DIN 933 Full Threaded Bolts: Selection for Tightening and Positioning

The DIN 933 standard was withdrawn in October 1991, and its technical content has been superseded by ISO 4017. Technical documentation from Böllhoff and PGB Europe confirms: "DIN standard withdrawn | Wrench widths for M10, M12, M14, and M22 changed | Bolts can mate with 6h tolerance internal threads."

In actual purchasing, DIN 933 is still widely used as a product name, but note that DIN 933 and ISO 4017 differ in width across flats for M10, M12, M14, and M22. DIN 933 specifies fully threaded hex head bolts, with threads extending all the way to the underside of the head, corresponding to the partially threaded DIN 931. The standard covers metric thread sizes, commonly from M3 to M30, with manufacturers such as Würth supplying up to M64.

Key Technical Features

Full Thread Design: The Core of Securing and Positioning

The essential difference between DIN 933 and DIN 931 is the full thread structure. The thread extends from the end of the shank all the way to the head bearing surface, which means:

Adjustable along the full length: The bolt can mate with a nut at any thread position, with clamping length not limited by an unthreaded shank. This is critical for positioning applications requiring precise adjustment.

Threads never "run out": In thin panel connections or where multiple washers are needed for adjustment, full thread ensures sufficient thread engagement length at all times.

Shear bearing trade-off: On a fully threaded bolt, the shear plane falls in the threaded section, and the thread root is a stress concentration point. Therefore, DIN 933 is better suited to tension-dominated connections and applications requiring positioning adjustment. Structural connections carrying transverse shear loads should preferentially use DIN 931 partially threaded bolts.

Property Grades and Mechanical Properties

Per the ISO 898-1 system, DIN 933 covers grades from general to ultra-high strength. The minimum tensile strength and 0.2% yield strength for each grade are as follows:

Grade 8.8 alloy steel material must contain at least one of the following elements at the minimum content: chromium 0.30%, nickel 0.30%, molybdenum 0.20%, vanadium 0.10%. Grade 10.9 is tempered at a lower temperature than Grade 8.8, resulting in different stress relaxation behavior at elevated temperatures.

For stainless steel grades, A2-70 (304) and A4-70 (316) have minimum tensile strength of 700 N/mm² and yield strength of 450 N/mm², suitable for corrosive environments.

Dimensional Parameters (Common Sizes)

Per Böllhoff and Fastenal dimensional data, key DIN 933 parameters are as follows:

Bolded sizes are those where width across flats differs from ISO 4017. Product grade is A (≤M24) and B (>M24), with Grade A having tighter tolerances.

Surface Finishes

Zinc flake coating (ZFSHL) is the recommended corrosion protection for high-strength grades (10.9 and above). Würth technical documentation shows the coating is Cr(VI)-free and adjusts the friction coefficient to the 0.09-0.14 μ range through integrated lubricating particles in the top coat, supporting stable preload control. Würth's supply range shows Grade 10.9 ZFSHL coating covers M6-M48, and Grade 12.9 covers M6-M30.

Hydrogen embrittlement boundary conditions: For electroplated steel parts with tensile strength ≥1000 MPa or hardness ≥320 HV, there is a risk of hydrogen-induced brittle fracture. Therefore, Grade 10.9 and above should avoid electroplated zinc, preferring zinc flake coating.

Application Areas

Equipment Manufacturing and Machinery Assembly

DIN 933 fully threaded bolts are widely used for component connections in equipment manufacturing, construction machinery, and agricultural machinery. The full thread design is particularly convenient in assemblies requiring clamping length adjustment or multiple washer adjustments. Würth's supply data shows Grade 8.8 carbon steel covers a wide range from M3 to M64, meeting assembly needs from precision instruments to heavy equipment.

Positioning and Adjustment Applications

The full thread structure makes DIN 933 suitable for applications requiring precise position adjustment. The bolt can be locked at any thread position, unrestricted by an unthreaded shank, making it suitable for fixtures, molds, guide rails, and other assemblies requiring fine adjustment.

Molds and Tooling

Grade 12.9 DIN 933 bolts (tensile strength 1220 N/mm², hardness HRC 39-44) are used for mold repair and tooling fixtures, meeting high-stress requirements in mold clamping and ejection mechanisms. Würth's supply range shows Grade 12.9 plain finish covers M4-M36, and ZFSHL coating covers M6-M30.

General Industrial and Construction

Grade 8.8 DIN 933 bolts are used in power transmission and distribution, construction, marine, and other industrial and civil fields. Hot-dip galvanized versions (marked 8.8U) suit outdoor and corrosive environments. Per the ISO 10684 thread fit system, HDG bolts marked U indicate the thread was reduced to 6az before coating and must mate with 6H/6G nuts; if the bolt has no U marking (6g/6h before coating), the nut must be oversize-tapped 6AZ/6AX after coating (marked Z or X). The two paths must not be mixed—mixing causes thread stripping or assembly interference.

Selection Guide

Summary

DIN 933 specifies fully threaded hex head bolts, covering common sizes M3-M30 (extendable to M64), with property grades from 5.6 to 12.9 plus stainless steel A2/A4-70. The core value of its full thread design lies in adjustable clamping length and threads that never run out, making it particularly suited to positioning, adjustment, and assemblies requiring multiple washer adjustments. DIN 933 was withdrawn in 1991 and superseded by ISO 4017, with M10/M12/M14/M22 differing in width across flats. Grade 8.8 carbon steel is the general choice; Grade 12.9 alloy steel is used for molds and main components. Zinc flake coating (ZFSHL) provides 480-720 hour salt spray protection and an adjustable 0.09-0.14 μ friction coefficient, offering a Cr(VI)-free, hydrogen-embrittlement-free corrosion solution for Grade 10.9 high-strength bolts. These products are widely used in equipment manufacturing, machinery assembly, molds and tooling, and general industrial fields, making them the foundational choice for fully threaded connectionsin the metric fastener system.