to hold the plug. Many sockets use a rubber insert, which can stay stuck on the plug at the bottom of the well. A magnet holds the plug without that problem, so it doesn't drop as you lift it out or start it in the hole.
- A swivel. Handy for plugs hidden behind pipes, brackets or covers. Keep the socket straight when starting the threads, though, so the plug doesn't go in at an angle.
- The right drive size. Many spark plug sockets are 3/8-inch drive, which suits both a ratchet and a torque wrench.
The JUSMIR 16 mm spark plug socket combines all four: a 1 mm wall, a magnet, a 360-degree swivel and a 3/8-inch square drive, in chrome vanadium steel. It's a 16 mm socket only, so if your engine uses 14 or 21 mm plugs, you need a different size.
Mistakes to avoid
- Buying by thread size. A "14 mm plug" usually means the thread, and many of those take a 16 mm socket.
- Using an ordinary deep socket. It may not fit the well, can't hold the plug, and if it tilts it can crack the plug's ceramic insulator.
- Accepting a sloppy fit. A socket that rocks on the hex can round off the corners, which makes removing the plug much harder.
- Using an impact wrench. Spark plugs should be removed and fitted with hand tools, finishing with a torque wrench.
FAQ
What size socket is a spark plug socket?
The most common is 16 mm (5/8 inch). Other common sizes are 14 mm, used on many newer engines, and 21 mm (13/16 inch), used on older engines and many small engines. Check your plug's hex before buying.
Is a 5/8 spark plug socket the same as 16 mm?
In practice, yes. The difference is about 0.1 mm, and the two names are used for the same plug hex.
What is 14 mm in inches for a spark plug socket?
The nearest inch size is 9/16 inch (about 14.29 mm), which is slightly loose on a 14 mm hex. A metric 14 mm socket, in 12-point if your plugs have a 12-point head, is the better fit.
Can I use a regular socket on spark plugs?
Sometimes, if it's deep enough and fits the well, but it's a poor choice. A regular socket doesn't hold the plug, often won't fit down a narrow well, and makes it easier to crack the insulator.
What size spark plug socket for a lawn mower?
Many lawn mower plugs use a 13/16-inch (21 mm) socket, but some take 5/8 inch or 3/4 inch. Check the plug or the owner's manual.
Found your size? Our step-by-step guide on how to change spark plugs covers the rest, from cold engine to final torque.
Browse all sockets and wrenches.
