Know the size.
Get the fit right.

Modern ISO 4014 / 4017 M10 standard hex bolts use a 16 mm wrench. Older DIN 931 / 933 dimensions use 17 mm, and that hardware is still common. M12 changes from 19 mm DIN to 18 mm ISO; M14 from 22 to 21 mm. M22 goes the other way: 32 mm DIN versus 34 mm ISO. Identify the standard or measure the actual head.

Use the correct size whenever possible. Some nominal sizes are very close: 3/4 inch equals 19.05 mm, just 0.05 mm from 19 mm. But tolerance, wear and loading affect the fit. A smaller socket may not seat; a larger socket may round the corners. Never force a tool or use one that rocks on the fastener. The conversion labels describe numerical closeness, not a guarantee of interchangeability.

No. M10 describes a nominal 10 mm thread diameter, not a 10 mm head. A modern ISO M10 standard hex bolt takes a 16 mm wrench; an M10 full-height socket head cap screw takes an 8 mm hex key. The head style matters as much as the thread size.

It is the straight-line distance between two opposite flat faces of a hexagonal head or nut. Place the caliper jaws against the flats, not the corners. Clean off dirt first and avoid damaged edges. This distance corresponds to the nominal wrench or socket opening.

Both come in metric millimeters and SAE inches, but they engage different surfaces. An Allen key fits an internal hexagonal recess; a normal socket fits around the outside of a head. An M8 ISO hex bolt uses a 13 mm socket, while an M8 full-height socket head cap screw uses a 6 mm hex key. Button, flat and low-profile heads need their own charts. Use the exact key size.

Use calipers to measure across the head flats for an initial tool choice. To identify the bolt itself, measure the outside diameter of the threads and use a thread-pitch gauge to find metric pitch or inch threads per inch. Measure length from under the head to the tip for a standard hex or socket cap screw. Compare the head style and measurements with a verified reference; a wrench size alone cannot identify the thread.

Metric pitch is the distance between adjacent thread peaks in millimeters. TPI counts thread peaks per inch. Measure across several gaps or use a thread gauge; confirm before ordering a replacement.

No. A tap drill makes the pilot hole for cutting internal threads. A clearance hole is larger than the bolt thread so the bolt passes through freely. Check your tap type, material and fit requirement before drilling.