Threaded Hex Bolts Specifications: Grades, Thread Types, and Sizing Charts

What Is a Threaded Hex Bolt

A hex bolt is a fastener with a six-sided (hexagonal) head, designed to be tightened with a wrench or socket rather than a screwdriver. "Fully threaded" hex bolts (often called tap bolts) have threads running the entire length of the shank, while standard hex bolts leave a smooth, unthreaded section near the head, called the shank, before the threading begins.

The choice between fully threaded and partially threaded bolts depends on the application: fully threaded bolts work well in tapped holes or thin materials where maximum thread engagement matters, while partially threaded bolts are preferred in shear applications, since the smooth shank resists shearing forces better than a threaded section would.

Understanding Bolt Grades and Strength Markings

Bolt grade indicates the material and heat treatment used, which directly determines tensile strength. In the U.S., grade is identified by raised markings on the bolt head; internationally, the ISO metric system uses a different numbering scheme.

Thread Types: Coarse vs. Fine Threading

Beyond grade, hex bolts differ in thread pitch — the distance between threads — which is standardized under two main systems in the U.S. and one dominant system internationally.

• UNC (Unified National Coarse) — fewer threads per inch, faster to install, more resistant to stripping in softer materials; the default choice for most general applications.
• UNF (Unified National Fine) — more threads per inch, providing greater tensile strength and finer adjustment, commonly used in automotive and precision machinery.
• Metric (ISO) — designated by diameter and pitch together (e.g., M10 x 1.5), used globally outside the U.S. and increasingly in imported machinery and vehicles.

Thread Class Fit

Thread class (1A, 2A, or 3A for external threads) indicates how tightly the bolt fits its mating nut. Class 2A is the standard commercial fit used in most hardware, while Class 3A provides a tighter, more precise fit for critical or high-vibration assemblies.

How to Read a Hex Bolt Sizing Chart

Bolt size is described using a standardized format: diameter – threads per inch x length (imperial) or diameter x pitch x length in millimeters (metric). For example, "1/2-13 x 2" describes a 1/2-inch diameter bolt with 13 threads per inch and a 2-inch length, while "M10 x 1.5 x 40" describes a 10mm diameter bolt with 1.5mm thread pitch and 40mm length.

Material Options and Corrosion Resistance

Beyond grade, the base material and coating determine how a bolt performs in specific environments.

• Zinc-plated steel — the most common finish, offering basic corrosion resistance for indoor or dry outdoor use.
• Hot-dip galvanized steel — a thicker zinc coating suited to outdoor and high-moisture environments, typically lasting 20+ years before significant corrosion.
• Stainless steel (18-8 or 316) — used where corrosion resistance is critical, such as marine environments or food processing equipment; 316 grade offers superior resistance to saltwater and chlorides.
• Black oxide — a thin protective coating providing mild corrosion resistance with minimal dimensional change, common on precision fasteners.

Matching Bolt Length and Diameter to Your Application

1. Measure the total thickness of the materials being joined, then add the nut height and roughly 1.5 thread pitches of extra length so the nut can be fully tightened with visible thread protrusion.
2. For tapped holes rather than through-bolting, aim for thread engagement equal to at least the bolt's diameter in steel, or 1.5 to 2 times the diameter in softer materials like aluminum.
3. Choose diameter based on the load path and shear/tensile requirements of the joint — undersized bolts are the most common cause of joint loosening under vibration or cyclic load.
4. When replacing an existing bolt, always match both diameter and thread pitch exactly; a coarse-thread bolt will not seat properly in a fine-thread tapped hole even if the diameter matches.

Common Specification Mistakes to Avoid

• Mixing imperial and metric bolts or nuts, which can appear to thread together initially but strip after a few turns due to slightly mismatched pitch.
• Selecting a bolt grade based on diameter alone rather than the actual tensile or shear load the joint will experience.
• Using standard zinc-plated bolts in marine or high-moisture environments, leading to rust and joint failure well before the expected service life.
• Overtightening a lower-grade bolt to compensate for an undersized diameter, which can cause the bolt to yield or snap under torque rather than provide reliable clamping force.

Specifying the right threaded hex bolt means aligning three factors: a grade rated for the actual tensile or shear load the joint will bear, a thread type and class that matches the mating nut or tapped hole exactly, and a diameter/length combination sized to the material thickness with adequate thread engagement. Whether it's a Grade 5 bolt for general construction or a Grade 8 bolt for a high-stress structural connection, matching these specifications correctly is what determines whether a joint holds reliably for decades or fails prematurely under load.