High-Strength Fish Bolts vs Ordinary Fish Bolts: Key Differences
Nov 06, 2025
Fish bolts are the fasteners that clamp the fishplates (joint bars) to the rail web at a rail joint, holding the two rail ends in alignment so the wheel passes smoothly from one rail to the next. They are available in different strength grades, commonly described as ordinary (low-to-medium strength) and high-strength. The grade is defined by the property class of the bolt, which determines the tension the joint can be tightened to and therefore the capacity of the joint.
How a Fish Bolt Works
At a joint, the fishplates are clamped to the rail web by the fish bolts. The bolt is tensioned so that the friction between the fishplate and the rail web carries the bending and shear loads of the joint. The higher the bolt tension, the higher the friction and the stiffer the joint. Bolt tension is limited by the bolt material: tightening beyond the yield point relaxes the bolt or breaks it. This is why the strength grade of the bolt matters directly for the joint capacity.
Property Classes Compared
Fish bolt grades follow the ISO 898-1 property class system. The table shows the general requirements of the classes commonly used for fish bolts.
| Property class | Tensile strength (MPa) | Yield strength (MPa) | Elongation | Typical use |
|---|---|---|---|---|
| 4.6 | 400 | 240 | 22% | Ordinary joints, light traffic |
| 5.6 | 500 | 300 | 20% | Standard joints, medium traffic |
| 8.8 | 800 | 640 | 12% | High-strength joints, heavy traffic |
| 10.9 | 1000 | 900 | 9% | Very high strength joints |
Ordinary fish bolts are normally supplied in grades 4.6 and 5.6, made from plain carbon steel. High-strength fish bolts are supplied in grades 8.8 and above, made from quenched and tempered alloy steel such as 40Cr or similar, which gives the higher tensile and yield values in the table.
Materials and Manufacture
Ordinary fish bolts are produced from carbon steel, formed by forging or machining, then threaded. High-strength fish bolts are heat treated (quenched and tempered) after forming to reach the mechanical properties of the higher property classes. The heat treatment must be controlled so that the hardness, tensile strength and toughness are within the standard's limits, and the thread is normally rolled after heat treatment to keep the required thread strength and accuracy.
Dimensions and Standards
Fish bolts are specified by diameter and length, commonly in metric sizes such as M20 or M22 with lengths defined by the fishplate and rail web combination. Standards such as DIN 5903-2 define the bolt types, dimensions and grades for rail joints; other national and international standards cover equivalent products. The bolt head type (for example square head) and the surface finish (zinc plated, hot-dip galvanized, plain) are selected for the track and the environment.
Which Grade to Choose
The choice of grade follows the joint design. For light lines and low traffic, grade 4.6 or 5.6 is sufficient and economical. For heavy-haul, high-speed or other lines where the joint must carry high bending moments without loosening, a high-strength grade such as 8.8 is specified. High-strength bolts must be tightened with controlled torque or tension to the value given in the joint design, using suitable nuts and washers of matching grade.
FAQ
What is the difference between high-strength and ordinary fish bolts?
The difference is the property class: ordinary fish bolts are usually grades 4.6 and 5.6 made of carbon steel, while high-strength fish bolts are grades 8.8 and above made of quenched and tempered alloy steel, giving higher tensile and yield strength.
What grade of fish bolt should be used on a heavy-haul line?
Heavy-haul joints are normally specified with high-strength bolts, typically grade 8.8 or higher, tightened to a controlled torque so the joint carries the loads without loosening.
What does grade 8.8 mean?
8.8 is an ISO 898-1 property class: minimum tensile strength 800 MPa and minimum yield strength 640 MPa. The bolt is quenched and tempered alloy steel.
What standard covers fish bolts?
Standards such as DIN 5903-2 define fish bolt types, dimensions and grades. Rail joints may also be specified against national standards or the project specification.
Can ordinary fish bolts be replaced by high-strength bolts on site?
Only if the joint design allows it. The higher bolt tension changes the forces in the fishplate and the rail web; the replacement should be checked against the joint design and the torque specification.
What surface finishes are available for fish bolts?
Common finishes are plain, zinc plated and hot-dip galvanized. Galvanized bolts are used where the joint is exposed to moisture and corrosion.
