Hex Head Bolt dimensions

Metric

(ISO 4017 / DIN 933)

Imperial

ASME B18.2.1 - Finished Hex bolts.

Design Parameters

Hex head bolts (ISO 4017 / DIN 933) are the most common industrial fastener, requiring an external wrench or socket for installation. Length is measured from under the head to the tip.

Engineering Note: Hex bolts are often available in different property classes (e.g., 8.8, 10.9). Always ensure the wrench or socket is fully seated before applying torque to prevent "rounding" the corners of the hex head.

Technical Guidance for Hex Head Fasteners

Hexagon head bolts and screws are the workhorses of industrial machinery. Unlike socket-driven fasteners, hex heads allow for the application of massive amounts of torque via external wrenches or sockets. However, their ubiquity often leads to oversight regarding standards, material grades, and assembly physics.

The ISO vs. DIN Discrepancy (Wrench Sizes)

A frequent point of confusion in the workshop is the "Width Across Flats" (AF) for certain metric sizes. While many assume that DIN 933 and ISO 4017 are identical, there are critical differences in wrench sizes for M10, M12, and M14 fasteners:

• M10: DIN uses a 17mm wrench; ISO uses 16mm.
• M12: DIN uses a 19mm wrench; ISO uses 18mm.
• M14: DIN uses a 22mm wrench; ISO uses 21mm.

When designing for high-volume production or international maintenance, it is safer to provide clearance for the larger DIN wrench sizes to ensure tool compatibility regardless of the specific hardware sourced.

Understanding Property Classes and Grades

The numbers or marks on a hex head are not serial numbers; they are structural ratings.

Metric Property Classes (e.g., 8.8, 10.9, 12.9): The first number represents 1/100th of the nominal tensile strength in MPa (e.g., "10" in 10.9 means 1000 MPa). The second number represents the ratio of yield strength to tensile strength. A 10.9 bolt has a yield strength that is 90% of its tensile strength.

• 8.8: "Standard" high-strength steel.
• 10.9 / 12.9: Used in automotive and heavy machinery where high clamping force is required. These are sensitive to hydrogen embrittlement if plated incorrectly.

Imperial Grades (ASME/SAE): Instead of numbers, imperial bolts use radial lines on the head.

• Grade 2: No lines. Low carbon steel.
• Grade 5: Three radial lines. Equivalent to Metric 8.8.
• Grade 8: Six radial lines. Equivalent to Metric 10.9.

Bolt vs. Screw: The Thread Length Nuance

Technically, a "Hex Head Bolt" (ISO 4014) has a partially threaded shank, while a "Hex Head Screw" (ISO 4017) is threaded all the way to the head.

In structural engineering, the unthreaded portion (the grip) should ideally cross the shear plane of the joint. Threads are significantly weaker in shear than the solid shank. If your assembly is subject to lateral loads, specify a partially threaded bolt so that the interface between the two parts rests on the smooth shank of the fastener.

Clamping Force and Preload

A bolt is essentially a very stiff spring. When you tighten a hex head, you are stretching the bolt to create preload. This preload creates friction between the clamped parts, which is what actually holds the assembly together.

One of the biggest mistakes in assembly is assuming that "tight is tight." If a bolt is under-torqued, it won't stretch enough to maintain tension under vibration, leading to fatigue failure. If over-torqued, the bolt enters the "plastic" region where it permanently deforms and loses its clamping ability. For critical joints, always refer to a torque chart based on the fastener's diameter and property class.

Best Practices for Installation

1. Washer Usage: Always use a flat washer under the head when fastening into softer materials like aluminum. This prevents the hex corners from digging into the surface and "plowing" the material, which causes a loss of preload.
2. Thread Lubrication: Be aware that "Dry" torque and "Lubricated" torque values differ by as much as 20-30%. If you use anti-seize or oil, you must reduce your target torque to avoid snapping the bolt.
3. Tool Engagement: Because hex heads are external, they are prone to "rounding" if the wrench is at an angle. In high-torque applications, use 6-point sockets rather than 12-point sockets to maximize surface contact on the flats.
4. Galvanic Corrosion: If using stainless steel hex bolts in aluminum, use a barrier like Tef-Gel or Zinc Duster. Without it, the two metals will effectively "weld" together over time due to electrolyte exposure.

When to Use Hex vs. Socket Heads

Use hex heads when you need to use a standard open-ended wrench (where vertical space is limited) or when the fastener will be exposed to heavy mud, paint, or debris. A hex head is much easier to clean and turn with a wrench than a socket head, which can become clogged and impossible to engage with an Allen key.

Common Standards Reference