Mushroom Head Carriage Bolts & Dome Head Bolts: Manufacturer & Stockist
A dome head bolt shows only a smooth, rounded head on the finished side. There is nothing to grip, so it cannot be undone from the outside; a shaped neck under the head holds it still while the nut is tightened from behind.
Exotic Alloy manufactures dome head bolts in Mumbai, India, for trailer and truck-body builders, playground, fencing and furniture makers, and timber construction. We stock across India and supply Sri Lanka, the Gulf, Africa and Europe. Heads and necks are cold formed in one piece, and threads are rolled.
One head, six necks
ASME B18.5 uses the same round head with a plain, square, short square, ribbed or fin neck, plus a step bolt. DIN 607 adds a metric cup head with a nib. The square neck version is the classic carriage bolt, covered in depth on its own page together with DIN 603.
- Head
- Smooth dome, flat bearing face
- Anti-turn
- Square, ribbed or fin neck; nib (DIN 607)
- Length
- From under the head
- Thread
- 2D + 1/4″ up to 6″ long (B18.5)
- Grades
- 18-8, 316, low-carbon, Grade 5
- Related
- Carriage bolts, mushroom head, elevator bolts
Dome Head Bolt Specification
The range we manufacture and supply.
Dome Head Bolt Neck Styles Compared
The head stays the same; the neck decides what the bolt grips and how it stops turning.
| Neck | Standard | Holds by | Best in |
|---|---|---|---|
| Plain (no neck) | ASME B18.5 Table 1 | – | Where the head can be held |
| Square neck | ASME B18.5 Table 2, DIN 603 | Square seated in the part | Wood, square holes |
| Short square neck | ASME B18.5 Table 3 | Shorter square | Thinner parts |
| Ribbed neck | ASME B18.5 Table 4 | Ribs cut into the hole wall | Round holes in wood or soft material |
| Fin neck | ASME B18.5 Table 5 | Two fins, 180° apart | Thin plywood |
| Nib | DIN 607 | Nib under the head | Parts with a notch, timber |
| Step bolt | ASME B18.5 Table 6 | Square neck | Wood, with a larger head |
Dome Head Bolt Dimensions
| Size | UNC | W head dia. max / min | H head height max / min | B body dia. max / min | Square neck O × P max (carriage) |
|---|---|---|---|---|---|
| No. 10 | #10-24 | .469 / .438 | .114 / .094 | .199 / .182 | – |
| 1/4″ | 1/4-20 | .594 / .563 | .145 / .125 | .260 / .237 | .260 × .156 |
| 5/16″ | 5/16-18 | .719 / .688 | .176 / .156 | .324 / .298 | .324 × .187 |
| 3/8″ | 3/8-16 | .844 / .782 | .208 / .188 | .388 / .360 | .388 × .219 |
| 7/16″ | 7/16-14 | .969 / .907 | .239 / .219 | .452 / .421 | – |
| 1/2″ | 1/2-13 | 1.094 / 1.032 | .270 / .250 | .515 / .483 | .515 × .281 |
| 5/8″ | 5/8-11 | 1.344 / 1.219 | .344 / .313 | .642 / .605 | .642 × .344 |
| 3/4″ | 3/4-10 | 1.594 / 1.469 | .406 / .375 | .768 / .729 | .768 × .406 |
ASME B18.5 round head, common to the neck styles, inches. Square neck width O and depth P for comparison (no square neck data for No. 10 and 7/16″ here).
| Size | Ribs approx. | V over ribs min | M head to ribs ≤ 7/8 / ≥ 1, max | D depth over ribs ≤7/8 / 1–1-1/8 / ≥1-1/4 | R fillet max |
|---|---|---|---|---|---|
| No. 10 | 9 | .210 | .062 / .094 | .250 / .407 / .594 | .031 |
| 1/4″ | 10 | .274 | .062 / .094 | .250 / .407 / .594 | .031 |
| 5/16″ | 12 | .340 | .062 / .094 | .250 / .407 / .594 | .031 |
| 3/8″ | 12 | .405 | .062 / .094 | .250 / .407 / .594 | .031 |
| 7/16″ | 14 | .470 | .062 / .094 | .250 / .407 / .594 | .031 |
| 1/2″ | 16 | .534 | .062 / .094 | .250 / .407 / .594 | .031 |
| 5/8″ | 19 | .660 | .125 / .125 | .313 / .438 / .625 | .062 |
| 3/4″ | 22 | .785 | .125 / .125 | .313 / .438 / .625 | .062 |
ASME B18.5 round head ribbed neck bolts, inches. M and D depend on the bolt length: 7/8″ and shorter, 1 to 1-1/8″, 1-1/4″ and longer.
| Thread | P | dk head dia. max | k head height max / min | ds min | g max (nib) | i min (nib) | DIN 603 head dk for comparison |
|---|---|---|---|---|---|---|---|
| M8 | 1.25 | 16.55 | 6.38 / 5.62 | 7.42 | 3 | 2.3 | 20 |
| M10 | 1.5 | 19.65 | 7.45 / 6.55 | 9.42 | 3.2 | 2.7 | 24 |
| M12 | 1.75 | 24.65 | 9.65 / 8.75 | 11.3 | 3.6 | 3.2 | 30 |
| M16 | 2 | 30.65 | 11.75 / 10.65 | 15.3 | 4.2 | 3.5 | 38 |
DIN 607:2010 cup head nib bolts, product grade C, mm; g and i are the nib dimensions as lettered in the standard. DIN 603 head shown for comparison.
Dome Head Bolt Grades & Tensile Loads
| Grade | Tensile min | Yield min |
|---|---|---|
| Low-carbon | 60,000 psi | 36,000 psi |
| Grade 5 (1/4–1″) | 120,000 psi | 92,000 psi |
| 18-8 / 316 | ASTM F593 values on request | |
Low-carbon and Grade 5 values from the ASME B18.5 ribbed neck bolt chart.
| Size | Stress area in² | Low-carbon min tensile, lbf | Grade 5 min tensile, lbf | Grade 5 min tensile, kN |
|---|---|---|---|---|
| No. 10 | 0.0175 | 1,050 | – | – |
| 1/4″ | 0.0318 | 1,910 | 3,820 | 17.0 |
| 5/16″ | 0.0524 | 3,150 | 6,290 | 28.0 |
| 3/8″ | 0.0775 | 4,650 | 9,300 | 41.4 |
| 7/16″ | 0.1063 | 6,380 | 12,760 | 56.7 |
| 1/2″ | 0.1419 | 8,510 | 17,030 | 75.7 |
| 5/8″ | 0.2260 | 13,560 | 27,120 | 120.6 |
| 3/4″ | 0.3345 | 20,070 | 40,140 | 178.5 |
Our calculation: UNC stress area × minimum tensile strength. Bolt values only; the head, neck and the material under the head can limit the joint first.
Fitting Dome Head Bolts
Drill to the body size
Drill a round hole for the body; ribs and fins cut their own seat.
Seat the neck
Tap the head down so the square, ribs or fins bed fully into the part.
Washer under the nut
Use a washer on the nut side, especially in timber.
Tighten from behind
Only the nut is turned; the neck holds the head still.
Choose the length
Part thickness + washer + nut + a little thread showing.
Re-tighten timber joints
Check the nuts after the wood has settled or dried.
Ribbed Neck Bolt Data & Load Calculator
Pick the size, length and grade to see the rib zone, head size, thread length and the bolt’s minimum loads.
Bolt
Head and rib data from ASME B18.5. Loads are our calculation: UNC stress area × minimum tensile or yield strength. Stainless values on request (ASTM F593).
Result
Dome Head Bolt Materials & Finishes
| Material | Finish | Best for |
|---|---|---|
| 18-8 stainless (304) | Plain, passivated | Outdoor furniture, fencing, play equipment |
| 316 stainless | Plain, passivated | Coastal structures, boats, docks |
| Low-carbon steel | Zinc plated, hot-dip galvanised | Timber framing, general assembly |
| Grade 5 steel | Zinc plated, hot-dip galvanised | Trailers, truck bodies, heavier joints |
How Our Dome Head Bolts Are Made
Wire
Stainless or carbon steel wire, certified by heat.
Heading
Dome and neck (square, ribs, fins or nib) formed together.
Thread rolling
UNC or metric threads rolled to length.
Heat treatment
Quench and temper for Grade 5.
Finishing
Passivation, zinc plating or hot-dip galvanising.
Inspection
Head, neck, ribs and thread checked; lot sealed.
Where Our Dome Head Bolts Are Used
Trailers & truck bodies
Floorboards and side panels on steel frames.
Play equipment
Smooth heads with nothing to catch fingers or clothing.
Timber construction
Decks, pergolas, fencing and gates.
Outdoor furniture
Benches and tables in stainless.
Security fixings
Heads that cannot be gripped from the outside.
Docks & marinas
316 stainless in wet timber.
Dome Head Bolt Inspection & Certification
| Check | Method | Applied to |
|---|---|---|
| Head diameter & height | Callipers: W, H | Every lot |
| Neck | Square O × P; ribs V, M, D; fins; nib | Every lot |
| Thread | GO / NO-GO ring gauges | Every lot |
| Length | From under the head | Every lot |
| Mechanical | Hardness; tensile on Grade 5 | Every heat-treatment lot |
| Material | PMI | Stainless lots |
Every shipment includes
- Mill certificate with heat number
- Dimensional report
- Lot number on every box
On request
- EN 10204 3.1 certificate
- ASTM F593 test report (stainless)
- Galvanising thickness report