If you've spent any time sourcing fasteners from China, you've seen both DIN 933 and ISO 4017 listed on the same spec sheets. Are they interchangeable? Mostly — but not always.

The short answer

DIN 933 and ISO 4017 are both fully-threaded metric hex bolts. They share the same basic geometry, the same property classes (4.8, 5.8, 8.8, 10.9, 12.9), and the same surface finish options. For the vast majority of applications, they are functionally interchangeable.

The key difference is the across-flats (wrenching) dimension on a few specific sizes. The most commonly cited example is M10:

  • DIN 933 M10: across-flats = 17 mm
  • ISO 4017 M10: across-flats = 16 mm

This 1 mm difference matters when you're standardizing on a wrench size or when an existing assembly was built with one and you're sourcing replacements.

When the difference matters

For new construction or new equipment design, just specify whichever standard your local market uses (Europe tends toward DIN; international and ISO-aligned markets toward ISO 4017) and stick with it consistently.

For replacement parts, look at what's already on the machine. If the existing bolts have 17 mm flats on M10, source DIN 933. If 16 mm, source ISO 4017. The threads are identical — but you don't want a service technician showing up with the wrong wrench.

What about US standards?

The closest US equivalent is the hex cap screw to ASME B18.2.1. It uses inch threads (UNC or UNF) instead of metric and has its own across-flats schedule. For metric projects in the US, both DIN 933 and ISO 4017 are widely available.

Sourcing notes for buyers in China

Most factories in Yongnian, Handan produce both DIN 933 and ISO 4017 from the same production lines, switching the heading dies for the affected sizes. If your order is mixed (some DIN 933, some ISO 4017), no factory will refuse — but make sure the spec is unambiguous on each line item of your purchase order. We've seen problems trace back to a buyer specifying "M10 hex bolt" without naming the standard, and getting whichever the factory had on hand.

If you need help sourcing either standard, or any other fastener, get in touch — we work directly with Yongnian factories on both DIN and ISO lines.