What Are the Essential Differences Between Full Thread and Partial Threaded Hex Bolts?

In the world of industrial fasteners, the Threaded Hex Bolt is a cornerstone of mechanical assembly. However, one of the most common points of confusion for procurement managers and engineers is deciding between a Full Thread and a Partial Threaded design.

While they may look similar at a glance, the choice between them can significantly affect the structural integrity, load distribution, and longevity of your project.

1. Defining the Core Anatomy

To understand the performance differences, we must first define the structural components of a Threaded Hex Bolt.

1.1 Full Threaded Hex Bolts (Fully Threaded)

A fully threaded bolt has threads that extend from the very tip of the bolt all the way to the underside of the hex head.

• Primary Use: These are designed for applications where the grip length is short or where the bolt is being used to fasten into a tapped hole (a hole with pre-existing internal threads).
• Load Characteristic: The load is distributed along the entire length of the bolt.

1.2 Partial Threaded Hex Bolts (Partially Threaded)

A partially threaded hex bolt features a smooth, unthreaded portion immediately below the head, known as the Shank or Grip. The threaded portion only exists at the tip.

• Primary Use: These are standard for heavy-duty structural applications and machinery where the bolt passes through two or more unthreaded components.
• Load Characteristic: The unthreaded shank is designed to bear the “shear” forces of the assembly.

2. Technical Comparison: Performance and Mechanics

When selecting a Threaded Hex Bolt, engineers must look at the mechanical forces at play: Tension, Shear, and Grip.

Technical Specification Matrix

3. Why Shear Strength Matters

The most critical difference lies in how the bolt handles Shear Stress (forces pushing the joined materials in opposite directions).

3.1 The Vulnerability of Threads

In a Threaded Hex Bolt that is fully threaded, the “root” of the thread (the narrowest part of the bolt) is where the shear force is applied. Because the diameter is smaller at the thread root, the bolt is technically weaker at every point along its length.

3.2 The Advantage of the Solid Shank

A partially threaded bolt ensures that the solid, unthreaded shank sits exactly where the two materials meet (the shear plane). Since the shank has a larger cross-sectional area and no “notches” from threads, it provides maximum resistance against snapping or bending under heavy lateral loads.

4. Grip Length and Alignment Precision

In precision machinery and structural engineering, the Threaded Hex Bolt must do more than just hold things together; it must keep them perfectly aligned.

4.1 Reducing “Play” in the Assembly

Because threads are slightly smaller than the hole they pass through, a fully threaded bolt can allow for microscopic shifting over time. In contrast, the unthreaded shank of a partially threaded bolt can be manufactured to a very tight tolerance, filling the hole more completely and reducing “slop” or vibration-induced movement.

4.2 Optimizing Clamping Force

Full-threaded bolts are excellent for Tension applications where you need to pull two items together tightly using the maximum amount of thread engagement. They are less likely to loosen under constant vertical tension compared to a partial thread if the nut is not properly torqued.

5. Selection Guide: Which One Do You Need?

5.1 Choose Full Threaded Hex Bolts if:

• You are fastening into a Tapped Hole (e.g., a bolt going into an engine block).
• Your assembly is prone to Vibration and you need maximum thread-to-material contact.
• You are working with thin materials where a shank would prevent the nut from tightening (avoiding “bottoming out”).

5.2 Choose Partial Threaded Hex Bolts if:

• The application involves Heavy Shear Loads (e.g., vehicle suspension, bridges, or rotating machinery).
• You need Perfect Alignment between two heavy plates.
• You are using a Nut and Washer on the opposite side, and the combined thickness of the materials exceeds the length of the shank.

6. Engineering the Perfect Connection

While both types of Threaded Hex Bolts are essential, they are not interchangeable. For high-stress industrial and structural projects, the partially threaded bolt is often the superior choice due to its shear-resistant shank. For general-purpose assembly and tapped-hole applications, the fully threaded bolt offers the versatility and tensioning power needed for a secure fit.

7. FAQ: Frequently Asked Questions

Q1: Can I use a fully threaded bolt if the specs call for a partial thread?
A: Caution is advised. If the application involves high shear force, replacing a partial thread with a full thread significantly lowers the safety margin, as the threads act as “stress concentrators” and may lead to bolt failure.

Q2: How do I measure the “Grip Length” of a partial thread?
A: The grip length is the distance from the underside of the hex head to the start of the threads. Ideally, the grip length should be slightly shorter than the thickness of the materials you are joining to ensure the nut can be tightened securely.

Q3: Are stainless steel threaded hex bolts available in both styles?
A: Yes. However, be aware of Galling (cold welding) with stainless steel. Using an anti-seize lubricant is highly recommended for both full and partial threaded stainless fasteners.

8. References

1. ASME B18.2.1: Square, Hex, Heavy Hex, and Askew Head Bolts and Hex, Heavy Hex, Hex Flange, Lobed Head, and Lag Screws.
2. ASTM A325: Standard Specification for Structural Bolts, Steel, Heat Treated, 120/105 ksi Minimum Tensile Strength.
3. Fastener Quality Act (FQA) Guidelines on Strength and Testing.
4. Machinery’s Handbook: Threaded Fastener Mechanics and Shear Planes.