Verified by Rachel Mayfield, Supply Chain Analyst - July 2026
Photo by Daniel Smyth on Pexels
The 5/16-18 is a Unified National Coarse (UNC) bolt with a 0.3125-inch (7.94 mm) major diameter and 18 threads per inch. The tap drill for a standard 75% thread engagement is F (0.257"), the free-fit clearance drill is 21/64", and the hex head takes a 1/2" wrench. The fine-thread version is 5/16-24 UNF at 24 TPI.
5/16-18 specifications table
| Property | Value | Reference |
|---|
| Nominal major diameter | 0.3125" (7.94 mm) | ASME B1.1 |
| Threads per inch - UNC (coarse) | 18 TPI | ASME B1.1 |
| Threads per inch - UNF (fine, 5/16-24) | 24 TPI | ASME B1.1 |
| Tap drill - UNC (75% engagement) | F (0.257") | Standard tap drill chart |
| Tap drill - UNF (5/16-24) | I (0.272") | Standard tap drill chart |
| Clearance hole (free fit) | 21/64" | Standard clearance chart |
| Hex head / wrench size (A/F) | 1/2" | ASME B18.2.1 |
| Tensile stress area | 0.0524 in² | ASME B1.1 |
| Typical torque, Grade 5, dry | 17 ft-lb | Published torque charts |
| Typical torque, Grade 8, dry | 25 ft-lb | Published torque charts |
Torque figures are typical mid-range values for clean, dry threads. Lubricated assemblies need roughly 25-30 percent less torque for the same preload; critical joints should always be calculated for the actual fastener, finish and lubrication, not taken from a generic chart.
5/16-18 tap drill size
Drill F (0.257") for a 5/16-18 UNC tapped hole at 75% thread engagement - the standard for general work in steel and aluminium. In tough materials (stainless, tool steel) many machinists drop to 65-70% engagement with the next drill up to reduce tap breakage; in soft or thin material, higher engagement adds strength.
UNC or UNF - which thread?
UNC (18 TPI on this diameter) is the default for general fastening: faster assembly, more tolerant of nicks and dirt, less prone to cross-threading. UNF (24 TPI) gives finer adjustment, marginally higher tensile area and better vibration resistance, which is why it appears in automotive and instrument work. If you are matching an existing tapped hole, count the threads per inch against a rule before ordering.
Grade markings
SAE grade is marked on the bolt head: no radial lines means Grade 2 (low carbon, general purpose), three lines means Grade 5 (medium carbon, quenched and tempered - the workhorse), six lines means Grade 8 (alloy steel, highest standard strength). Metric equivalents run roughly Grade 5 to class 8.8 and Grade 8 to class 10.9, though the systems are not interchangeable in a controlled joint.
Metric near-equivalent
The closest metric size to 5/16-18 is M8. Near is not equal: threads will not interchange, and a metric bolt in an imperial hole (or vice versa) will strip under load. See MLC's metric bolt specification cards for the metric equivalents.