What Is a Partial‑Thread Hex Bolt?
A partial‑thread hex bolt is a headed fastener with a six‑sided head and an unthreaded shank (also called the grip) between the head and the threaded portion. The grip length is defined to position smooth material within the joint, so that bearing and shear loads are carried by the unthreaded shank rather than by threads.
The most common international designations are DIN 931 and ISO 4014 (metric), with inch‑series equivalents covered by ASME B18.2.1. Partial‑thread geometry is preferred in structural joints, precision alignments, and any assembly where threads in the shear plane would reduce bearing area or cause fretting.
Key Features & Benefits
- Improved shear performance: The smooth shank in the shear plane provides more bearing area, lowering contact stresses compared with threads in the same plane.
- Better hole alignment: The unthreaded section acts as a dowel, helping align components and reducing the risk of thread damage during insertion.
- Reduced galling and wear: Contact occurs on the shank rather than the threads, minimizing surface damage in cyclic or vibrating joints.
- Consistent preload: With hardened washers and correct procedures, partial‑thread bolts achieve predictable clamp load while protecting joint surfaces.
- Design flexibility: Grip length can be selected to place threads entirely outside of the shear plane, meeting structural codes and best practices.
Standards & Types
Partial‑thread hex bolts are defined by multiple standards. The most cited are:
- DIN 931 / ISO 4014 (Metric) — Hex head bolts with a defined unthreaded shank. Property classes include 4.6, 8.8, 10.9, 12.9 (carbon/alloy) and A2‑70, A4‑70/80 (stainless).
- ASME B18.2.1 (Inch) — Hex cap screws and bolts covering inch sizes (UNC/UNF). Partial‑thread versions provide specified thread length relative to bolt length.
- Heavy Hex Structural Bolts — ASTM A325 / A490 (now F3125 for consolidated spec) are commonly used in structural steel; typically partial‑thread to keep smooth shank in shear planes.
Thread forms: metric coarse (e.g., M20 × 2.5), UNC (e.g., 3/4″‑10) for structural applications, and UNF where finer adjustment or thin materials require it.
Materials & Strength Classes
Carbon & Alloy Steel (ISO Property Classes)
- 4.6 / 5.6 / 5.8 — General purpose for light duty; commonly zinc plated.
- 8.8 — Workhorse grade for machinery and construction; balanced strength and ductility.
- 10.9 — High strength for compact designs; needs careful torque control.
- 12.9 — Very high strength for critical machinery; note limitations at elevated temperatures.
Inch Grades (SAE/ASTM)
- SAE J429 Grade 5 / 8 — Widely used for machinery; partial‑thread variants available by length.
- ASTM F3125 (A325/A490) — Heavy hex structural bolts for steel construction; heat‑treated alloy/carbon steels.
Stainless Steel (ISO 3506)
- A2‑70 (304) — General corrosion resistance for outdoor/industrial use.
- A4‑70 / A4‑80 (316) — Better chloride resistance for marine/chemical environments.
Other Alloys
- Duplex / Super Duplex — High strength with superior chloride SCC resistance; popular for offshore and desalination.
- Nickel Alloys — Elevated temperature and aggressive media; specialized projects.
Coatings & Surface Treatments
- Electro‑Zinc (Blue/Clear/Yellow) — Economical, bright finish; moderate corrosion resistance.
- Hot‑Dip Galvanized (HDG) — Thick zinc for long‑term outdoor exposure; use compatible oversize nuts to accommodate coating thickness.
- Mechanical Galvanizing — Uniform zinc without hydrogen embrittlement risk; common in structural kits.
- Phosphate & Oil — Improves torque repeatability; limited corrosion resistance by itself.
- Black Oxide — Dark finish for indoor machinery; add oil/wax for protection.
- PTFE / Fluoropolymer — Low friction and chemical resistance; reduces K‑factor variability in corrosive environments.
- Passivation (Stainless) — Enhances corrosion resistance by removing free iron from the surface.
Dimensional & Material Standards
- ISO 4014 / DIN 931 — Hex head bolts with partial thread (metric).
- ISO 4017 / DIN 933 — Full‑thread counterpart, for comparison.
- ISO 898‑1 — Mechanical properties of carbon and alloy steel fasteners (property classes).
- ISO 3506‑1 — Mechanical properties of stainless steel fasteners.
- ASME B18.2.1 — Inch hex bolts and cap screws, thread length rules.
- ASTM F3125 (A325/A490) — Structural bolting specification (heavy hex) frequently used in steel construction.
For construction projects, reference local building codes and specify documentation (EN 10204 3.1), hardness/tensile tests, and coating thickness checks as required.
Manufacturing Process
- Material Selection: Certified wire rod or bar based on target property class or stainless grade; review heat chemistry and cleanliness for fatigue‑critical applications.
- Cold Heading: Form the hex head; control fillet radius and head height per standard for proper wrench engagement and stress distribution.
- Shank & Threading: Turn or roll to define shank diameter; roll‑form threads to improve fatigue resistance and dimensional accuracy.
- Heat Treatment: Quench and temper (8.8/10.9/12.9) to achieve specified mechanical properties; avoid decarburization.
- Surface Treatment: Apply coating (zinc, HDG, phosphate, PTFE) or passivation for stainless; verify hydrogen embrittlement risk for high‑strength bolts with electroplating.
- Inspection: Dimensional checks (AF, head height, thread gauge), hardness, tensile/proof load, wedge tensile where required, and coating thickness.
- Marking & Packaging: Head markings (class/grade/manufacturer); lot traceability on labels with MTRs included for QA.
Common Applications
- Structural Steel Connections: Beams, columns, and bracings where smooth shank should occupy the shear plane.
- Bridges & Infrastructure: Expansion joints, bearings, and connection plates under cyclic loads.
- Heavy Machinery & Equipment: Bearing housings and frames where alignment and wear control are critical.
- Rail & Marine: Connections facing vibration and dynamic loading; stainless variants for corrosion exposure.
- Energy & Utilities: Turbine casings, transformers, and outdoor structures; HDG or stainless for durability.
- Automotive & Off‑Highway: Suspension brackets and chassis points designed with specific shear/bearing requirements.
How to Select the Right Partial‑Thread Hex Bolt (Checklist)
- Joint Design: Identify whether a shear plane crosses the fastener. Keep threads out of the shear plane by choosing appropriate grip length.
- Strength Class / Grade: 8.8/10.9/12.9 for carbon/alloy; A2‑70 or A4‑80 for stainless. Balance strength with ductility and temperature limits.
- Environment: Indoor, outdoor, marine, chemical? Select coating (HDG/PTFE) or stainless (A4) to suit.
- Thread Form: Metric coarse vs fine, UNC vs UNF; match mating components and service needs.
- Dimensions: Diameter, total length, thread length b, head clearance, washer size; ensure 1–3 threads protrusion after tightening.
- Documentation: Request material certificates (3.1), coating specs, and QA inspection records for critical projects.
Frequently Asked Questions (FAQs)
Why choose partial‑thread over full‑thread?
To keep threads out of the shear plane, improve alignment, and reduce wear. It is the preferred approach for structural and cyclic shear joints.
What is DIN 931 / ISO 4014 exactly?
International standards for hex head bolts with partial thread in metric sizes, defining dimensions, tolerances, and thread lengths.
Are heavy hex structural bolts the same as DIN 931?
Not exactly. Structural bolts (e.g., ASTM F3125 A325/A490) have different head geometry, mechanical properties, and installation procedures. Both are often partial‑thread, but they follow different standards and use cases.
How many threads should protrude beyond the nut?
A common visual standard is 1–3 threads visible after tightening to verify full engagement.
What torque should I apply?
Depends on diameter, grade, coating, and lubricant. Use torque tables calibrated for your exact finish/lube or measure elongation for critical joints.
Can I use stainless partial‑thread bolts in structural applications?
Stainless offers corrosion resistance but different strength behavior and may not meet structural codes. Verify requirements before substitution.
Can I trim bolt length on site?
Possible for general use, but for certified projects cutting may affect compliance and thread quality; re‑chamfer and chase threads if trimming.
Do I need washers?
Hardened flat washers are recommended, especially under nuts on softer or coated surfaces, to reduce embedment and improve torque repeatability.
