Metric and inch fasteners are two separate systems, and they don’t mix. The trouble is that a few sizes are so close in diameter that one will start threading into the other — then bind and strip both. Knowing the lookalikes (and how to read a head) saves the part.
An M8 bolt is 0.315″ across; a 5/16″ is 0.3125″ — just two-and-a-half thousandths apart. They’ll thread together a turn or two, feel “close enough,” then jam and chew up the threads. If a bolt starts but suddenly fights you, stop and check the system — don’t force it.
These inch and metric sizes sit close enough in diameter to be confused. Same diameter doesn’t mean same thread — the pitches differ, so they won’t actually mate.
| Inch size | Dia. | Metric near-match | Dia. | Apart by |
|---|---|---|---|---|
| #10 | 0.190″ | M5 | 0.197″ | 0.007″ |
| 1/4″ | 0.250″ | M6 | 0.236″ | 0.014″ |
| 5/16″ | 0.3125″ | M8 | 0.315″ | 0.0025″ — watch out |
| 3/8″ | 0.375″ | M10 | 0.394″ | 0.019″ |
| 1/2″ | 0.500″ | M12 | 0.472″ | 0.028″ |
Diameters are the nominal major diameter (metric sizes converted from millimeters). Close diameters, different threads — never interchangeable.
This is the quickest tell. Metric bolts carry a property class number stamped on the head — like 8.810.912.9. Inch (SAE) bolts use radial lines instead — three for Grade 5, six for Grade 8. See the grades chart.
Metric is written diameter × pitch in millimeters — M8×1.25 means the threads are 1.25 mm apart. Inch is diameter–TPI — 5/16–18 means 18 threads per inch. Different language, different system.
If a nut or bolt doesn’t spin on freely by hand, stop — you may have crossed the two systems. A clean measurement helps: a diameter that lands on a tidy millimeter (6, 8, 10 mm) is probably metric; one that lands on a clean fraction is probably inch. When in doubt, bring both pieces in and we’ll gauge them in seconds.
Not sure if you’ve got metric or inch? Bring the bolt, the nut, or the part it came out of. We’ll gauge it and match it exactly — no cross-threaded surprises.
