Hammer bolts (T-bolts) — how to read them, how to choose one
Called a hammer bolt or T-bolt in the Gulf, a channel bolt or hammer-head bolt in Europe — the same fastener. It is the only one you fit by turning it. Get the head geometry wrong and it either will not enter the slot, or it enters and never locks. This page explains the principle, the three head forms and what to measure — then gives the material and finish limits from the standards.
On this page: hammer bolt, T-bolt, T-head bolt, hammer-head bolt and channel bolt all mean the same product. The hook-head bolt is a related form covered below. The channel or rail is the section it fixes into.
The principle: enter narrow, bear wide
Every hammer bolt works the same way. The head is rectangular. Its width is small enough to drop through the slot; its length is longer than the slot but shorter than the channel's internal width. Insert it, turn it a quarter turn, and the head bears on the underside of both lips.
It must turn
If the head length exceeds the internal width, it cannot rotate to the locked position. It will feel tight and be doing nothing.
It must bear
If the head length barely exceeds the slot, the bearing area on each lip is tiny. The lips deform under load and the joint relaxes.
It must be seen
A quarter turn is invisible once the fixture is on. Check every one before the bracket goes over it.
The two numbers in the name
A hammer bolt is usually called by two numbers — something of the form 28/15. They are the head dimensions in millimetres: length / width. The first number is what bears on the lips; the second is what has to pass the slot.
This is why a bolt named for one rail will not necessarily suit another of the same nominal size. The name describes the bolt head, not the channel. Two channels of the same catalogue size from different makers can have different slot openings and different internal widths. Thread size is quoted separately and is independent of the head.
How it is fitted
The quarter turn is the whole job, and it is the one thing you cannot see once the bracket is on. Below it runs in two views at once: section shows the head seating under the lips, plan shows it turning.
Three head forms
Hammer head
Plain rectangular head. Enters the slot, turns a quarter turn, bears on both lips. The general-purpose form. Relies entirely on the clamping force to stay put.
Serrated hammer head
Teeth on the underside bite into the lips as it tightens. Resists slip along the channel and resists the bolt turning back. For loads that push along the rail, or where vibration is present.
Hook head
Engages one lip only, so it can be hooked in from the side of an already-installed run without threading it from the end. Used where access is limited.
The teeth on a serrated head are part of the load path. Never fit a serrated bolt to a channel whose maker has not approved it — the teeth are designed against a specific lip thickness and material.
Three ways it goes wrong
Every one of these passes a visual check from above once the bracket is fitted. That is why the orientation has to be confirmed before, not after.
Head too long
The head length exceeds the channel's internal width, so it cannot complete the quarter turn. It jams at an angle and feels tight while carrying nothing.
Head too short
The head only just passes the slot, so the bearing area on each lip is a sliver. Under load the lips bend down, the bolt relaxes, and the joint loosens.
Never turned
Inserted, the bracket fitted over it, the nut run down — but the head was never rotated. It holds while nothing pulls on it, then lifts straight back out through the slot.
Choose by the channel, not the label
Site labels fall off, and the same nominal channel size differs between makers. Four measurements settle it. Take them off the channel in front of you, with a vernier, not from a delivery note.
Head width must be less than A, with clearance for the coating.
Head length must be less than B, or the head cannot turn.
Head thickness plus any spring or locator must fit inside C.
Decides how much bearing the lips can take, and whether serrations are appropriate.
Add the coating to the head, not to the channel: hot-dip galvanizing builds roughly 85 µm a surface, and on a head that was a sliding fit before coating, that is enough to stop it entering.
Material classes — mechanical
| Class | Standard | Material and condition | Tensile Rm min, MPa | Yield min, MPa | Elongation min | Hardness |
|---|---|---|---|---|---|---|
| 4.6 | ISO 898-1:2013 | Carbon steel, not heat treated | 400 | 240 ReL | 22 % | 120–220 HV |
| 8.8 ≤ M16 | ISO 898-1:2013 | Carbon or alloy steel, Q&T | 800 | 640 | 12 % | 250–320 HV |
| 8.8 > M16 | ISO 898-1:2013 | Carbon or alloy steel, Q&T | 830 | 660 | 12 % | 255–335 HV |
| A2-70 · A4-70 | ISO 3506-1 | Austenitic, cold worked | 700 | 450 | 0.4 d | — |
| A2-80 · A4-80 | ISO 3506-1 | Austenitic, cold worked | 800 | 600 | 0.3 d | — |
Stainless elongation is expressed as a multiple of d on the whole fastener, not as a percentage on a machined test piece. ISO 3506 was revised in 2020 — confirm which edition the project specifies.
Material classes — chemical
| Class | Standard | C % | Cr % | Ni % | Mo / other % | P max | S max |
|---|---|---|---|---|---|---|---|
| 4.6 | ISO 898-1 Table 2 | 0.55 max | — | — | — | 0.050 | 0.060 |
| 8.8 ≤ M16 | ISO 898-1 Table 2 | 0.15 – 0.40 | — | — | — | 0.025 | 0.025 |
| 8.8 > M16 | ISO 898-1 Table 2 | 0.25 – 0.55 | — | — | — | 0.025 | 0.025 |
| A2 · 1.4301 | EN 10088-3 | 0.08 max | 17.0 – 19.5 | 8.0 – 10.5 | — | 0.045 | 0.015 |
| A4 · 1.4401 | EN 10088-3 | 0.08 max | 16.5 – 18.5 | 10.0 – 13.0 | 2.00 – 2.50 Mo | 0.045 | 0.015 |
Finishes for Gulf exposure
| Finish | Standard | Where it belongs | What to watch |
|---|---|---|---|
| Zinc electroplated | ISO 4042 | Indoor, dry | Thin coating; baking required above class 10.9 or 39 HRC (ISO 4042) |
| Hot-dip galvanized | ISO 10684 · ISO 1461 | External, general | Nuts tapped oversize after coating — order galvanized nuts with galvanized bolts |
| Zinc flake | ISO 10683 | External, thin film needed | No hydrogen embrittlement route; check the channel maker accepts it |
| A2 · 1.4301 | ISO 3506-1 · EN 10088-3 | Indoor, fresh water, food | Pits in chlorides — not the Gulf coastal default |
| A4 · 1.4401 | ISO 3506-1 · EN 10088-3 | Coastal, marine, chemical | The molybdenum buys chloride pitting resistance |
A hammer bolt sits in a slot where water collects and does not drain. Treat it as a more severe exposure than the surrounding steelwork, not a milder one.
What this page does not give you — and why
You will not find tightening torques, permissible loads or design resistances here. Those values belong to the channel system, not to the bolt: they depend on the lip geometry, the channel material and the anchor behind it, and they are established by the channel manufacturer's own testing and approval documents.
Quoting them out of that context is how a joint gets designed to a number that does not apply to the channel actually on site. For a load-bearing connection, work from the channel manufacturer's approval document (ETA or equivalent) and the project structural engineer — and ask us for the certificates covering the fasteners themselves.