Metric vs. Inch Fasteners
They share the same shelf and some of them look almost identical — which is exactly how a metric bolt ends up cross-threaded into an inch nut. Here’s how to keep them straight.
Metric and inch are two separate systems, and they don’t mix. A few sizes are so close that one will start threading into the other — then bind and strip both.
The dangerous twin: M8 and 5/16″
An M8 is 0.315″ across; a 5/16″ is 0.3125″ — just two-and-a-half thousandths apart. They thread together a turn or two, feel “close enough,” then jam and chew up the threads. Here’s why:
First six threads, enlarged. Each M8 thread is about 0.006″ shorter than a 5/16″ thread — the red gap grows every turn.
If a bolt starts but suddenly fights you, stop and check the system — don’t force it.
The lookalike sizes
These sizes sit close in diameter. 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.
How to tell them apart
1. Read the head markings — the quickest tell
Inch (SAE) — lines
Metric — numbers
Metric bolts carry a property class number on the head — like 8.810.912.9. Inch (SAE) bolts use radial lines — three for Grade 5, six for Grade 8. See the grades chart.
2. Check how the thread is called out
Metric is diameter × pitch in millimeters. Inch is diameter–TPI. Different language, different system.
The simple rule
When in doubt, bring both pieces in and we’ll gauge them in seconds.
Bring the part, skip the guesswork
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.