Square head and hex head machine bolts that pass through clearance holes and tighten with a nut. We make ASME B18.2.1 bolts from ¼″ to 4″ and galvanized pole-line bolts to IEEE C135.1, in A307, SAE J429, A449 and F593 stainless. Every bolt ships with its matched square or hex nut.
A machine bolt is a square or hex head bolt that passes through clearance holes in the parts and is tightened with a nut. The head is held while the nut is turned. That is the opposite of a cap screw, which is turned by its head into a tapped hole.
ASME B18.2.1 calls these products square bolts and hex bolts; the trade calls both machine bolts. Exotic Alloy manufactures them in Mumbai, India, with a plain underhead and rolled threads, and supplies each bolt with its square or hex nut to ASME B18.2.2. We also make the zinc-coated square head machine bolts used on wood poles and crossarms, to IEEE C135.1. We supply India and export to Kuwait, the UAE, Saudi Arabia, Qatar, Oman, Africa and the Americas.
Standard A307 hex and square machine bolts, plain or hot-dip galvanized, are made in volume at factory prices. Long through-bolts for poles, docks and timber bridges, SAE Grade 5 and A449 bolts, and F593 stainless or silicon bronze bolts are made to your length and thread length.
The range we manufacture. Lengths, thread lengths and points outside it are made to order.
The head shape, the point and the thread length change with the job; the principle stays the same: through the parts, nut on the far side.
The original bolt form. The square head gives a large bearing face and grips timber, so it does not turn while the nut is tightened. Used for wood construction, docks, rail, farm equipment and restoration work, often with a square nut.
The everyday through-bolt for steel, equipment and structures. It has a plain underhead with no washer face and wider tolerances than a hex cap screw, and is held at the head while the nut is turned.
Long square head bolts with one square nut and a cone point. They go through wood poles to hold crossarms, insulator brackets and hardware. They are zinc coated with the nut tapped after galvanizing, to the tensile strengths in IEEE C135.1.
Timber and pole bolts are often threaded 4″ to 8″ or fully, so one bolt length suits several grip thicknesses as the wood shrinks or the layers vary.
Metric product grade C hex bolts in classes 4.6 to 8.8 are the metric equivalent of the A307 hex machine bolt. Square head bolts come with a collar (DIN 478), a short dog point (DIN 479) or both (DIN 480).
For docks, boats, coastal timber, food plants and water treatment, where galvanizing would not last. Supplied with matching F594 stainless or bronze nuts.
| Point | Machine bolt (this page) | Hex cap screw | Machine screw |
|---|---|---|---|
| Standard | ASME B18.2.1 (hex bolt, square bolt) | ASME B18.2.1 (hex cap screw) | ASME B18.6.3 |
| Head | Square or hex, plain underhead | Hex with washer face | Slotted or recessed pan, flat, round, fillister |
| Tolerances | Wider on head and body | Closer; chamfered point | Small-screw tolerances |
| Head height, ½″ hex | 11⁄32″ (0.344″) | 5⁄16″ (0.313″) | — |
| Tightened by | Turning the nut, head held | Turning the head, into a nut or tapped hole | Screwdriver, into a nut or tapped hole |
| Size range | ¼″ to 4″ | ¼″ to 3″ | No. 0 to ¾″ |
| Usual grade | A307 A, galvanized | SAE J429 Gr. 5 / 8, A449 | Low carbon steel, stainless, brass |
The definitions follow the long-standing ASME and Industrial Fasteners Institute usage. A bolt is meant to be tightened by turning a nut; a screw is meant to be tightened by turning its head. Hex cap screw dimensions are listed in the ASME B18.2.1 table on our hex bolts page.
Inch dimensions from ASME B18.2.1. Thread lengths follow the standard rule of 2D + ¼″ for bolts up to 6″ long and 2D + ½″ above. Pole and timber bolts are often threaded longer, to order.
| Size | UNC | E body dia. max / min, in | F across flats max / min, in | G across corners max / min, in | H head height max / min, in | Thread length L ≤ 6″ / L > 6″ |
|---|---|---|---|---|---|---|
| 1/4″ | 20 | 0.260 / 0.237 | 0.375 / 0.362 | 0.530 / 0.498 | 0.188 / 0.156 | 0.75 / 1.00 |
| 5/16″ | 18 | 0.324 / 0.298 | 0.500 / 0.484 | 0.707 / 0.665 | 0.220 / 0.186 | 0.88 / 1.12 |
| 3/8″ | 16 | 0.388 / 0.360 | 0.562 / 0.544 | 0.795 / 0.747 | 0.268 / 0.232 | 1.00 / 1.25 |
| 7/16″ | 14 | 0.452 / 0.421 | 0.625 / 0.603 | 0.884 / 0.828 | 0.316 / 0.278 | 1.12 / 1.38 |
| 1/2″ | 13 | 0.515 / 0.482 | 0.750 / 0.725 | 1.061 / 0.995 | 0.348 / 0.308 | 1.25 / 1.50 |
| 5/8″ | 11 | 0.642 / 0.605 | 0.938 / 0.906 | 1.326 / 1.244 | 0.444 / 0.400 | 1.50 / 1.75 |
| 3/4″ | 10 | 0.768 / 0.729 | 1.125 / 1.088 | 1.591 / 1.494 | 0.524 / 0.476 | 1.75 / 2.00 |
| 7/8″ | 9 | 0.895 / 0.852 | 1.312 / 1.269 | 1.856 / 1.742 | 0.620 / 0.568 | 2.00 / 2.25 |
| 1″ | 8 | 1.022 / 0.976 | 1.500 / 1.450 | 2.121 / 1.991 | 0.684 / 0.628 | 2.25 / 2.50 |
| 1-1/8″ | 7 | 1.149 / 1.098 | 1.688 / 1.631 | 2.386 / 2.239 | 0.780 / 0.720 | 2.50 / 2.75 |
| 1-1/4″ | 7 | 1.277 / 1.223 | 1.875 / 1.812 | 2.652 / 2.489 | 0.876 / 0.812 | 2.75 / 3.00 |
| 1-3/8″ | 6 | 1.404 / 1.345 | 2.062 / 1.994 | 2.917 / 2.738 | 0.940 / 0.872 | 3.00 / 3.25 |
| 1-1/2″ | 6 | 1.531 / 1.470 | 2.250 / 2.175 | 3.182 / 2.986 | 1.036 / 0.964 | 3.25 / 3.50 |
ASME B18.2.1 square bolts, maximum and minimum limits in inches. E is the body diameter, F the width across flats, G the width across corners and H the head height. Square bolts are standardised up to 1½″.
| Size | Threads / in UNC | F across flats basic, in | G across corners min, in | H head height basic, in | Thread length L ≤ 6″, in | Thread length L > 6″, in |
|---|---|---|---|---|---|---|
| 1/4″ | 20 | 7/16 | 0.484 | 0.172 | 0.75 | 1.00 |
| 5/16″ | 18 | 1/2 | 0.552 | 0.219 | 0.88 | 1.12 |
| 3/8″ | 16 | 9/16 | 0.620 | 0.250 | 1.00 | 1.25 |
| 7/16″ | 14 | 5/8 | 0.687 | 0.297 | 1.12 | 1.38 |
| 1/2″ | 13 | 3/4 | 0.826 | 0.344 | 1.25 | 1.50 |
| 5/8″ | 11 | 15/16 | 1.033 | 0.422 | 1.50 | 1.75 |
| 3/4″ | 10 | 1-1/8 | 1.240 | 0.500 | 1.75 | 2.00 |
| 7/8″ | 9 | 1-5/16 | 1.447 | 0.578 | 2.00 | 2.25 |
| 1″ | 8 | 1-1/2 | 1.653 | 0.672 | 2.25 | 2.50 |
| 1-1/8″ | 7 | 1-11/16 | 1.859 | 0.750 | 2.50 | 2.75 |
| 1-1/4″ | 7 | 1-7/8 | 2.066 | 0.844 | 2.75 | 3.00 |
| 1-3/8″ | 6 | 2-1/16 | 2.273 | 0.906 | 3.00 | 3.25 |
| 1-1/2″ | 6 | 2-1/4 | 2.480 | 1.000 | 3.25 | 3.50 |
| 1-3/4″ | 5 | 2-5/8 | 2.893 | 1.156 | 3.75 | 4.00 |
| 2″ | 4-1/2 | 3 | 3.306 | 1.344 | 4.25 | 4.50 |
| 2-1/4″ | 4-1/2 | 3-3/8 | 3.719 | 1.500 | 4.75 | 5.00 |
| 2-1/2″ | 4 | 3-3/4 | 4.133 | 1.656 | 5.25 | 5.50 |
| 2-3/4″ | 4 | 4-1/8 | 4.546 | 1.813 | 5.75 | 6.00 |
| 3″ | 4 | 4-1/2 | 4.959 | 2.000 | 6.25 | 6.50 |
| 3-1/4″ | 4 | 4-7/8 | 5.372 | 2.188 | 6.75 | 7.00 |
| 3-1/2″ | 4 | 5-1/4 | 5.786 | 2.313 | 7.25 | 7.50 |
| 3-3/4″ | 4 | 5-5/8 | 6.198 | 2.500 | 7.75 | 8.00 |
| 4″ | 4 | 6 | 6.612 | 2.688 | 8.25 | 8.50 |
ASME B18.2.1 hex bolts, basic dimensions in inches. Across flats is 1½D for ⅝″ and above. The head heights are those of the hex bolt, which are taller than hex cap screw heads for most sizes. 1⅝″, 1⅞″ and other sizes are available in 8UN thread.
| Size | Threads / in | Min. tensile strength lbf (kN) | Stress area in² | Equivalent stress ksi (MPa), calculated | Head & nut | Finish |
|---|---|---|---|---|---|---|
| 1/2″ | 13 | 7,800 (34.7) | 0.142 | 55.0 (379) | Square head, one square nut | Hot-dip galvanized, ASTM A153 |
| 5/8″ | 11 | 12,400 (55.2) | 0.226 | 54.9 (378) | Square head, one square nut | Hot-dip galvanized, ASTM A153 |
| 3/4″ | 10 | 18,350 (81.6) | 0.334 | 54.9 (379) | Square head, one square nut | Hot-dip galvanized, ASTM A153 |
| 7/8″ | 9 | 25,400 (113.0) | 0.462 | 55.0 (379) | Square head, one square nut | Hot-dip galvanized, ASTM A153 |
Minimum tensile strengths from IEEE C135.1, Table 10, as cited in utility material specifications. The equivalent stress column is our calculation on the UNC stress area: every size works out at about 55 ksi. Lengths, thread lengths and head and nut sizes follow C135.1 and the utility's material specification. Utilities typically require rolled threads, and nuts are tapped oversize after zinc coating.
Most machine bolts are A307 Grade A, the 60 ksi general-purpose grade. Specify a higher grade only where the joint needs it, and a stainless grade where corrosion rules out galvanized steel.
| Grade | Diameter | Tensile ksi (MPa) | Yield min ksi (MPa) | Hardness | Grade mark | Typical machine bolt use |
|---|---|---|---|---|---|---|
| ASTM A307 Grade A | ¼–4″ | 60 (414) min | — | B69–B100 HRB | 307A (grade charts) + maker's mark | General through-bolting, timber, equipment, secondary steel |
| ASTM A307 Grade B | ¼–4″ | 60 (414) – 100 (689) | — | B69–B95 HRB | 307B (grade charts) + maker's mark | Flanged piping joints with cast-iron flanges |
| IEEE C135.1 | ½–⅞″ (tabled) | ≈ 55 on As (see table) | — | — | Maker's mark | Overhead line and pole hardware |
| SAE J429 Grade 2 | ¼–¾″ | 74 (510) min | 57 (393) | B80–B100 HRB | None | Light machinery and agricultural equipment |
| SAE J429 Grade 5 | ¼–1″ | 120 (827) min | 92 (634) | C25–C34 HRC | 3 radial lines | Vehicles, machinery, higher-load through-bolts |
| ASTM A449 Type 1 | ¼–1″ | 120 (827) min | 92 (634) | C25–C34 HRC | 3 radial lines | A325-strength bolts outside the F3125 range |
| ASTM F593 Group 1 / 2, CW1 | ¼–⅝″ | 100 (689) – 150 (1034) | 65 (448) | — | F593C (304) / F593G (316) | Stainless machine bolts: marine, food, water, coastal timber |
| ASTM F593 Group 1 / 2, CW2 | ¾–1½″ | 85 (586) – 140 (965) | 45 (310) | — | F593D (304) / F593H (316) | Larger stainless machine bolts |
ASTM A307 Grade A and B both have 18 % minimum elongation. Grade C was withdrawn in 2007 and replaced by ASTM F1554 Grade 36 for anchor bolts. A307 requires the manufacturer's identification on every head; the 307A / 307B grade marks are shown as they appear in industry grade-marking charts. Higher-strength SAE J429 grades above 1″ have lower values; see the heavy hex grade tables.
Every grade links to its datasheet on this site.
1010 to 1022 type steels hot or cold forged into square and hex heads. They are tough and easy to galvanize, and make up the bulk of all machine bolts.
For poles, fences, docks and outdoor timber the low carbon bolt is hot-dip galvanized with its nut. That is the standard outdoor machine bolt.
1035 to 1045 type steels, quenched and tempered to 120 ksi for machine bolts that carry real load in vehicles and machinery.
Machine bolts are made in volume, so the process is about consistency: the same head, thread and coating on every piece of a lot.
Low and medium carbon steel wire rod or bar received by heat with mill certificates; stainless and bronze PMI-checked.
Square and hex heads cold headed on multi-blow headers up to about ¾″. Larger diameters and long pole bolts are hot forged, with a plain bearing face and no washer face.
Hex heads trimmed; square heads forged to size. Ends pointed flat, chamfered or cone pointed for pole-line bolts that are driven into bored holes.
UNC threads rolled to class 2A, to the standard 2D + ¼″ length or to the longer thread a pole or timber bolt needs. Rolling keeps the grain unbroken at the root.
None for A307 and Grade 2. Quench and temper for SAE Grade 5 / 8, A449 and B7. Stainless is cold worked to the F593 condition.
Bolts hot-dip galvanized and spun to clear the threads; nuts galvanized as blanks and tapped oversize afterwards, so the zinc on both still lets them assemble.
Tensile and hardness tests per lot, thread and dimension checks, coating thickness. Nuts assembled or packed with the bolts, then labelled by size, length and quantity.
Hot-dip galvanizing adds a zinc layer to the thread flanks of the bolt. A standard nut will not run onto it. So nuts are galvanized as blanks and tapped oversize afterwards, which leaves room for the zinc on the bolt while the nut's own threads stay bare. They are protected by the zinc on the bolt.
That is also why galvanized and plain parts must not be mixed. An overtapped nut on a plain bolt is a loose fit and loses strength. We ship galvanized machine bolts with their own nuts, assembled or in the same pack.
Most machine bolts work outdoors in timber and steel, so the finish usually decides their life. See also our coated fasteners page.
| Finish | Specification | Use | Notes for machine bolts |
|---|---|---|---|
| Plain (self colour) | As manufactured, lightly oiled | Indoor equipment, painted after assembly | Lowest cost; rusts outdoors |
| Hot-dip galvanized | ASTM A153 / ASTM F2329 | Poles, timber, fencing, outdoor steel | The standard outdoor finish and the one IEEE C135.1 calls for; nuts tapped oversize after galvanizing |
| Mechanically galvanized | ASTM B695 | Where a smoother, more even thread coat is wanted | No molten zinc; thickness chosen by class |
| Zinc electroplate | ASTM F1941 / B633 | Indoor and sheltered equipment | Thin and bright; not for outdoor timber |
| Passivated stainless | ASTM A967 | Marine, food, water treatment | Anti-seize recommended to stop galling between stainless bolt and nut |
| Bright bronze | As machined | Boats and marine timber | Needs no coating; weathers to a dark brown |
Enter the grip and washers to get the bolt length you need. The tool also checks that the thread starts inside the grip and estimates the weight per bolt and per 100 for shipping.
Minimum length = grip + washers + nut (≈ 0.875 D, ASME B18.2.2 basic) + projection. It is rounded up to the next ½″ up to 6″ and the next 1″ above, our usual stock steps; any length is made to order. Weight counts the head from ASME B18.2.1 dimensions and the shank as a full-diameter cylinder (threads make it slightly lighter). Nut not included.
Galvanized square head machine bolts to IEEE C135.1 for crossarms, insulator brackets, transformer mounts and hardware on wood poles.
Through-bolts for heavy timber frames, decks, docks, piers, timber bridges and retaining walls, with plate or square washers under head and nut.
Hex and square machine bolts in A307 and Grade 5 for implements, conveyors, hoppers and chutes, where simple, replaceable bolting is wanted.
A307 hex machine bolts for purlins, girts, stairs, handrails and other connections that do not need structural A325 bolts.
A307 Grade B machine bolts with heavy hex or square nuts, whose capped tensile strength protects brittle cast-iron flanges from over-tightening.
316 stainless and silicon bronze machine bolts for boat building, pontoons, sea walls and coastal timber structures.
| Test | Method | Applied to |
|---|---|---|
| Chemical analysis | Mill certificate; OES on request | Every heat; PMI on stainless and bronze |
| Tensile / wedge tensile | ASTM F606, full-size bolt | Every lot, to the grade or to IEEE C135.1 minimum tensile |
| Hardness | Rockwell B or C | Every lot (A307: B69–B100; Grade 5: C25–C34) |
| Proof load | ASTM F606 | SAE Grade 5 / 8 and A449 |
| Thread fit | Go / no-go gauges to ASME B1.2; nut run-on after galvanizing | Every lot |
| Dimensions | Head across flats and corners, head height, length, thread length | Every lot, to ASME B18.2.1 limits |
| Coating | Magnetic thickness gauge; ASTM A153 / F2329 requirements | Every galvanized lot |
Tell us the head, diameter, length, thread length, grade, finish and quantity, and whether nuts and washers are needed. Our engineers respond within 2 to 4 business hours.