MF FASTENER INDUSTRIES LLC
Emirates Modern Industrial Area, Umm Al Quwain, U.A.E. · P.O. Box 7071 · ISO 9001:2015 · Technical Library
TECHNICAL LIBRARY
Guardrail bolts
Button head, oval shoulder · 5/8-11 UNC and M16 × 2 · AASHTO M180 · ASTM A307 Grade A
The bolt that holds a W-beam crash barrier together. Its head is domed and smooth so nothing projects into the traffic face, and its neck is oval so it locks into the slot in the rail and cannot spin while the nut is tightened. Both features are dictated by the road, not by the fastener — which is why this is a specification item and not a generic carriage bolt.
Three views — outline, dimensioned drawing with the oval-shoulder end view, and isometric. Dimensions in mm [inch], geometry per AASHTO M180.
Geometry per AASHTO M180 · dimensions A and B to the M180 figure
At a glance
Thread5/8-11 UNC
M16 × 2Class 2A inch · 6g metric · both to AASHTO M180
M16 × 2Class 2A inch · 6g metric · both to AASHTO M180
MaterialASTM A307 Gr A60 ksi min tensile · low carbon
FinishHDG A153 Class Cor ASTM F2329 · nuts tapped oversize
Governing specAASHTO M180geometry, material and nut
Two families
- Splice bolts — short, typically 5/8 × 1¼ in / M16 × 32 mm. Eight per rail splice. They join one W-beam panel to the next.
- Post bolts — long, 2–46 in / 50–1 170 mm. Through the rail, the blockout and the post. Common: 8, 10, 14, 18, 22, 25 in — 200, 250, 356, 460, 560, 635 mm.
Dimensions
| Feature | Inch | Metric | Source |
|---|---|---|---|
| Nominal diameter | 5/8 in | 16 mm | AASHTO M180 |
| Thread | 5/8-11 UNC-2A ANSI B1.1 | M16 × 2, 6g ANSI B1.13M | M180 · both editions |
| Head diameter, A | ≈ 1-5/16 in CONFIRM | ≈ 33 mm | Read from the M180 figure |
| Oval shoulder, B | Per the M180 figure CONFIRM | — | Fills the 5/8 × 1¼ in [16 × 32 mm] slot |
| Fillet radii, min | 1/16 in | 2 mm | M180 drawing note |
| Nut thread | 5/8-11 UNC-2B | M16 × 2, 6h | M180 |
| Nut recess | 1 × Ø, 1/16 in deep | 1 × Ø, 1.5 mm deep | Both faces · M180 figure |
| Bolt projection past nut, max | 1/4 in | 6 mm | M180 drawing note |
Bolt projection. The bolt must pass right through the nut and stand no more than 1/4 in (6 mm) proud of it. Longer than that and it is trimmed back — a projection on a crash barrier is a hazard, which is the whole reason the head is domed in the first place.
Length and thread length
Thread length is set by the bolt length, not chosen. These are the values on the AASHTO standard drawings.
| Bolt length L, in | 1¼ | 1⅜ | 2 | 8 | 10 | 14 | 18 | 21 | 24 |
|---|---|---|---|---|---|---|---|---|---|
| Bolt length L, mm | 32 | 35 | 50 | 200 | 250 | 356 | 460 | 533 | 610 |
| Thread length T, min, in | 1 | 1⅛ | 1¾ | 4 | 4 | 4-1/16 | 4 | 4-1/16 | 4 |
| Thread length T, min, mm | 25 | 29 | 45 | 100 | 100 | 103 | 100 | 103 | 100 |
The metric sizes are the standard’s own, not a conversion. AASHTO M180 is published in both editions and the metric drawings call out M16 × 32 mm for the splice bolt and M16 × 460 mm for the long post bolt — exactly the 1¼ in and 18 in bolts. Where a metric drawing specifies a hex-head bolt instead, it is M16 × 38 mm; the high-strength hex bolts in an end anchorage are M22 × 200, 300 or 450 mm.
L is measured under the head to the end of the bolt. Bolts longer than about 2 in carry a nominal 4 in of thread; the 14 in and 21 in entries are from the AASHTO / state DOT sheets, which specify 4-1/16 in minimum. Other lengths are made to order.
Mechanical properties
| Property | Inch | Metric | Note |
|---|---|---|---|
| Tensile strength, min | 60 ksi | 414 MPa | Full-size bolt, A307 Grade A |
| Yield strength | Not specified | 240 MPa* | *A307 Grade A sets no yield. The 36 ksi / 240 MPa on DOT drawings comes from the metric equivalence, ASTM F568M class 4.6 — not from A307. |
| Elongation, min | 18 % | 18 % | In 2 in / 50 mm |
| Hardness | B69–B100 HRB | 121–241 HB | Bolts under 3 × diameter carry the minimum as well as the maximum |
| Stress area | 0.226 in² | 157 mm² (M16) | 5/8-11 UNC · M16 × 2 per ISO 898-1 |
| Minimum tensile load | 13,560 lbf | 60.3 kN | 0.226 in² × 60 ksi |
| Metric material equivalent | — | ASTM F568M cl. 4.6 | 400 MPa tensile / 240 MPa yield · head marked "4.6" |
| Corrosion-resistant option | A325 Type 3 | F568M cl. 8.8.3 | 120 ksi / 830 MPa tensile — weathering steel, where the drawing calls for it |
Chemical composition
| Element | Heat analysis, max | Product analysis, max |
|---|---|---|
| Carbon | 0.29 % | 0.33 % |
| Manganese | 1.20 % | 1.25 % |
| Phosphorus | 0.04 % | 0.041 % |
| Sulfur | 0.15 % | — |
ASTM A307 Table 1. Resulfurized steel is not subject to rejection on product analysis for sulfur. Older state DOT training documents quote a 0.90 % manganese limit; that is a superseded edition of A307 — check the edition your specification calls for.
Nuts, washers and coating
What goes with it
- Recessed nut — the AASHTO M180 nut, recessed on both faces so the bolt end sits inside it. 5/8-11 UNC-2B.
- Nut grade — ASTM A563 Grade A hex is the A307 pairing. Several DOTs specify A563 DH heavy hex for guardrail; follow the project.
- Washers — ASTM F844 plain under general hardware; F436 hardened where the drawing calls for it; a rectangular plate washer behind the rail at post bolts.
- Galvanizing — ASTM A153 Class C, or F2329. Zinc-coated nuts are tapped oversize per A563, so they still run on the coated thread.
Why the shape
A carriage bolt has a square neck. A guardrail bolt has an oval one, because the slot in a W-beam is oval — 5/8 × 1¼ in — and the oval fills it. That does two things: it stops the bolt turning while the nut is tightened from behind, and it lets the rail move slightly along the slot as the barrier is set out, which matters over a 6 ft 3 in post spacing.
The domed head is there so that a vehicle sliding along the barrier meets a smooth surface. That is why the projection limit is 1/4 in and why no hex head is permitted on the traffic face.
Two things to get right when you order. First, guardrail bolts on federally funded highway work usually carry a domestic origin requirement — check the project before sourcing. Second, "guardrail bolt" alone is not enough: state the length, because the thread length follows from it, and say whether you need the recessed nut and the plate washer, which are separate items.
How it fits the rail
Four steps. Press play, or step through them.
Step 1 of 4
What MF supplies
Stock and supply
- Guardrail splice bolts and post bolts to AASHTO M180, ASTM A307 Grade A, hot-dip galvanized to A153 Class C.
- Recessed nuts and washers to match, supplied as a set.
- Plate washers manufactured at Umm Al Quwain to the rectangular sizes the standard drawings call for.
- EN 10204 3.1 certificate with every delivery, traceable to the heat.
Ask us for
- A take-off from your guardrail layout — splice bolts per splice, post bolts per post, nuts and washers.
- Non-standard lengths, made to order.
- The A325 Type 3 weathering-steel version where the specification calls for it.