A rail fish plate is a mechanical coupling device that allows separate rail lengths to behave as a continuous structural member by transferring loads through bolted steel interfaces. This is fundamental in jointed rail systems without welding. The bolts that clamp the fish plates to the rail web are called fish bolts, and their number is determined by the joint length and the load class of the line.
Standard Bolt Configuration
A standard railway joint uses four fish bolts per pair of fish plates, with two bolts passing through each rail end. Some longer or heavy-duty joints use six bolts. The bolts, together with nuts, washers and spring washers, secure the fish plates on both sides of the rail web.
| Configuration | Bolt Count | Use |
|---|---|---|
| Standard joint | 4 bolts | Normal rail sections and speeds |
| Long joint | 6 bolts | Heavier tracks and longer joint bars |
| Hardware per joint | 4 bolts, 4 nuts, 4 washer sets | Matching sets for assembly |
The purpose of fish bolts is to connect rail ends, ensure horizontal and vertical alignment and allow controlled expansion at the joint gap.
Fish Bolt Grades and Mechanical Properties
Fish bolts are manufactured in several strength grades, each matched to a material and mechanical property requirement.
| Grade | Material | Tensile Strength | Yield Strength | Elongation |
|---|---|---|---|---|
| 4.6 | Q235 | not less than 400 MPa | not less than 240 MPa | not less than 22 percent |
| 5.6 | 35# | not less than 500 MPa | not less than 300 MPa | not less than 20 percent |
| 8.8 | 45# | not less than 800 MPa | not less than 640 MPa | not less than 12 percent |
| 10.9 | 40Cr | not less than 1000 MPa | not less than 900 MPa | not less than 9 percent |
All grades pass a cold bending test of 90 degrees without cracking. Common surface finishes are plain oiled, oxide black, zinc plated, hot-dip galvanized, waxed, bitumen coated, Dacromet coated and sherardized. The applicable standard is UIC 864-2.
Why Fish Bolts Are Essential
Structural integrity: holding the fish plates firmly against the rail web and preventing misalignment that could lead to derailment
Load distribution: helping transfer wheel loads from one rail to the next and reducing fatigue damage at the joint
Safety and stability: maintaining track gauge and a smooth rail top surface under intense vibration and impact
Thermal movement: bolt holes are designed to allow rails to expand and contract with temperature without breaking the connection
Easy maintenance: bolted joints are simpler to install, remove and maintain than welded joints
Installation and Inspection
Install bolts through the matching holes of both fishplates, fit the washers and spring washers, then tighten the nuts evenly in sequence to the specified torque. Retighten after the first weeks of traffic because seating movement is normal. Inspect bolts regularly for loosening, corrosion and fatigue, and replace any bolt with visible damage immediately.
Frequently Asked Questions
How many fish bolts are used per fish plate? Each fishplate receives two bolts per rail end, so a standard pair of fishplates uses four bolts in total.
When are six fish bolts used? Six bolts are used for longer joint bars on heavier tracks that demand higher clamping force.
What grade of fish bolt should I choose? Grade 8.8 and 10.9 bolts are recommended for heavy-haul, high-speed and mining railways, while grade 4.6 suits light lines.
Do fish bolts allow rail expansion? Yes, the bolt holes are designed so the rails can expand and contract with temperature changes without breaking the joint.
Which standard applies to fish bolts? UIC 864-2 covers the requirements for rail fish bolts and joint hardware.
