Hexagon Head Bolts – Coarse Thread Series (DIN 931 / DIN 933)
In applications involving cast iron, aluminum housings, or field-assembly environments, fine threads often fail due to cross-threading or stripping (thread shear). Sunhy Coarse Thread Hex Bolts are engineered to solve this assembly bottleneck. With a larger thread pitch and deeper flank engagement, these bolts provide superior resistance to fatigue cycles and stripping forces compared to fine threads. Designed for structural steel, heavy machinery, and automotive chassis, they ensure reliable preload retention even when assembled with impact wrenches or in conditions where thread damage is a risk.
- Resists stripping in soft metals.
- Rapid assembly, tolerates minor damage.
- High fatigue strength (Grade 10.9).
- ISO 4014/4017 dimensional precision.
- Consistent torque-tension relationship (K-factor).
- Plating options: Zinc, HDG, Geomet.
Technical Specifications
Standards
DIN 931 (Partial Thread), DIN 933 (Full Thread), ISO 4014, ISO 4017, ANSI B18.2.1
Material Grades
Carbon Steel: Class 4.8, 8.8 (Med. Carbon), 10.9 (Alloy Steel SCM435), 12.9
Stainless: A2-70 (SS304), A4-80 (SS316)
Tolerance Class
Product Grade A (d≤M24 and l≤10d), Product Grade B (>M24)
Thread Type
Metric Coarse (M), UNC / BSW
Diameter Range
M6 – M64 (1/4″ – 2-1/2″)
Surface Finish
Black Oxide, Zinc Plated (Cr3+), Hot Dip Galvanized (HDG), Geomet 500A, Dacromet
Certifications
ISO 9001:2015, EN 10204 3.1 Mill Test Certificate
Why Specify Coarse Thread (UNC/Metric Coarse)?
Stripping Resistance in Soft Materials: When bolting into lower-strength materials like Cast Iron (engine blocks) or Aluminum (casings), Coarse Thread is mandatory. The larger thread pitch results in a greater volume of material between threads, significantly increasing the shear area. This prevents the internal threads of the expensive casting from stripping before the bolt reaches its yield point.
Mitigation of Galling (Cold Welding): For Stainless Steel (A2/A4) applications, galling is a frequent failure mode during installation. Coarse threads have a larger helix angle and more clearance than fine threads, reducing the friction heat generation per rotation. This makes them less susceptible to seizing during high-speed assembly.
Fatigue & Impact Durability: While fine threads have a slightly larger stress area, coarse threads offer better fatigue distribution in imperfectly aligned joints. In structural steel connections subject to slight settlement or vibration, the robust thread profile of a Grade 10.9 coarse bolt handles cyclic loading without crack initiation at the thread root.
Based on ISO 4017 / DIN 933 (Full Thread)
| Thread Size (d) | Pitch (P) | Head Width (s) | Head Height (k) | Proof Load (8.8 Grade) |
| M6 | 1.00 mm | 10.00 mm | 4.00 mm | 11,600 N |
| M8 | 1.25 mm | 13.00 mm | 5.30 mm | 21,200 N |
| M10 | 1.50 mm | 17.00 mm | 6.40 mm | 33,700 N |
| M12 | 1.75 mm | 19.00 mm | 7.50 mm | 48,900 N |
| M16 | 2.00 mm | 24.00 mm | 10.00 mm | 91,000 N |
| M20 | 2.50 mm | 30.00 mm | 12.50 mm | 147,000 N |
Torque Control & Lubrication:
Do not rely solely on dry torque charts. The Torque Coefficient (K-factor) varies drastically by finish. For example, a dry zinc-plated bolt has K\approx 0.20, whereas adding Molybdenum Disulfide paste drops K to 0.12. Applying dry torque values to lubricated bolts will cause yield failure. We recommend establishing torque procedures based on ISO 16047.
Hole Clearance (ISO 273):
Proper clearance is critical to prevent shear stress on the shank. For an M12 coarse thread bolt, use a 13.5mm clearance hole (Medium Series). Tight tolerances (13.0mm) should only be used where alignment is precision-machined to avoid bolt bending.
Washer Selection:
Always use hardened washers (HV 200 min) under the turned element (nut or bolt head) to prevent embedding into the clamped part. For Grade 10.9 bolts, utilize ISO 7089 (DIN 125A) washers with 300 HV hardness to maintain preload.
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FAQ
What is the difference between coarse and fine thread hex bolts?
Coarse thread bolts have a larger pitch (fewer threads per inch) compared to fine threads. Coarse threads are the industrial standard for general assembly because they are more durable, faster to install, and less prone to stripping in soft materials like aluminum or cast iron.
How do I identify the strength grade of a metric hex bolt?
The strength grade is embossed on the bolt head. Common markings are “8.8”, “10.9”, or “12.9”. The first number represents ultimate tensile strength (X100 MPa), and the second number indicates the yield strength ratio (e.g., 8.8 means 800 MPa tensile and 640 MPa yield).
Are stainless steel hex bolts as strong as Grade 8.8 steel bolts?
Generally, no, unless specifically treated. Standard A2-70 or A4-70 stainless bolts have a tensile strength of 700 MPa, which is lower than Grade 8.8 (800 MPa). However, A4-80 high-strength stainless bolts offer equivalent tensile strength to Grade 8.8 carbon steel.
What is the correct hole size for an M12 hex bolt?
According to ISO 273 (Medium Series), the standard clearance hole for an M12 bolt is 13.5 mm. Using a hole that is too small can cause alignment issues, while a hole that is too large reduces the bearing area under the bolt head.
Can I reuse high-strength hex bolts (Grade 10.9)?
It is generally not recommended to reuse Grade 10.9 bolts in critical structural connections. Once tightened to their yield point, the bolt undergoes plastic deformation. Reusing them can result in fatigue failure or an inability to hold the correct preload.