Coach Bolts DIN 603: Everything You Need to Know

Coach Bolts DIN 603: Everything You Need to Know

When it comes to construction and DIY projects, there are few things more exciting than choosing the right bolts. Exciting may not be the first word that comes to mind for most people, but for those who appreciate the beauty of a well-crafted fastener, coach bolts DIN 603 are a must-have in their toolbox!

These versatile fasteners have been around since the mid-19th century and have become a staple in many applications from wooden structures to automotive manufacturing.

Despite their humble origins as bolts used for connecting wheels to horse-pulled carriages, coach bolts DIN 603 have become one of the most popular choices for fastening metal to wood or connecting timber joists and decking. Their original use is why they are also known as cup square carriage bolts, which is their name that provokes less confusion when compared to DIN 571 coach screws.


horse and carriage bolts from 19th century


Their strength and durability make them ideal for heavy-duty applications, while their sleek design and domed mushroom head adds a touch of elegance to any project.

Whether you're a seasoned professional or just starting out with your first DIY project, this comprehensive guide will provide you with everything you need to know about coach bolts, including their dimensions, available materials and installation techniques.


What are Coach Bolts DIN 603?

Coach bolts DIN 603, also known as cup square carriage bolts, are a type of double-component fixing with an enlarged dome head and nut. Their double-component aspect provides greater holding strength than screws due to the larger surface area of the nut providing resistance to the materials being held together.

Coach bolts are often confused with coach screws, but they are very different. Their names are similar because they were both used in coaches in the 19th century, but aesthetically and functionally they are unique.

They are typically used to fasten metal to wood or connect pieces of timber and can withstand heavy loads such as old coaches, where they were first used. 

Coach bolts have a domed head for reaching tight spots along edges and avoiding pertrusion, making them suitable for applications where space is limited. They are tightned with a nut at the thread end for long-lasting projects, while coach screws may become loose over time when timber moves because they don't use a nut for added resistance.


furniture workshop with coach bolts


Coach bolts come in multiple material compositions, but the most popular are Bright Zinc Plated (BZP), A2 stainless steel, and marine-grade A4 stainless steel. 

BZP has a higher tensile strength than stainless steel, but are prone to rusting and have little flexibility. A2 stainless steel is perfect for damp and wet conditions due to its corrosion resistance, and A4 stainless steel is for use with saltwater, particularly on boats. Most outdoor projects don't require A4 stainless steel to prevent rusting.

Sizes and Materials of Coach Bolts DIN 603

When selecting the appropriate fastener for a specific application, it's important to consider the dimensions and materials of carriage bolts in accordance with their DIN-number specification (DIN 603) as this can impact the durability and reliability of the final product.

The DIN 603 standard specifies that these bolts have a mushroom-shaped head with a square neck below it, which allows them to be tightened without spinning in their hole. They also have a machine thread that can be fully threaded up to their head, particularly in shorter sizes. In longer sizes, they occasionally come with a shank for maximum strength.

Carriage bolts come in various sizes, primarily ranging from M6 to M12, with varying lengths depending on the application. There are also M16 coach bolts available on our website for larger projects.

When choosing carriage bolts, it's also important to consider the materials they are made of since this will affect their strength and corrosion resistance.

The most common materials used for manufacturing coach bolts are:

  1. Carbon steel: This material is strong but prone to rusting over time.
  2. Bright Zinc Plated (BZP) steel: Higher tensile strength and mild corrosion resistance.
  3. Stainless steel: A popular choice due to their high corrosion resistance and strength.
  4. Brass: Offers good corrosion resistance but are softer than steel so are used more for decoration than practicality.

By considering the appropriate size and material when selecting coach bolts you can ensure that your construction project will be secure and long-lasting.


Applications of Coach Bolts DIN 603

There are many applications for coach bolts. They are utilised in various industries and projects due to their strength and durability, making them a reliable choice for long-term use.

One of the most common applications is in woodworking and carpentry, where they are used to fasten pieces of timber together or connect wood to metal. 

Coach bolts can be found in construction projects because they provide greater holding strength than screws due to the larger surface area of the nut offering pressure resistance. They are often employed at critical structural joins and can always be tightened (with multiple nuts if required) for long-lasting projects.


timber construction with coach bolts


Advantages of Using Coach Bolts DIN 603

One advantage of using DIN 603 carriage bolts is their holding strength. As they are a double-component fixing with an enlarged dome head and nut, they can be easily tightened and adjusted over time, ensuring that the connection remains secure even after prolonged use.

In the same vein, they are able to be easily disconnected without leaving grooves in the material they have been taken out of. In comparison, a screw would leave a threaded hole if they were removed. 



To properly install DIN 603 carriage bolts, simply drill a hole through both materials that is slightly larger than the bolt's shank diameter. The hole should be deep enough to accommodate the full length of the bolt, including its dome head, and have some of the thread exposed through the end to leave enough room for the hexagon nut to secure the connection.

Next, place the bolt into the drilled holes and use a spanner to tighten a nut onto the threaded end of the bolt until it is snug against both surfaces.

It is important not to overtighten as this can cause damage to either material or strip the threads on the bolt itself. Once tightened, check for any movement or wobbling of either surface to ensure a solid connection.


Choosing the Right Coach Bolts for Your Project

Selecting the appropriate DIN 603 coach bolts is crucial for ensuring a secure and long-lasting connection in various construction projects.

The size, material composition, and finish of the coach bolt should be considered when choosing the right one for your project. M12 coach bolts are a popular choice of heavy duty applications that need to bear heavy loads.

To help with selecting the right coach bolt, use the table below that displays the different sizes, thread pitches, head diameters and thread diameters available.

This can provide an easy reference guide when comparing different options and making a decision on which one is best suited for your project. 


Thread Pitch

Head Diameter

Thread Diameter

















M16 2mm 38mm 16mm


m12 x 100mm coach bolt in a2 stainless steel

Frequently Asked Questions

Can They be Used Outdoors or with Pressure-Treated Wood?

Coach bolts can be used in outdoor environments and with pressure-treated wood. Stainless steel is always the best option for ourdoor projects due to their corrosion resistance. If bolts rust while supporting structures, it could compromise the integrity of the strucuture and become a safety concern.

How do You Determine the Appropriate Size to Use?

The appropriate size of a coach bolt needed for your project can be determined by measuring the thickness of the material being fastened and selecting a bolt with a length that accommodates it. The diameter of the bolt should also match the size of the hole drilled for it. For stronger holds, use a larger size coach bolt


Buy Coach Bolts

At Fixabolt, we have a wide range of stainless steel M8 coach bolts available at low every day prices with quantity discounts. Buy 1 to 1000 or visit our bolts blog for more information about fixings and fasteners.

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