Anchor bolts — long parts, rack-only plating, and a galvanizing question
Anchor bolts are too long to barrel, corrode at the concrete–air interface, and are usually specified for hot-dip galvanizing — a different process from electroplating. ASTM F1554 grades, oversize nuts and bend-radius risk.
At a glance
| Thread / dimensional standard | ISO 965-1 · 6g · ASTM F1554 |
|---|---|
| Tolerance class | 6g |
| Barrel window | not available above ≈ 150 mm total length |
| Rack / basket | M12 – M36, 200 – 1 000 mm |
| Strength specification | ISO 898-1 4.6 – 8.8 · ASTM F1554 Gr 36/55/105 |
| ISO 4042 relief bake | Triggered by class — 10.9 and above |
| Reviewed | 2026-08-04 |
An anchor bolt is a structural part that spends its life half in concrete and half in air. Its length rules out a barrel, its corrosion life is decided at the interface between those two halves, and the finish most often written on its drawing — hot-dip galvanizing — is a different process from the electroplating described here.
Why this shape is processed the way it is
L-bolts, J-bolts and straight rods with a plate-and-nut assembly run 200 to 1 000 mm long and are threaded at one end only. Nothing that shape tumbles. They are hung from the threaded end on a rack, and current distribution along a long rod becomes the whole plating problem: deposit thickness falls away with distance from the contact point, so the end furthest from the hook is the end that matters at inspection.
Grades we run in this shape
Where these parts go
Threads, tolerances and dimensional standards
Threads are the ISO 261 coarse series at ISO 965-1 class 6g on metric rod, or the UNC series where the drawing is American. ASTM F1554 is the anchor bolt standard proper, in grades 36, 55 and 105; ASTM A307 covers lower-strength carbon bolts and ASTM A563 the mating nuts. DIN 529 covers masonry and foundation bolt forms. The anchorage design itself — edge distance, embedment depth, concrete cone failure — belongs to EN 1992-4 or ACI 318 Chapter 17 and has nothing to do with the coating; a coating specification cannot rescue an anchorage that is too shallow.
Size band and handling route
M12 to M36, 200 to 1 000 mm, rack only. Barrel processing stops at roughly 150 mm of total length, well short of any real anchor bolt, so the cost model is per piece rather than per kilogram and rack density decides the price more than mass does. Bent forms take more rack space than straight rod of the same weight, which is why an L-bolt and a stud of identical mass do not quote the same.
Strength specification
Metric anchor rod is usually class 4.6, 5.6 or 8.8 to ISO 898-1. Inch anchor bolts are graded by ASTM F1554, where grade 36, grade 55 and grade 105 name the specified yield strength in ksi rather than a tensile-to-yield ratio, so the numbering looks nothing like ISO. Grade 105 is the one that changes the process: it is quenched and tempered, it sits above the embrittlement threshold, and it must not be cold bent after heat treatment. Grades 36 and 55 tolerate bending; grade 105 does not, and a J-bolt bent from grade 105 rod after hardening is a crack waiting for a load.
Hardness: class 8.8: 250–320 HV; ASTM F1554 grade 105 is quenched and tempered and must not be cold bent after heat treatment
Coating thickness and what limits it
Two coating worlds meet on this part and confusing them is the most expensive mistake on the page. Hot-dip galvanizing to ISO 1461 or ASTM F2329 puts a coating on an anchor bolt an order of magnitude heavier than electroplating, which is what most structural specifications assume — and because that thickness cannot fit inside a 6g/6H thread pair, the mating nut is tapped oversize after galvanizing, which is why a galvanized nut feels loose and is supposed to. Electroplated zinc is a thin coating by comparison and keeps a normal 6g fit at M20, where the allowance leaves room for a heavier service condition than a small bolt could take. Hot-dip galvanizing is a separate process and is not what is described on this page.
| Feature | M20 × 2.5 external thread, 6g |
|---|---|
| Available band | |es| = 42 µm |
| Coating budget | ≈ 10 µm electroplated — hot-dip is 45–85 µm and needs an oversize nut |
- ASTM B633 SC 3 — Fe/Zn 13
- SC 4 — Fe/Zn 25
- ISO 1461 / ASTM F2329 hot dip 45–85 µm (a different process)
Hydrogen embrittlement relief
Class 8.8 rod sits under the ISO 4042 trigger; ASTM F1554 grade 105 and any quenched-and-tempered anchor at or above class 10.9 sits over it and needs relief started as soon as possible and normally within 4 hours of plating. Geometry makes the risk worse than the class number suggests: a cold-formed bend in a J-bolt is a locked-in stress concentration, and hydrogen collects where residual tension is highest. On bent high-strength anchors the safer specification is usually a coating route that introduces no hydrogen.
Hydrogen de-embrittlement bake checkerHow this part fails
- Crack at the bend radius
A J- or L-bolt cracks at the inside of the bend, either during forming after hardening or later under load. Bending a quenched and tempered anchor cold leaves residual tension exactly where the section is already worked hardest.
- Thickness falls off down the rod
On a long racked part the deposit is heaviest near the contact and lightest at the far end. If thickness is only measured at the threaded end, the buried end can be well under specification and nobody finds out until the structure is years old.
- Corrosion at the concrete–air interface
The classic anchor failure. The steel is passivated inside the concrete and exposed above it, and the boundary between the two is where moisture, oxygen and chloride all meet. Coating loss at that band drives the service life of the whole anchor.
- Thread damaged before the nut arrives
Anchor bolts are cast in, walked on, splashed with concrete and left exposed for weeks. Damaged or contaminated threads are found at erection, long after the coating has been accepted, and a thread chaser through a coated thread removes the coating with the concrete.
What to check on the lot
- 1Thickness measured at both ends of the rodISO 3497 readings at the contact end and the far end, not an average. On a long rack part these are genuinely different numbers and the specification should name both positions.
- 2Thread gauging over the full threaded lengthISO 1502 ring gauging run the whole length of the thread, not just started. On a long thread a local high spot from handling or a heavy deposit near the hook will stop a gauge partway.
- 3Straightness, projection and bend dimensionsFor an L- or J-bolt the leg length and the projection above concrete are what the erector needs. These are dimensional acceptance characteristics and they are checked far too rarely before shipping.
- 4Hardness, and for the high grades the bake recordISO 6508-1 hardness to confirm the delivered condition, plus the plating-finish and oven-entry times on the lot record for any grade above the relief threshold. The clock is the control.
Test methods
Standards cited on this page
Questions engineers ask
Do you hot-dip galvanize anchor bolts?+
No — hot-dip galvanizing to ISO 1461 or ASTM F2329 is a separate process and not one of ours. What we run is electroplated zinc with trivalent passivation, phosphate coating, heat treatment and lab testing. If your specification calls for hot dip, that is a different supplier; if it allows electroplated zinc at a stated thickness, we can quote it. It is worth checking which the drawing actually requires, because the two are often written interchangeably and they are not.
Why is the nut on a galvanized anchor bolt loose?+
Because it was tapped oversize on purpose. A hot-dip coating is far too thick to fit inside a standard 6g/6H thread pair, so the nut is over-tapped after galvanizing to give the coating somewhere to go. The looser feel is the specification working, not a defect — although it does mean the nut and bolt are a matched pair and should not be mixed with electroplated hardware.
Can a J-bolt be bent after hardening?+
Not on the high-strength grades. ASTM F1554 grade 105 and any quenched-and-tempered anchor should be bent before heat treatment, not after; cold bending a hardened rod leaves residual tension at the bend and, if the part is then electroplated, that is precisely where hydrogen collects. Grades 36 and 55 are ductile enough to bend without that risk. Send the grade with the drawing and we will say whether the sequence works.
We reply to quote requests within 24 hours.
Guidance based on published standards and general fastener metallurgy. Every figure is a typical range that depends on the part, the material and the governing specification — the customer drawing and the applicable standard always prevail. Coating designations describe what a drawing can call for, not a result guaranteed on your parts without testing.