Fish Bolt Guide: How to Choose the Right Grade, Size and Standard for Railway Projects

Selecting the correct fish bolt is far more important than many people realize. While rails and sleepers often receive the most attention, a rail joint is only as reliable as the bolts that hold the fish plates firmly against the rail. A poorly selected or incorrectly installed fish bolt can lead to joint movement, accelerated rail wear, higher maintenance costs, and, in severe cases, compromise track safety.

As a manufacturer of railway fastening components, we’ve worked with customers on metro systems, freight railways, mining railways, and heavy-haul projects around the world. One thing we’ve learned is that many procurement issues don’t come from poor-quality bolts—they come from choosing the wrong specification.

The questions we hear most often include:

• Should I choose Grade 8.8 or Grade 10.9?
• Which fish bolt size matches UIC 60 rail?
• Can UIC and AREMA fish bolts be used interchangeably?
• Which surface coating is best for coastal or mining environments?
• Are all railway fish bolts manufactured the same way?

This guide answers these questions and explains how to select the right fish bolt based on railway standards, operating conditions, and engineering requirements.

What Is a Fish Bolt?

A fish bolt, also known as a railway fish bolt, rail bolt, or track bolt, is a high-strength fastener used to connect two fish plates (joint bars) across a rail joint.

Unlike ordinary structural bolts, fish bolts are specifically engineered for railway applications where they must withstand:

• Continuous vibration from passing trains
• Repeated impact loading at rail joints
• Dynamic tensile and shear forces
• Temperature-related expansion and contraction
• Long-term exposure to rain, humidity, dust, and corrosive environments

A typical bolted rail joint consists of:

• Two fish plates
• Four or six fish bolts (depending on the joint design)
• Matching nuts
• Spring washers or flat washers (depending on the applicable standard)

Each component works together to maintain proper clamping force throughout the service life of the track.

Learn how fish plates work in a rail joint in our Rail Fish Plate Guide.

Why Choosing the Right Fish Bolt Matters

From an engineering perspective, a fish bolt is much more than a simple fastener. Its primary function is to generate sufficient clamping force so that the fish plates and rail act as a single structural assembly under repeated train loads.

When the wrong bolt is selected—or when the correct bolt is improperly installed—the consequences can extend far beyond the fastener itself.

Loose Rail Joints

An undersized or low-strength bolt may gradually lose preload under cyclic loading. As the clamping force decreases, the fish plates begin to move slightly with every wheel passing over the joint.

This movement accelerates wear on both the fish plates and the rail, eventually increasing maintenance frequency.

Fatigue Failure

Railway joints experience millions of loading cycles throughout their service life. If the bolt material, thread quality, or heat treatment is inadequate, fatigue cracks often initiate at the thread root.

For this reason, railway fish bolts are almost always manufactured with rolled threads, which provide significantly better fatigue resistance than cut threads.

Increased Maintenance Costs

Replacing a fish bolt is inexpensive.

Replacing worn fish plates, damaged rails, or repairing a failed joint is not.

In our experience, selecting the correct bolt specification during procurement is one of the simplest ways to reduce long-term maintenance costs.

Fish Bolt Grades Explained: Which Strength Grade Should You Choose?

One of the first questions customers ask is:

“Should I choose Grade 8.8 or Grade 10.9?”

The answer depends on the railway application rather than simply choosing the highest strength available.

Grade 8.8 Fish Bolts

Grade 8.8 is the most widely used strength class for conventional railway applications.

Typical applications include:

• Metro systems
• Light rail transit
• Passenger railways with moderate axle loads
• Secondary railway lines

Advantages include:

• Good strength-to-cost ratio
• Excellent ductility
• Easy availability
• Reliable performance under normal railway conditions

For many urban rail systems, Grade 8.8 provides sufficient clamping force when properly installed and maintained.

Grade 10.9 Fish Bolts

Grade 10.9 is commonly specified for more demanding railway environments.

Typical applications include:

• Heavy-haul freight railways
• Mainline railway networks
• High-speed rail
• Mining railways
• Industrial rail systems with high traffic density

Compared with Grade 8.8, Grade 10.9 offers:

• Higher preload capacity
• Better resistance to bolt loosening
• Improved fatigue performance
• Greater safety margin under heavy cyclic loading

From our engineering experience, Grade 10.9 is the preferred choice for most heavy-duty railway projects because it provides an excellent balance between strength, fatigue resistance, and ductility.

Grade 12.9 Fish Bolts

Although Grade 12.9 offers even higher tensile strength, it is not automatically the best option.

Higher-strength bolts are generally less ductile and more susceptible to hydrogen embrittlement if improper surface treatments are used.

Unless specifically required by the project specification, Grade 12.9 is rarely necessary for conventional railway joints.

Many customers initially assume that “stronger is always better.” In practice, selecting the appropriate strength grade based on the operating environment usually delivers better long-term reliability.

Quick Selection Guide

Standard Fish Bolt Sizes for Railway Applications

Selecting the correct fish bolt size is just as important as choosing the appropriate strength grade. Even a high-strength bolt cannot perform properly if its diameter or length does not match the fish plate and rail profile.

Unlike general-purpose fasteners, railway fish bolts are designed to fit specific rail standards. The bolt diameter, thread type, head dimensions, and overall length are determined by the rail section and the corresponding fish plate.

Before ordering fish bolts, always confirm:

• Rail profile (UIC 54, UIC 60, 136 RE, etc.)
• Fish plate drawing or standard
• Bolt hole diameter
• Joint thickness
• Applicable railway standard (UIC, EN, AREMA, or others)

This information ensures the bolt provides the required preload without excessive clearance or installation issues.

Common UIC Fish Bolt Sizes

Most railway projects in Europe, Asia, the Middle East, and Africa follow UIC or EN standards, where metric fasteners are used.

Among these sizes, M24 is by far the most commonly specified railway fish bolt for UIC-standard railways.

For many UIC 54 and UIC 60 installations, M24 provides sufficient clamping force while remaining compatible with standard fish plate designs.

However, some heavy-haul railways adopt M27 bolts to achieve a higher preload and improved resistance to joint movement under extreme axle loads.

Common AREMA Fish Bolt Sizes

Railways in North America generally follow AREMA standards, which use imperial dimensions rather than metric sizes.

Although the functions are the same, AREMA fish bolts are not interchangeable with UIC fish bolts because their thread systems and dimensions are completely different.

How Bolt Length Is Determined

Diameter is only one part of the specification.

Bolt length depends on the total thickness of the assembled joint, including:

• Left fish plate
• Rail web
• Right fish plate
• Washer(s)
• Nut

A bolt that is too short may not allow full thread engagement.

A bolt that is too long can reduce installation efficiency and increase the exposed thread length, making corrosion more likely over time.

For this reason, railway engineers normally specify both bolt diameter and bolt length together rather than selecting them independently.

How to Match Fish Bolts with Rail Fish Plates

One of the most common mistakes we see is selecting a fish bolt before confirming the fish plate.

In reality, the fish plate determines almost every critical dimension of the bolt.

Each fish plate is designed with a specific hole diameter and spacing. Once the fish plate standard is confirmed, the matching fish bolt specification becomes straightforward.

A typical selection process looks like this:

Rather than asking, “Which fish bolt should I buy?”, a better engineering question is:

“Which fish bolt matches my fish plate?”

This approach minimizes installation problems and ensures the joint achieves the required clamping force.

Looking for matching joint bars? Read our Rail Fish Plate Buying Guide to understand how rail profile, bolt hole pattern, and joint design work together.

Don’t Rely on Bolt Diameter Alone

It is a common misconception that bolts with the same diameter are interchangeable.

For example, two M24 fish bolts may have different:

• Overall lengths
• Thread lengths
• Head dimensions
• Neck designs
• Surface treatments

These differences can affect installation, preload, and long-term reliability.

Whenever possible, provide your supplier with the fish plate drawing, rail standard, or existing bolt specification instead of specifying only the diameter.

This allows the manufacturer to verify compatibility before production.

UIC vs AREMA Fish Bolts: Can They Be Interchanged?

The short answer is no.

Although both systems serve the same purpose, they are designed according to different engineering standards.

The main differences include:

Attempting to install a UIC fish bolt into an AREMA fish plate—or vice versa—can result in improper thread engagement, incorrect preload, and installation failure.

If your railway project follows UIC standards, all fastening components—including fish plates, bolts, nuts, and washers—should follow the same standard.

The same principle applies to AREMA systems.

As a rule of thumb:

Never mix fastening systems within the same rail joint.

Maintaining compatibility across all components is the safest and most reliable approach.

Choosing the Right Surface Coating for Fish Bolts

The service life of a fish bolt depends not only on its strength grade but also on its resistance to corrosion.

Railway fasteners are exposed to rain, snow, coastal salt spray, industrial pollution, and repeated wet-dry cycles throughout their service life. Without appropriate corrosion protection, rust can reduce thread performance, make future maintenance difficult, and eventually affect the integrity of the rail joint.

The most suitable coating depends on the operating environment rather than simply selecting the thickest protective layer.

Black Oxide

Black oxide is one of the simplest surface treatments used for steel fasteners.

It provides a uniform appearance and offers light corrosion protection when combined with protective oil.

Typical applications include:

• Dry indoor storage
• Temporary protection before installation
• Components that will receive additional protective treatment later

Because the coating is extremely thin, black oxide should not be considered a long-term corrosion solution for outdoor railway projects.

Zinc Electroplating

Electroplated zinc is widely used because it offers a good balance between corrosion resistance and cost.

It is commonly specified for:

• Conventional railways
• Inland railway lines
• Dry or moderate climates

However, engineers should pay particular attention when using electroplated coatings on high-strength bolts.

During the electroplating process, hydrogen may be introduced into the steel. If it is not properly removed through post-plating baking, Grade 10.9 and Grade 12.9 fish bolts can become susceptible to hydrogen embrittlement, which may lead to delayed brittle failure under service loads.

Hot-Dip Galvanizing

Hot-dip galvanizing provides a much thicker zinc coating than electroplating.

It is often selected for:

• Coastal railways
• Ports and terminals
• Tropical climates
• High-humidity environments

The thicker coating significantly improves corrosion resistance and extends service life in aggressive environments.

However, because the coating adds considerable thickness, both the bolt and nut threads must be manufactured specifically for hot-dip galvanizing to ensure proper assembly.

Geomet and Dacromet Coatings

Modern non-electrolytic coatings such as Geomet and Dacromet have become increasingly popular for high-strength railway fasteners.

These coatings offer several advantages:

• Excellent corrosion resistance
• Uniform coating thickness
• No hydrogen embrittlement risk during coating
• Stable friction characteristics during tightening

For many heavy-haul and high-speed railway projects, these coating systems have become the preferred solution for Grade 10.9 fish bolts.

In our experience, they provide an excellent combination of durability and installation reliability, particularly where long maintenance intervals are required.

Sherardizing

Sherardizing is a thermal diffusion zinc process that creates a uniform zinc-iron alloy layer on the bolt surface.

Compared with conventional galvanizing, it offers:

• Excellent coating uniformity
• Better coverage on threads
• Good wear resistance
• Stable dimensional accuracy

Although less common than hot-dip galvanizing, it is a practical option for railway fasteners operating in corrosive environments.

Surface Coating Comparison

Which Coating Do We Recommend?

There is no single coating suitable for every railway project.

Instead, we recommend selecting the coating based on the operating environment and bolt strength.

As a general guideline:

• Metro and conventional railway lines – Zinc electroplating is usually sufficient.
• Coastal, mining, or high-humidity environments – Hot-dip galvanizing or Sherardizing provides better long-term protection.
• Grade 10.9 and Grade 12.9 fish bolts – Geomet or Dacromet coatings are generally preferred because they minimize the risk of hydrogen embrittlement while providing excellent corrosion resistance.

Selecting the appropriate coating at the procurement stage can significantly reduce future maintenance costs and extend the service life of the entire rail joint.

Common Mistakes When Purchasing Fish Bolts

After supplying railway fastening components for customers in different countries, we’ve noticed that most purchasing problems are not caused by manufacturing defects. Instead, they usually result from incorrect specifications during the procurement stage.

Here are some of the most common mistakes—and how to avoid them.

Mistake 1: Choosing the Highest Strength Grade Without Considering the Application

Many buyers assume that a Grade 12.9 fish bolt is always better than Grade 10.9.

In reality, higher strength does not necessarily mean better performance.

Grade 12.9 bolts are less ductile and require stricter control of heat treatment and surface coating. For most heavy-haul, freight, and passenger railway applications, Grade 10.9 provides a more balanced combination of strength, fatigue resistance, and long-term reliability.

Mistake 2: Specifying Only the Bolt Diameter

A request such as “We need M24 fish bolts” does not provide enough information for manufacturing.

Different railway systems may use the same diameter but require different:

• Bolt lengths
• Thread lengths
• Head dimensions
• Neck designs
• Surface treatments

Whenever possible, provide the rail profile, fish plate drawing, or project standard instead of only the bolt diameter.

Mistake 3: Mixing UIC and AREMA Components

Although UIC and AREMA fish bolts perform the same function, they are designed according to different standards.

Mixing metric and imperial components can lead to installation difficulties, improper preload, and unnecessary replacement costs.

Always use fish bolts, nuts, washers, and fish plates that comply with the same railway standard.

Mistake 4: Ignoring Corrosion Protection

Some buyers focus only on the bolt material and overlook the surface coating.

However, corrosion is one of the leading causes of maintenance issues in rail joints, especially in coastal regions, ports, mining railways, and humid climates.

Choosing the correct coating at the beginning of a project is far less expensive than replacing corroded fasteners later.

Mistake 5: Purchasing Cut-Thread Bolts

From the outside, rolled threads and cut threads may appear similar.

Their fatigue performance, however, is very different.

Rolled threads preserve the grain flow of the steel and introduce beneficial compressive stresses at the thread root, making them far more resistant to fatigue cracking.

For safety-critical railway applications, thread rolling should always be preferred over thread cutting.

Fish bolts are only one part of a complete fastening system. Learn how Rail Clips maintain rail clamping force and work together with fish bolts and fish plates in modern railway track systems.

How Fonyo Manufactures High-Quality Fish Bolts

A railway fish bolt is expected to remain secure under millions of load cycles during its service life. Achieving that level of reliability depends not only on the material grade but also on consistent manufacturing and rigorous quality control.

At Luoyang Fonyo Heavy Industries Co., Ltd., every production stage is designed to ensure that fish bolts meet the mechanical performance and dimensional accuracy required by railway standards such as EN, UIC, and AREMA.

Rather than relying on final inspection alone, we control quality throughout the entire manufacturing process.

1. Raw Material Verification

The manufacturing process begins with certified steel from qualified suppliers.

Before production starts, the chemical composition of each batch is verified using a spectrometer to confirm that the material meets the required specification.

This step helps ensure consistent mechanical properties after heat treatment and reduces the risk of material variation between production batches.

2. Precision Hot Forging

The bolt head is formed through hot forging rather than machining.

Hot forging improves grain flow inside the steel, resulting in higher strength and better impact resistance compared with bolts produced from machined bar stock.

The forging process also produces a more accurate bolt head and minimizes material waste.

3. Controlled Heat Treatment

Heat treatment is one of the most critical steps in manufacturing railway fish bolts.

Temperature, holding time, and cooling rate are carefully controlled to achieve the required mechanical properties for each strength grade.

Proper heat treatment provides the balance of:

• Tensile strength
• Yield strength
• Toughness
• Fatigue resistance

that railway applications demand.

4. Thread Rolling Instead of Thread Cutting

One feature that distinguishes railway-grade fish bolts from ordinary industrial fasteners is the use of rolled threads.

Unlike cut threads, thread rolling forms the thread profile by cold working the steel.

This process preserves the natural grain flow and introduces compressive residual stress around the thread root, significantly improving fatigue performance under repeated railway loading.

For components subjected to millions of load cycles, rolled threads have become the industry standard.

5. Mechanical Testing and Dimensional Inspection

Every production batch undergoes routine inspection before shipment.

Depending on customer requirements, testing may include:

• Tensile testing
• Yield strength verification
• Hardness testing
• Dimensional inspection
• Thread gauge inspection
• Surface coating thickness measurement

Material certificates and inspection reports can also be provided to support project documentation and quality assurance requirements.

6. Surface Treatment and Final Packaging

After inspection, fish bolts receive the specified surface treatment, such as zinc plating, hot-dip galvanizing, or Geomet coating.

Finished bolts are then packaged together with matching nuts and washers to simplify installation on site and reduce the possibility of component mismatch.

Raw Material → Hot Forging → Heat Treatment → Thread Rolling → Inspection → Packaging

Use real factory photos where possible instead of icons. Original manufacturing images strengthen EEAT and help differentiate the article from generic industry content.

Installation Best Practices

Even the highest-quality fish bolt cannot perform as intended if it is installed incorrectly.

Proper installation ensures that the fish plates remain firmly clamped and that the rail joint maintains sufficient preload throughout its service life.

The following practices are recommended for most railway projects.

Use the Specified Tightening Torque

Each bolt size and strength grade has a recommended tightening torque.

Applying too little torque may allow the joint to loosen under vibration, while excessive torque can overstress the bolt or damage the threads.

Always follow the torque values specified by the applicable railway standard or project documentation.

Tighten Bolts Evenly

For rail joints using four or six bolts, tightening should be carried out gradually in a balanced sequence rather than fully tightening one bolt at a time.

This helps distribute the clamping force evenly across the fish plates and minimizes uneven stress within the joint.

Recheck Bolt Tightness After Initial Service

During the first period of operation, slight settlement may occur as the contact surfaces between the fish plates, rail, and fasteners stabilize.

Many railway maintenance procedures therefore recommend checking the bolt preload after the line has been placed into service.

Regular inspections also help identify loose fasteners before they develop into larger maintenance issues.

Never Mix Different Fastener Components

A fish bolt assembly should always consist of compatible components manufactured to the same standard.

Mixing bolts, nuts, or washers from different systems may result in incorrect thread engagement or uneven preload.

When replacing fish bolts during maintenance, it is good practice to replace the complete assembly rather than individual components.

Internal Link: Fish bolts work together with Rail Clips, Rail Pads, and Fish Plates to form a complete railway fastening system. Understanding how these components interact helps improve joint reliability and reduce maintenance requirements.

Frequently Asked Questions

What is the difference between a fish bolt and a standard bolt?

A fish bolt is specifically designed for railway rail joints. Compared with general-purpose structural bolts, it is manufactured to railway standards, uses higher-strength materials, and is designed to withstand continuous vibration, impact loading, and fatigue stresses. It is not recommended to replace a railway fish bolt with an ordinary commercial bolt.

How many fish bolts are used in one rail joint?

The number of fish bolts depends on the fish plate design.

A standard 4-hole fish plate uses four fish bolts, while a 6-hole fish plate uses six fish bolts.

Each bolt passes through both fish plates and the rail web, securing the complete joint as a single assembly.

Can fish bolts be reused?

In most railway maintenance applications, reusing fish bolts is not recommended.

After long-term service and repeated loading, both the bolt and nut may experience wear that affects the ability to maintain the required preload.

Replacing the complete bolt assembly during maintenance provides greater reliability and helps reduce the risk of future joint loosening.

Should I choose Grade 10.9 or Grade 12.9 fish bolts?

For most freight, passenger, metro, and heavy-haul railway projects, Grade 10.9 provides an excellent balance between strength, toughness, and fatigue resistance.

Grade 12.9 is generally reserved for special engineering applications where exceptionally high strength is specified.

Choosing the highest grade is not always the best engineering solution.

Can UIC fish bolts be used on AREMA railways?

No.

UIC and AREMA fastening systems use different dimensions, thread standards, and component specifications.

To ensure proper installation and long-term performance, all components within a rail joint should comply with the same railway standard.

Which surface coating lasts the longest?

There is no universal answer because the best coating depends on the service environment.

For inland railways, zinc electroplating is often sufficient.

For coastal, mining, or high-humidity environments, hot-dip galvanizing, Sherardizing, or Geomet coatings generally provide better long-term corrosion protection.

Manufacturer

Although a fish bolt is one of the smallest components in a railway fastening system, it plays a critical role in maintaining the integrity of every rail joint.

Selecting the correct bolt involves much more than choosing a diameter or strength grade. Rail profile, fish plate design, operating conditions, corrosion environment, and applicable railway standards all influence the final specification.

Understanding these factors at the procurement stage helps reduce installation problems, extend maintenance intervals, and improve the long-term reliability of the track.

At Luoyang Fonyo Heavy Industries Co., Ltd., we manufacture railway fish bolts, fish plates, rail clips, and other railway fastening components for projects worldwide. Whether you require standard products or customized fasteners for a specific railway system, our engineering team can help you identify the most suitable solution based on your drawings, technical specifications, or project requirements.

If you’re planning a railway project or sourcing replacement fish bolts, feel free to contact us. We’ll be happy to recommend the appropriate fish bolt size, strength grade, surface coating, and matching fastening components for your application.

In addition to fish plates, we also supply fish bolts, rail clips, rail pads, railway sleepers, and other railway track components.

Contact us today for technical support, engineering consultation, and quotations.