Here's a buyer's guide to getting the right bolt for your job.
There are many types of bolt on the market, but choosing the right one for the job is so important. Bolts are a type of mechanical fastening, manufactured with external threads and when used with nuts, they make a strong fastening.
Bolts are made to be inserted into pre-drilled holes, which makes them different from screws. Most bolt fastenings also require a nut to combine the objects together in a tight and strong fix, although not all bolts require nuts.
Fasteners like bolts can provide a strong and stable fastening and come in a range of materials suitable for all types of connections and weather uses.
Advantages of Nut & Bolt Fastenings
The advantage of using nuts, washers and bolts as a fastening method means you can unscrew them anytime, making them easier to replace. Unlike riveting fasteners, bolts and nuts are far more flexible. Riveting is a more permanent method of fixing, whereas bolting items together with a nut and bolt still makes a good, solid fixing, but is far more adaptable.
A wide variety of surfaces can be connected together, including; wood, steel, iron, plastics, concrete and sheet metals. Using a washer means you spread the tension over a much wider area and can also protect the surface from being damaged.
Materials & Grade Options For Metric Bolts & Nuts
Depending on your required use, bolts come in a variety of finishes so you have to chose the right bolt material for the intended job. All bolts, nuts, screws and washers are assigned a grade which sets out the minimum acceptable standard of performance and mechanical properties of the particular fastener.
Watch this video to understand more about bolts and fixings:
About Bolt Grades
Grade identifications for bolts appear on the head of the bolt and the larger the grade the higher the strength characteristics of that bolt. The most popular grade for metric bolts is ISO 898-1 which is for nuts, bolts and studs made from carbon steel and alloy steel. This grade, or property designation, has a two digit number separated by a dot stamped onto the head of the bolt. For example 8.8. The figures mean:
- 1/100th of the parts nominal tensile strength (Megapascals or MPa’s)
- 1/10th of the percentage of the parts nominal yield strength compared to the parts nominal tensile strength
What does this all mean? This property class bolt has a tensile strength of 800 MPa and a yield strength of 80% of that tensile strength - 640 MPa.
Bolt Materials
Steel is the most common material for fasteners as it has so many variations depending on your requirements. The surface treatment of steel means it can be galvanised, or plated with zinc or chrome.
With enhanced corrosion characteristics, stainless steel is an alloy of low carbon steel and chromium, making it highly resistant to corrosion even if scratched or damaged during installation. However, stainless steel is not as strong as hardened steel as it cannot be hardened during the heating process due to its alloy content.
Stainless steel fixings are also less magnetic than regular hardened steel fasteners.
What Does 18-8 Stainless Steel Mean?
18-8 stainless steel refers to the mix of steel and alloys. In this case, 18% chromium and 8% nickel, which happens to be the most popular content for stainless steel fasteners.
A4 stainless steel is often called marine grade stainless steel as it less corrosive and ideal for many outdoor projects especially in salt water areas. A4 stainless steel is often more expensive but over time they will repay you in dividends as they will not need replacing as often.
Bolt Grades & Their Meanings
The most common grades for steel fixings are 2, 5, 8 and Alloy Steel. These are the ones that are used most regularly when it comes to fixings and fasteners. Grades 2, 5 and 8 usually look more 'silver-ish' in or yellow in colour due to their plating, whereas alloy steel that has not been plated will look darker.
Types of Bolt Fixings
Once again, depending on your desired usage, bolts come in different varieties, each with different characteristics, making them a very adaptable and purposeful fixing. Here’s a short list of the many varieties of bolts available.
- Through Bolts
- Socket Head Cap Screws
- Hex Bolts
- Carriage/Coach Bolts
- Coach Screws
- Eye Bolt Fixings
- Shoulder Bolts
- U-Bolts
Here’s a description of some of the bolt fixings available and in which scenario they would be ideal to use as a fixing.
Through Bolts
Through bolts, also known as through-fix anchors, are heavy-duty mechanical anchors used for fixing directly into solid concrete or dense masonry. They're designed to expand and grip firmly once tightened, making them ideal for structural steel fixings, baseplates, brackets, and heavy-duty railings.
Installation is simple: drill a hole, push the through bolt through your fixture and into the substrate, and then tighten the nut. When tightened, the lower section expands to grip the concrete and from a secure hold.
At Fixabolt, we stock stainless steel through bolts for use in industrial or coastal or industrial where standard zinc-plated fixings would corrode over time.
Socket Head Cap Screws
Socket head cap screws come in a variety of head shapes, including domed, countersunk, cylindrical and hex head. They have a recessed head socket that allows tightening using an Allan key or hex head socket tool, the deeper the recess the less chance you have of stripping the hex opening and producing a stronger tightening.
Hex Bolts
Hex bolts come with, as the name suggest, a hexagon-shaped head, making it ideal for tightening with a wrench or spanner. They do not always come with a fully threaded shaft, it really depends on the manufacturer, so always make sure you know what you order. The threaded part of the hex bolt protrudes beyond the jointed pieces and allows tightening with a nut or washer and nut. This type of bolt helps prevent against water damage to the joint as water will travel up the thread and into the jointed pieces, it stops at the end of the thread.
Coach Bolts/Carriage Bolts
Coach bolts are sometimes classed as a self-locking bolt due to its under head shape and its flush-mount domed head. This square under head section can be compressed into wood or a cut-out section of the material being fixed, to provide an anchor section for the bolt head to fix into. This means the carriage bolt can be tightened from the underside using a nut with the same M value as the thread diameter. For example, a M10 coach bolts require an M10 nut.
Coach Screws DIN 571
DIN 571 coach screws are heavy-duty wood screws with a hexagonal head, commonly used in timber construction, landscaping, and fencing. Unlike bolts, coach screws are partially threaded and have a pointed tip, allowing them to be driven directly into timber without the need for a nut or through-hole.
In the UK, they're often chosen for fixing timber to timber or timber to masonry (when used with a wall plug). The hex head allows for easy installation with a spanner or socket, delivering a secure hold.
At Fixabolt, we supply stainless steel coach screws for maximum corrosion resistance in outdoor or damp environments where regular carbon steel screws would quickly degrade.
Threaded Rod
Threaded rod, also known as threaded bar, stud bar, allthread, or studding, is a versatile fixing used across construction, engineering, and fabrication. Unlike standard bolts, a threaded rod has a continuous thread running along its entire length, allowing you to cut it to the desired size and secure it with nuts and washers at both ends.
Stainless steel threaded rods are especially popular for outdoor or corrosive environments, as they offer excellent resistance to rust and long-term degradation, making them suitable for many outdoor applications.
Eye Bolt Fixings
The head of the eye bolt comes either fully connected as one continuous loop or partially closed with a slight gap in the loop. They can be bolted in place and used to thread cables for lifting, fixing garden gates to posts, or for running cables through and keeping them in place. Some structures also use eye bolts to attach wire cables to strengthen the fencing. Other options include having a base plate underneath the head loop to allow for more pressure to be spread evenly whilst tightening in place.
Set Screws (DIN 933 Full Thread Bolts)
DIN 933 set screws, also known in the UK as full thread bolts or hex sets, are a core fixing used across construction, metalwork, and machinery assembly. Unlike standard hex bolts which have an unthreaded shank, set screws are fully threaded from head to tip, offering maximum grip and adjustability through the entire length of the fastener.
Set screws are ideal when you're bolting directly into a threaded hole or when you need precise clamping over long thread engagement. The hexagonal head makes them easy to install with a spanner or socket, and the full threading ensures a secure fix even in thinner materials.
At Fixabolt, we stock DIN 933 stainless steel set screws in sizes from M6 to M16, with a wide range of thread lengths.
Shoulder Bolt Fixings
Shoulder bolts have a plain cylindrical shaft directly underneath the head to allow the unthreaded part of the bolt to be used as an axle containing a rotating part or bearing. The shoulder bolt is often referred to as a stripper bolt.
U-Bolt Fixings
As the name suggests, a U-bolt is a 'U' shaped bar with screw threads at either end for the attachment of washers and nuts. Not being fully threaded allows for the U bolt to clamp or support specific parts, such as pipes and cables. They can also be square shaped.