DIN 933 and DIN 931 are two of the most ordered hex bolt standards in the world - and the difference between them comes down to a single feature: how far the thread runs along the shank. Understanding when to use each standard helps buyers avoid a common and costly mismatch on assembly lines and construction sites.
DIN 933 covers hexagon head bolts with a metric thread running the full length of the shank. It has been technically replaced by ISO 4017, but the DIN 933 designation remains universally understood by manufacturers, distributors, and importers, so both standards are quoted interchangeably on purchase orders.
DIN 931 covers hexagon head bolts with a partial (half) thread: a smooth, unthreaded grip section sits under the head, and the thread occupies only the end portion. Its ISO successor is ISO 4014, again fully interchangeable in dimension.
When should you choose the full-thread DIN 933? Because every millimeter of the shank is threaded, the bolt can be adjusted to almost any grip length and the nut can be run close up under the head. This makes DIN 933 bolts the default choice for tapped holes, threaded flanges, thin-sheet assemblies, brackets, clamps, jigs, and any joint where a standard nut is used rather than a bearing shank.
When does the partial-thread DIN 931 win? The smooth shank has two advantages. First, its full cross-section is stronger than a threaded section, so shear and fatigue loads are carried better. Second, when the unthreaded portion fills the clearance hole, the joint is more accurate and less prone to loosening under sideways loads. DIN 931 bolts are therefore standard for structural steel connections, machinery, agricultural equipment, and high-load pivots.
Designers also use partial-thread bolts to control grip: the nut tightens onto the threaded end while the plain shank fills the hole, preventing over-clamping of thin components. For this reason, equipment drawings frequently state the required thread length as well as the overall length.
What thread length should you expect on a DIN 931 bolt? The standard formula is approximately twice the nominal diameter plus 6 mm for common sizes, with a defined minimum grip - for example an M16 x 80 bolt carries roughly 38 mm of thread. Manufacturers can also produce fully threaded versions of long bolts where the standard would otherwise leave a large plain section; these are usually marked "full thread to DIN 931" on the order.
Both standards share the same head dimensions across flats and corners, the same metric coarse pitch series (fine pitch versions are designated DIN 961 and DIN 960), and the same property classes 4.8, 8.8, 10.9, and 12.9. Switching between full and partial thread therefore never changes the wrench size, only the load behavior.
Sourcing tips: if you stock or distribute fasteners, DIN 933 zinc-plated class 8.8 in sizes M6 to M20 is the fastest-moving SKU range for maintenance and OEM customers. For construction and equipment exporters, DIN 931 hot-dip galvanized class 8.8 in M12 to M24 anchors a second core range. Confirming both thread type and standard on the packing list prevents receivers from rejecting visually "different" bolts.
Matrix Fasten manufactures DIN 933 / ISO 4017 and DIN 931 / ISO 4014 hex bolts from M5 to M48, in all standard property classes and finishes, with third-party inspection and customer-specific marking available. Send your RFQ with standard, size, length, grade, and finish, and we will return pricing within 24 hours.