M8 vs 5/16-18 UNC Screw Size Comparison
Overview
Reference data for M8 metric and 5/16-18 UNC imperial bolts. These two sizes are incredibly similar in diameter, often leading to dangerous confusion in structural assemblies.
Comparison Table
| Standard | Size | Diameter (mm) | Pitch / TPI | Tap Drill |
|---|---|---|---|---|
| Metric | M8 | 8.00 mm | 1.25 mm | 6.8 mm |
| Imperial | 5/16”-18 UNC | 7.94 mm | 18 TPI | 6.6 mm (F) |
FAQ
Are M8 and 5/16-18 the same?
No. While they are nearly identical in diameter (8.00 mm vs 7.94 mm), the thread pitches differ (1.25 mm vs 1.41 mm equivalent). Forcing a 5/16” bolt into an M8 hole will strip the threads almost immediately.
Source Standard
ISO 261 / ASME B1.1