A 1 5/16-inch socket converts to 33.34 mm, making 33 mm the closest common metric size. They are not exact equivalents: the inch socket is about 0.34 mm larger, and a socket conversion chart advises against substituting it for a 33 mm socket. Use a dedicated 33 mm socket for a 33 mm fastener when available.
How do you convert 1 5/16 inches to millimetres?
First convert the mixed fraction: 1 5/16 inches is 1.3125 inches. Since one inch equals 25.4 mm, the calculation is 1.3125 × 25.4 = 33.3375 mm, conventionally rounded to 33.34 mm. Engineering ToolBox’s conversion chart lists 1 5/16 inches as 33.34 mm; the Tien-i conversion sheet provides the inch-to-millimetre conversion values.
Is a 1 5/16-inch socket interchangeable with a 33 mm socket?
No—not as a direct, recommended substitute. A 33 mm size is 0.3375 mm smaller than the nominal 1 5/16-inch size, so a 1 5/16-inch socket can fit loosely on a 33 mm hex fastener. ElectricalCharts’ socket chart identifies 33 mm as the nearest metric size but marks the substitution “Do not substitute.”
A loose fit may slip or increase the chance of rounding the fastener’s flats under torque. Engineer Fix describes that risk and notes that a 6-point socket is preferable to a 12-point one when a near-size mismatch is being considered. That is general guidance, not a guarantee for every socket, fastener, tolerance, or load; it does not make the sizes equivalent.
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What should you check before choosing a socket?
- Match the size: For a fastener specified as 33 mm, choose a 33 mm socket when available.
- Match the drive: Confirm the socket’s drive size fits your ratchet or other tool.
- Check depth and rating: Choose a socket with suitable depth and a hand-tool or impact rating for the application.
The cited conversion and fit guidance do not establish a safe torque or application-specific approval for using a 1 5/16-inch socket on a 33 mm fastener. If the correct size is unavailable, avoid treating the near match as safe for a high-torque job.
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