In short: no. A 7/8-inch (78) socket is not a reliable substitute for a 22 mm hex fastener; use a 22 mm metric socket for a proper fit. The two sizes are close but not interchangeable, and using the wrong size can slip, round the corners, or strip the head.
Understanding metric and imperial sizing
Sockets come in metric sizes (millimeters) and imperial sizes (inches). A 22 mm hex head is measured across the flats and is roughly 0.866 inches. A 7/8-inch socket has an inner measure of 0.875 inches. Although the numbers are close, the difference—about 0.009 inches (roughly 0.23 mm)—is enough to prevent a precise grip in many cases. Relying on close but not exact sizing increases the risk of slippage or damage to the fastener.
Size math at a glance
Before choosing a socket, it helps to know these quick conversions and what they imply for fit.
- 22 mm is approximately 0.866 inches across the flats.
- 7/8 inch is 0.875 inches across the flats.
- The difference is about 0.009 inches (0.23 mm).
- In practice, a 7/8" socket is typically too large to grip a true 22 mm hex head securely.
Conclusion: The closest sizes are not interchangeable. An exact 22 mm socket is the safe choice for a 22 mm hex head, and a 7/8" socket should not be used as a substitute.
Practical guidance for ensuring the correct fit
Here are steps to ensure you’re using the right tool and to avoid damage or injury.
- Always verify the fastener size before choosing a socket. If the head is labeled 22 mm, use a 22 mm socket.
- When in doubt, measure with calipers or a precise ruler to confirm the across-flats dimension of the hex head.
- If you only have imperial sockets, do not force a 7/8" socket onto a 22 mm head; instead, obtain the corresponding metric size (22 mm) for a proper engagement.
- A socket adapter (drive size adapter) does not convert the hex size; it only changes the drive interface (e.g., from a square drive) and does not fix a mismatch in the hex-to-bolt size.
In practice, the safest approach is to use the exact size specified for the fastener. For a 22 mm hex head, that means a 22 mm metric socket. For larger or smaller heads, use the corresponding metric size rather than attempting a close-but-not-exact substitute.
Summary
For a 22 mm hex fastener, do not rely on a 7/8-inch (78) socket. The two sizes are not identical, and using the slightly oversized imperial socket can lead to poor engagement, rounding, or slippage. Always use the matching 22 mm metric socket, and verify measurements if there is any doubt about the fastener’s size.
Bottom line: precision matters in fastener work. When in doubt, measure, confirm the size, and use the correctly sized tool to ensure safety and torque accuracy.