Bolt grades serve the important purpose of communicating the mechanical properties of bolts. In this way, properties of fasteners such as strength, safety, reliability, consistency, and cost-effectiveness can be matched to required applications to achieve an optimal result. Like all things with nuts and bolts, things can be a little complex! In this article, we review bolt grade systems - what they are, how they work and how they differ from each other. Let's check it out!

Bolt Grades Overview

Bolts come in many different shapes and sizes. They're made of a variety of materials for a range of purposes. To understand their capabilities and determine the applications they are suitable for, they are assigned grades, which are like ratings that explain their properties.

The grades they are assigned come from standards (or systems). These standards are not identical - they are developed by different nations and industries to evaluate and present information about fasteners. Examples of different standards include:

ISO (International Organization for Standardization)
ASTM (American Society for Testing and Materials)
AS (Australian Standards)
DIN (Deutsches Institut für Normung)
BS (British Standards)

When you're talking about bolt grades, it's important also understand which standard it relates to, in order to properly understand what it means.

What do Grades Indicate?

Bolt standards help engineers and builders make informed decisions about which fasteners to use in different situations. Different standards provide different sets of information about fasteners, but generally they are concerned with strength, tensile strength, yield strength, materials, application suitability, performance under load, durability compatibility, and safety.

Which Grade is Best?

With fasteners, it's important to understand that suitability is more important than cost. In other words, you don't need the most expensive fastener - you only need the fastener which is mechanically suitable for the application. Since grades describe the properties of fasteners, the best grade is relative to the application.

Which Standard Should I Use?

Standards are not uniform in their conclusions. They are all essentially trying to resolve the same problem (the correct fastener to use, and how to use it) in ways that reflect the local (or industry-specific) practices and regulations, and heritage of practice. Accordingly, it's important to refer to the most current and relevant standards to ensure proper selection, installation, and performance of fasteners - it will depend on what you're working on and where you are working on it. If you're not sure, simply ask our team. We are here to help!

In Australia, fasteners commonly come in two systems, or standards: the metric system and the imperial system. If you're working with fasteners, It’s useful to at least be aware of the existence of these systems. In this brief article, bought to you by Bolt-In Co Sunshine Coast, we review the metric and imperial systems and come to terms with some of their conventions, similarities & differences. Let’s go!

Why is There More than One System? 

There are multiple measurement systems used in different countries around the world. The metric and imperial systems are not unique to fasteners. They are the two main systems used to measure the distance and weight of.. everything! Here are some common conventions used by these two systems:

Kilometers (Metric) Vs Miles (Imperial)
Kilograms (Metric) Vs Pounds (Imperial)
Centimetres (Metric) Vs Inches (Imperial) 

The reason these are the two main fastener systems in use has interesting historical considerations. The imperial system originated in Britain and made its way to America. Meanwhile, the metric system originated in France. The metric system became more popular because it is simple and easy to use, and easy to convert (multiply or divide). However, because the imperial system had already been adopted by the USA, one of the world’s greatest economies and manufacturers, it continues to stay in use. This is despite the rest of the world mainly using the metric system. 

Are there any Other Fastener Systems? 

Yes. There are multiple systems. In Australia, the metric and imperial systems are the most commonly encountered systems. Internationally, there are also other systems such as Unified Thread Standard (UTS), British Standard Whitworth (BSW) and British Standard Fine (BSF), Japanese Industrial Standard (JIS), and ISO Metric Screw Thread.

How Imperial and Metric Fasteners different?

Generally, metric and imperial fasteners come in slightly different sizes. While fasteners of both systems are designed to solve similar problems, they go about it in different ways. For example, they use slightly different diameters (width of the fastener) and thread pitches (number of threads on the fastener). Metric and imperial fasteners are not interchangeable. 

Systems within Systems

An interesting aspect of the difference between metric and imperial fasteners are the terminology they ascribe to their thread pitches. In the imperial system, they use Unified National Fine Thread (UNF) and Unified National Course Thread (UNC) to describe their fine and course thread pitches. Meanwhile, the metric system uses Metric Course Thread (M) and Metric Fine Thread (MF) to describe this differentiation in the metric system. 

Which Should I Use: Metric or Imperial? 

The correct fastener to use will depend on the context of the application. For example, if you are replacing a fastener, ideally it will be replaced with a fastener of the same system and dimensions. Then you can be certain that the new fastener will have the correct capability for the job. In other situations the significance of the fastener system may not be as relevant - for example, when the fastener being used is independent of any other components.  

Otherwise, you can refer to the technical documentation for the project to determine the correct type of fastener to use. If you’re not sure, you are welcome to enquire with our team. In addition to supplying a wide range of stainless steel and high tensile fasteners on the Sunshine Coast, we also offer great customer service and a wealth of knowledge and experience in working with fasteners. 

Mechanical fasteners, such as bolts and screws, have a ‘thread’ - the name for the ridges that wrap around the shaft of the fastener. In the case of a bolt and nut combination, the ridges on the bolt align with the grooves in the nut) which allow the bolt and nut to engage and create a mechanical bond. By engaging and tightening, the bolt and nut become a unit, fixed together. 

Threads come in two main types - course and fine threads. In this brief article, we take a look at these thread types and the implications they have on fastener capabilities and applications. For more information, or assistance with selecting appropriate fasteners for your application, please contact us here at Bolt-In Co Sunshine Coast - we are here to assist. 

A Quick Word on Metric and Imperial

There are two systems, or standards, commonly used for fasteners: metric and imperial. These systems use different names for the course and fine threads within their system:
Metric: Course Threads / Fine Threads
Imperial: UNC (Unified National Coarse) / (Unified National Fine)
For the purpose of this article, which is general in nature, we are just going to use the words ‘course’ and ‘fine’ to refer to the course and fine variants in each system.

TPI & Thread Pitches

Before we dive into the differences between course and fine threads, it is useful to understand thread pitches and TPI. ‘Thread Pitch’ is really what we are referring to when we talk about course and fine threads. It refers to the number of threads on a fastener. A fine thread pitch means more threads, a course thread pitch means less threads. TPI stands for ‘Threads per inch’, and is used to specific exactly how many threads exist per measured inch. This specification is useful to exactly describe different types of fasteners.

What’s Better: Course or Fine Thread Pitches?

Like many things in fasteners, ‘better’ depends entirely on the application. There are many variables with fasteners and the best result is really the least expensive fastener that is suitable for the engineering required. For example, many automotive fasteners need fine thread pitches to ensure a strong connection amidst the vibration and movement inside an engine or vehicle. Alternatively, a course fastener may create a better bond with timber, where the space created between the threads creates a better locking capability. In other words, neither is ‘better’ - both are necessary in different applications.

Advantages of Fine Threads

1. Vibration Resistance
Fine threads have greater resistance to loosening that can be caused by vibration, due to the greater number of threads making contact.

2. Precision
Fine threads can be adjusted much more precisely due to the greater number of threads and commensurate options.

3. Thread Strength
Fine threads distribute their load more evenly along the fastener. This makes them suitable for applications requiring high tensile strength.

4. Working in Limited Space
With more threads per inch, fine threads can be used when limited space is available. The greater number of threads per inch enables them to make a better connection when space is an issue.

5. Weight Reduction
When weight reduction is necessary (aerospace / automotive) fine threads enables smaller fasteners to be used without sacrificing strength.

Disadvantages of Fine Threads

1. Weaker Tensile Strength:
Fine threads have a slightly smaller thread cross-sectional area compared to their coarse counterparts of the same diameter. This means they may have slightly lower tensile strength and may not handle heavy loads as effectively. In situations where you're dealing with high stress or significant loads, it's often better to opt for coarse threads as they offer greater strength.

2. Increased Susceptibility to Stripping:
Fine threads can be a bit more delicate and prone to stripping or damage compared to coarse threads. The narrower spacing between threads means there's less material supporting the load, making them more susceptible to wear, thread deformation, or cross-threading. It's important to be mindful of this when assembling or disassembling and when applying high levels of torque.

3. Heightened Sensitivity to Contamination:
Fine threads are a bit more sensitive to dirt, debris, or lubricant residues. Even tiny particles or foreign substances can cause friction or galling, making it a bit more challenging to tighten or loosen the fastener smoothly. Keep an eye out for cleanliness when working with fine threads.

4. Increased Difficulty in Alignment:
Because fine threads have a finer pitch, aligning them during assembly can be a tad trickier, especially in tight spaces or when visibility is limited. It may require a bit more patience and precision to ensure proper alignment, as there's a slightly higher risk of cross-threading or misalignment.

5. Limited Availability and Compatibility:
While fine threads are still widely used, they may not be as commonly available or as popular as coarse threads. This can make it a bit more challenging to find a wide range of compatible fasteners, tools, or accessories, particularly in certain industries or regions where coarse threads are more prevalent. It's worth keeping in mind and planning ahead if you specifically require fine-threaded options.

Advantages of Course Threads

1. Quick Assembly and Disassembly
Coarse threads are ideal for applications that involve frequent assembly and disassembly, as the wider pitch allows for faster threading and unthreading of fasteners. This can be advantageous in industries such as construction, where time efficiency is crucial.

2. High Clamping Forces
Coarse threads are preferred in applications where high clamping forces are required. The wider spacing between threads allows for increased thread engagement and a larger contact area, resulting in better load distribution and enhanced clamping capabilities.

3. Durability and Strength
The wider pitch of coarse threads provides increased thread depth and higher tensile strength, making them suitable for applications that demand robust and durable fastening solutions. Coarse threads can handle higher loads and exhibit better resistance to stripping or shearing.

4. Compatibility and Availability
Coarse threads are more commonly available and widely used than fine threads. Their popularity in construction, automotive, and general-purpose applications ensures easy access to a wide range of compatible fasteners, tools, and accessories.

Disadvantages of Course Threads

1. Reduced Precision
Coarse threads have a wider thread pitch, which means they provide less precision compared to fine threads. In applications requiring fine adjustments or precise positioning, coarse threads may not be the ideal choice as they do not offer the same level of accuracy.

2. Limited Resistance to Vibrations
While coarse threads generally offer better resistance to loosening due to vibrations compared to non-threaded connections, they are not as effective as fine threads in this regard. If your application is subjected to significant vibrations, such as in high-speed machinery or automotive components, fine threads might provide better stability and resistance against self-loosening.

3. Limited Load Distribution
Coarse threads have larger gaps between threads, resulting in a smaller contact area with the mating component. This can lead to uneven load distribution and potentially increase the stress concentration in localized areas. In applications that require a more evenly distributed load, fine threads may be preferred.

4. Limited Application in Thin Materials
In thin materials, such as sheet metal or thin plastic components, coarse threads may not be suitable. The wider pitch can cause the material to deform or crack more easily due to the larger forces applied during threading. Fine threads are often a better choice for such materials, as they distribute the load more evenly and reduce the risk of damage.

Summary

Fasteners come in different thread pitches which have implications for their fastening capability, efficiency, materials they can work with, cost and precision. Choosing the best fastener for the job isn't always a clear-cut matter. If you'd like some assistance with your fastener requirements, contact our team - we are here to assist.

What are high tensile fasteners? They are a specific type of fastener designed to withstand high levels of stress and tension. ‘Tensile strength’ refers to the ability of a material to resist forces without breaking or permanently deforming, and accordingly, high-tensile nuts and bolts represent a particularly strong type of fastener. This doesn’t always mean they’re the best option. Like all situations, the correct solution will depend on the engineering requirements of your project to assess and determine the correct type of fasteners for the job. On this page, we look further into High Tensile options to explore this important type of fasteners. For fasteners on the Sunshine Coast, please contact our team for a quality range of high tensile products in different grades and thread pitches. 

How are High Tensile Fasteners Different from other Fasteners?

Generally, fasteners are differentiated by the materials they are made with, the process by which they are made, the fastener dimensions, the measurement system (metric or imperial) and the thread pitch (course or fine). With that in mind, the difference between high tensile fasteners and other types of fasteners include:

Materials:

High tensile fasteners are made from carbon or alloy steel. Other types of fasteners are made from different metals; for example, stainless steel fasteners are made from a variety of stainless steel alloys, and titanium bolts from titanium. 

Process:

High tensile fasteners are heat treated to achieve their tensile strength. This process achieves strength by reducing internal stresses in the material, reducing brittleness, hardening and toughening. While other types of fasteners may also be heat treated, it is a mandatory aspect of high tensile bolts with a view to achieving enhanced strength. 

Dimensions

Bolts come in a wide range of shapes and sizes. Some of the characteristics which may be seen in high tensile bolts that differentiate them from lower strength bolts include a thicker shank & larger head diameter, although these larger aspects are not necessarily indicative of a high tensile bolt. 

Thread Pitch

While high tensile bolts are available in both course and fine thread pitches, fine thread pitche offers greater strength, higher resistance to vibration, better precision, and versatility (a fine thread pitch is more suitable for smaller diameter bolts). Fine thread pitches are commonly used in automotive bolts for these reasons. 

System

Bolts are available in metric and imperial systems, and high tensile fasteners are no exception. The system is important because when matching a bolt, the system must also be matched to ensure an accurate fit. Bolt-In Co stocks high tensile bolts in both metric and imperial systems, in course & fine thread pitches (metric) and UNF / UNC threads (imperial). 

When to Use High Tensile Fasteners

High tensile fasteners are used when superior strength and durability are necessary. Examples of high tensile fastener applications include:

Structural Applications

Infrastructure construction projects like bridges & buildings may utilise high tensile fasteners. 

Automotive & Aerospace

With high levels of stress and vibration, automotive and aeronautical engineering relies on high tensile fasteners. As mentioned above, a higher thread pitch is often utilised for a stronger result. 

Heavy Machinery & Equipment

The machinery and equipment utilised in industries such as mining, agriculture and construction utilise high tensile bolts, where high levels of stress and vibration are also a factor. 

Marine Applications

High Tensile fasteners may also be used in offshore and marine applications where high levels of stress, tension and vibration require a stronger fastener. 

When not to Use High Tensile Fasteners

Using high tensile fasteners should only be done when necessary - otherwise, they may be a more expensive solution for a project that doesn’t require them. Examples of when high tensile may not be the best solution include:

Low-Stress Applications

There are many less expensive fasteners which are suitable for low-stress applications. 

Corrosive Environments

Balancing the need for strength and corrosion resistance will depend on the specific application. Stainless steel fasteners offer a higher level of corrosion resistance than high tensile fasteners. 

High Temperature Environments

High tensile fasteners may lose their strength and durability at high temperatures. In these cases, titanium or nickel alloys may be more appropriate solutions. 

Grading the Strength of High Tensile Bolts

The SAE (Society of Automotive Engineers) grading system is a common system use to grade high tensile bolts. This system uses a numeric grade to indicate the tensile strength. The higher the grade, the stronger the fastener. It’s not the only system used. Other systems, such as ASTM (American Society for Testing and Materials) is another example. 

High Tensile Fasteners at Bolt-In Co

Enquire with Bolt-In Co Sunshine Coast for the fasteners for your project. We have a wide range of high tensile in stock, and can easily order products in if we don’t stock them (depending on quantity).