Thermite rail gap tolerance is where a rail weld begins. The gap between the two rail ends controls how much molten steel fills the joint, how the rail ends are heated, and how the weld solidifies. For the process in this guide, the working gap is 26 ± 2 mm, measured from the rail foot edge.
Get the gap wrong and the defect appears later. A gap that is too small can produce poor preheating at the foot, cold metal flow, lack of fusion and porosity. A gap that is too large can create micro-shrinkage in the weld centre, inclusions and even leakage of molten steel.
1. Why 26 ± 2 mm is the Thermite Rail Gap Tolerance
The gap is measured at the rail foot edge because that is the datum used by the process. The rail head may look The gap is measured at the rail foot edge because that is the datum used by the process. The rail head may look correct while the foot gap is out of tolerance. The mould, portion size and preheating behaviour are matched to the approved gap range, so thermite rail gap tolerance cannot be judged by eye.
The practical window is:
| Condition | Result |
| Gap below 24 mm | Poor foot preheating, incomplete fusion, porosity risk |
| Gap 24–28 mm | Approved working range |
| Gap above 28 mm | Extra metal to fill, shrinkage and inclusion risk |
The rail must be aligned before the final gap reading is taken. If the rail ends are not square and parallel, the gap varies around the section.
2. Align the Rail Before Setting the Thermite Rail Gap Tolerance
Alignment comes before measurement, because thermite rail gap tolerance depends on it. Use a one-metre steel straightedge. Check the rail head, web and foot on both sides of the joint. Work from the running surface when setting the head side of the rail.
The rail ends should be:
- Square to the rail axis
- Parallel to each other
- Free from cuts, impact damage, irregular flame-cut edges and bolt holes inside the weld zone
If a bolt hole cannot be cut away, fit a close-fitting steel plug into the first hole near the rail end. The plug keeps heat transfer stable during preheating.
Rail end alignment with a one-metre straightedge
3. Set the tip point with a one-metre straightedge
Tip point is the small controlled height difference between the two rail ends, and it works together with thermite rail gap tolerance. Place the straightedge with its midpoint over the weld centre. Check the running surface 0.5 m each side of the weld.
Target tip point: 1.8–2.3 mm.
This is a measurement, not a judgement call. A tip point that is too low or too high changes how the rail runs and how the weld cools. It also affects the final grinding allowance.
When the gap is difficult to hold, use a stretching machine with the welding procedure. Do not rely on the clamps alone to pull the gap into tolerance.
4. Worn rails and mixed old-new joints
A worn rail or a new rail welded to an old one needs a different alignment check. Work from the running surface, not from the original rail centre line.
For worn or mixed joints, the vertical step at the running surface should not exceed 8 mm. Above that, the mould seal at the foot becomes unreliable and the risk of molten steel leakage rises. Thermite rail gap tolerance alone is not enough here.
If the step is large, correct the alignment first. Do not try to compensate with sealing paste or extra mould pressure.
5. Thermite Rail Gap Tolerance Checks Before the Mould Goes On
BBefore mould installation, confirm:
| Check | Target |
| Rail gap | 26 ± 2 mm at the foot edge |
| Rail end squareness | Within the approved preparation tolerance |
| Running surface alignment | Straightedge check complete |
| Tip point | 1.8–2.3 mm |
| Rail end condition | Clean, dry, free of rust and oil |
| Foot area | Clean and dry |
Thermite rail gap tolerance and alignment are the foundation of the weld. Once the mould is sealed and preheating starts, these parameters cannot be corrected.
6. What goes wrong when the gap is wrong
6.1 Gap too small
- The foot receives less heat.
- The molten steel flows less freely.
- Lack of fusion and porosity become more likely.
- Static bend and ultrasonic inspection can fail.
6.2 Gap too large
- More metal is required to fill the joint.
- Shrinkage porosity forms in the weld centre.
- Slag and inclusions are harder to float out.
- Molten steel can leak through the mould seal.
The two failure modes are different, but both start with thermite rail gap tolerance that was not measured or checked properly.
Conclusion
Thermite rail gap tolerance is a field measurement discipline. Set the rail ends square, measure the gap at the foot edge, align the head with a one-metre straightedge, and confirm the tip point before the mould is installed. That is the cheapest point in the process to prevent a defective weld. Weld quality also depends on the other controlled parameters: tip point, flux grade, mould fit, preheating and cooling time. Each step should be measured and recorded against the approved process card.
FORGESAIL supply aluminothermic welding kits, consumables and technical support for railway, crane rail and industrial track projects.
