Deep Socket Sizes: Metric & Standard Deep Well Socket Chart
Look up deep socket lengths by size, and find the right deep socket for spark plugs, suspension, and recessed fasteners.
What Are Deep Sockets?
Deep sockets are longer than standard sockets, built to reach recessed fasteners and to fit over long protruding bolt shafts without bottoming out. The internal hex profile is identical to a standard socket of the same size. Only the body length changes, typically 60 to 80mm for metric deep sockets compared to 25 to 35mm for standard length, and the extra reach is the entire point of owning a deep socket set alongside a standard one.
The size conversion rules are exactly the same as for any other socket: a 14mm deep socket and a 14mm standard socket fit the same fastener, and the metric-to-SAE equivalents on the socket conversion chart apply equally to both lengths.
Deep Socket vs Standard Length
Deep sockets run roughly two to three times the body length of a standard socket at the same hex size.
| Size | Standard Length | Deep Length |
|---|---|---|
| 8mm | 24mm | 63mm |
| 10mm | 26mm | 63mm |
| 12mm | 28mm | 65mm |
| 13mm | 28mm | 65mm |
| 14mm | 28mm | 65mm |
| 15mm | 30mm | 70mm |
| 17mm | 32mm | 75mm |
| 19mm | 35mm | 80mm |
| 22mm | 38mm | 85mm |
| 24mm | 40mm | 90mm |
| 27mm | 45mm | 95mm |
| 30mm | 50mm | 100mm |
Applications Requiring Deep Sockets
- Spark plugs (5/8" or 13/16" deep socket with a rubber insert)
- Suspension nuts on threaded shafts
- Long bolts on bracket assemblies
- Nuts on all-thread rod
- Recessed fasteners in valve covers and timing covers
Spark Plug Sockets in Detail
A spark plug socket is a specialized deep socket with a rubber sleeve molded inside the hex opening. That rubber grips the plug's porcelain insulator, holding the plug steady during installation so it doesn't fall into the cylinder head, and cushioning the insulator against cracking from over-tight contact with the socket wall. Most gasoline engines use a 5/8 inch (or 14mm metric-labeled equivalent) spark plug socket; some older or larger engines use 13/16 inch (16mm). Always check the specific engine's service data, since using the wrong size can crack the porcelain or fail to seat on the plug's hex at all.
Metric Deep Socket Sizes
Common deep socket sets cover 6mm through 22mm in 3/8 inch drive and 10mm through 36mm in 1/2 inch drive. Spark plug deep sockets come in 14mm, 16mm, 5/8 inch, and 13/16 inch, each fitted with a rubber retainer.
SAE Deep Socket Sizes
SAE deep socket sets typically run from 1/4 inch to 1 inch in 3/8 inch drive and 3/8 inch to 1-1/2 inch in 1/2 inch drive. The two spark plug sizes, 5/8 inch and 13/16 inch, cover the overwhelming majority of gasoline engines sold in North America.
Deep Socket or Extension: Which One Do You Need
Use a standard socket when the fastener sits proud of the surface, since it engages faster and twists less under load than a deep socket at the same torque. Use a deep socket when the fastener itself is recessed, when a long bolt protrudes through a nut, or when the hex needs to clear a raised boss or casting around it. Reach for an extension bar instead when the fastener is at the surface but the ratchet handle needs to clear an obstruction like a fender, a bracket, or an intake manifold. Combining a deep socket with an extension is normal for deeply recessed work, though the added length increases flex, so keep the combination as short as the job allows. For impact wrench work specifically, see impact socket sizes for the material differences that matter under hammering torque.
Deep Socket FAQ
What size deep socket do I need for a spark plug?
Most spark plugs use a 5/8 inch or 13/16 inch deep socket with a rubber insert that grips and protects the porcelain insulator. Metric equivalents are 14mm and 16mm deep sockets with the same rubber retainer.
Is a deep socket the same size as a standard socket?
Yes, the hex opening size is identical between a deep and a standard socket of the same nominal size. Only the body length changes, typically 60 to 100mm for metric deep sockets compared to 24 to 50mm for standard length.
Should I use a deep socket or an extension?
Use a deep socket when the fastener itself is recessed below the surface. Use a standard socket with an extension bar when the fastener sits at the surface but the ratchet needs more reach to clear an obstruction. A deep socket adds length at the fastener end; an extension adds length between the ratchet and the socket.
Do deep sockets work with impact wrenches?
Yes, as long as the deep socket is impact-rated, meaning it's made from the same softer, shock-absorbing steel as a standard impact socket. A deep socket made for hand-ratchet use only should not be used on an impact wrench for the same reason a chrome standard socket shouldn't.
Why are deep sockets more likely to twist or flex?
The extra length between the drive end and the hex opening acts as a lever arm, so the same torque produces more twist in a deep socket than in a shorter standard socket. This is rarely a problem for hand tools but is worth knowing on stubborn, high-torque fasteners, where a standard socket and extension may hold up better.