DIN 933 vs. ISO 4017 and DIN 931 vs. ISO 4014

[Eurolink] DIN 933 vs. ISO 4017 and DIN 931 vs. ISO 4014 Four Sizes You Need to Know

Let’s begin with a moment of silence for DIN 933 and DIN 931.

Just kidding. Although DIN 933 and DIN 931 were withdrawn in favor of their ISO successors decades ago, try telling that to the metric fastener industry. These two DIN hex bolt standards are still widely requested, manufactured, and used. They have stayed relevant and continued to cause plenty of confusion.

Fortunately, the most important differences are easier to remember than they may seem. You only need to remember four sizes: M10, M12, M14, and M22. These are the nominal sizes where the DIN and ISO dimensions differ most noticeably.

Understanding the Four Hex Bolt Standards

Before comparing the dimensions, let’s clarify what each standard covers.

DIN 933 and ISO 4017

DIN 933 and ISO 4017 both specify fully threaded metric hex head cap screws with a coarse thread pitch. ISO 4017 is considered the successor to DIN 933.

DIN 931 and ISO 4014

DIN 931 and ISO 4014 both specify partially threaded metric hex head cap screws with a coarse thread pitch. ISO 4014 is considered the successor to DIN 931.

The easiest way to remember the pairings is:

DIN 933 → ISO 4017: Fully threaded
DIN 931 → ISO 4014: Partially threaded

What Is the Difference Between DIN 933 and ISO 4017?

For most nominal sizes, DIN 933 and ISO 4017 are dimensionally very similar. The primary differences involve the available size range and the width across the flats of the bolt head.

Available Nominal Sizes

DIN 933 technically covers nominal sizes from M1.6 through M52, while ISO 4017 extends the standard range through M64. This means ISO 4017 includes some larger nominal sizes that were not included in the original DIN 933 standard.

Width Across the Flats

Width across the flats, commonly abbreviated as WAF, is the distance between two opposite flat sides of the hex head. This measurement determines the wrench or socket size needed to install or remove the fastener.

The WAF dimensions differ at four nominal sizes:

Nominal Size	DIN 933 WAF	ISO 4017 WAF	Difference
M10	17 mm	16 mm	ISO is 1 mm smaller
M12	19 mm	18 mm	ISO is 1 mm smaller
M14	22 mm	21 mm	ISO is 1 mm smaller
M22	32 mm	34 mm	ISO is 2 mm larger

For M10, M12, and M14 fasteners, the ISO 4017 hex head is slightly smaller than the DIN 933 hex head. At M22, the relationship reverses: ISO 4017 specifies a larger width across the flats than DIN 933.

For example:

An M10 DIN 933 fastener has a 17 mm WAF.
An M10 ISO 4017 fastener has a 16 mm WAF.
An M22 DIN 933 fastener has a 32 mm WAF.
An M22 ISO 4017 fastener has a 34 mm WAF.

What Is the Difference Between DIN 931 and ISO 4014?

The comparison between DIN 931 and ISO 4014 follows the same dimensional pattern as DIN 933 and ISO 4017, except these standards apply to partially threaded metric hex head cap screws.

The same four nominal sizes have different widths across the flats: M10, M12, M14, and M22. ISO 4014 has a smaller WAF at M10, M12, and M14, while the relationship reverses at M22, where ISO 4014 has the larger WAF.

In other words, whether you need a fully threaded or partially threaded metric HHCS, the same four sizes are the ones to remember when comparing these DIN and ISO standards.

Are DIN 933 and ISO 4017 Interchangeable?

DIN 933 and ISO 4017 fasteners are often treated as interchangeable because their thread dimensions and most of their overall dimensions are the same or very similar. However, they should not automatically be considered identical.

The different WAF dimensions at M10, M12, M14, and M22 can affect:

Wrench and socket selection
Clearance around the bolt head
Automated assembly equipment
Existing tooling
Replacement parts
Applications with tightly controlled dimensional requirements

A one- or two-millimeter difference may not matter in every application, but it can become important when space is limited or when the fastener must match an existing drawing or specification. Always review the application requirements before substituting one standard for another.

Are DIN 931 and ISO 4014 Interchangeable?

DIN 931 and ISO 4014 are also commonly treated as equivalents, but the same warning applies. In addition to confirming the WAF dimensions, buyers and engineers should verify:

Nominal diameter
Fastener length
Thread length
Property class
Material
Finish or coating
Applicable tolerances
Customer or drawing requirements

When a purchase order or engineering drawing specifically calls for DIN 931 or ISO 4014, it is best to supply that requested standard unless an approved substitution has been confirmed.

Why Are DIN 933 and DIN 931 Still Used?

Although ISO 4017 and ISO 4014 replaced the older DIN standards, DIN 933 and DIN 931 remain common in the metric fastener industry.

There are several reasons:

Older equipment and drawings still reference DIN standards.
Customers have established specifications based on DIN dimensions.
Manufacturers continue to produce DIN-pattern fasteners.
Distributors and buyers are familiar with the DIN designations.
Replacement applications may require compatibility with an existing fastener.

This is why both DIN and ISO designations continue to appear in catalogs, purchase orders, technical drawings, and customer inquiries.

You may also see fasteners listed with both standards, such as DIN 933 / ISO 4017 or DIN 931 / ISO 4014. While these standards are often treated as equivalents, that does not necessarily mean the fastener meets every dimensional requirement of both standards. This distinction is especially important at M10, M12, M14, and M22, where the width across the flats differs between the DIN and ISO standards. If the specific standard matters for your application, don't rely on a dual DIN/ISO designation alone—confirm the actual dimensions of the fastener before ordering.

The Main Difference to Remember

You do not need to memorize an entire dimensional chart to understand the primary differences between these standards.

Just remember:

10, 12, 14, and 22.

At M10, M12, and M14, the ISO hex head is narrower across the flats. At M22, the ISO hex head is wider. Everything else begins with knowing whether you need a fully or partially threaded fastener:

DIN 933 and ISO 4017: Fully threaded
DIN 931 and ISO 4014: Partially threaded

Need Help Sourcing DIN or ISO Metric Fasteners?

Metric fastener standards can get confusing quickly, especially when an older DIN specification and its ISO successor appear almost identical.

Need the ISO version when everyone else is offering DIN? Or does your drawing specifically require the older DIN standard? Eurolink can source both. That's exactly the kind of problem we were built to solve.

We help North American industrial distributors source hard-to-find metric fasteners from our network of European stocking distributors. Whether the print calls for DIN 933, DIN 931, ISO 4017, ISO 4014, or something much harder to find, send us the requirement.

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