Inch has one strength level, alloy steel.
Metric has two strength levels, property classes 12.9 and 8.8.
- 8.8 is about 30% weaker.
- 8.8 is also 20 – 50% less expensive.
Caution: Don’t substitute 8.8 for 12.9.
For metric M5 and larger, the property class and manufacturer’s I.D. is marked on the head.