304 (A2) · 316 (A4) · A4-80 · Alloy Steel · Class 10.9 · Class 12.9
Socket Countersunk Screw
Socket countersunk screws (countersunk cap screws) in steel, 304 and 316 stainless and zinc flake, metric and imperial.
About the Socket Countersunk Screw
Countersunk socket screws sit flush in a countersunk hole, for hinges, machine guards, slides, trailers and anywhere nothing can stick up. Metric ones come to DIN 7991 and ISO 10642 from M2 to M24 in class 012.9 alloy steel (plain, zinc plated or zinc flake), class 010.9 (plain or zinc plated), 304 stainless with hex socket or Torx drive, 316 stainless, and Neptune and Bumax 88 high strength stainless; imperial ones run #4 to 1 inch in UNC, UNF and BSW alloy steel and 304 stainless. Choose the material, size and length above; the guide below covers why countersunk heads are reduced strength, head and key sizes and torques.
Also called countersunk bolts, flat head socket screws or CSK socket screws.
Features
- Flush countersunk head
- Hex-key and Torx drive
- High-tensile and stainless
- DIN 7991 / ISO 10642
Applications
- Flush fixing hinges, slides and guards
- Machinery and tooling
- Trailers and vehicle fit-out
What are countersunk bolts?
A countersunk socket screw has a cone-shaped head that pulls down into a countersunk hole and finishes flush with the surface, with a hex socket (Allen key) or Torx drive in the top. Builders and fitters usually call them countersunk bolts; Hobson's descriptions call them flat head socket screws (FHSS). Hobson lists them for jobs that need a flush or smooth finish with no protrusion, in the automotive, machinery and construction industries.
The head angle matters, because the countersink has to match it. Hobson's metric DIN 7991 sheet gives a head angle of 92/90 degrees up to M20, so use a 90 degree countersink for metric screws up to M20. Hobson's sheet for its zinc plated UNC countersunk screws gives an 82 degree taper for those screws; for the BSW, UNF and stainless imperial screws, and for M24, check the head angle before you countersink.
Countersunk heads are reduced strength: 010.9 and 012.9
A countersunk socket screw is not a full strength bolt. Hobson's engineering article explains that the stress area in a countersunk head is smaller than in the thread, so these screws cannot reach the full tensile capacity of their property class: Hobson puts their tensile strength at 80% of the class. The head marking carries a leading 0 to show it (08.8, 010.9, 012.9), although Hobson notes that in the Australian market the marking is often left off.
Ordinary bolts are designed to stretch and fail in the thread, which gives warning; a reduced loadability screw fails in the head, and Hobson warns the failure can be sudden. Hobson says countersunk and button head socket screws are used for their appearance and not for critical loading, and are not recommended for safety-critical connections, where a standard socket head cap screw should be used. Hobson's torque sheets give tensile, yield and clamp load figures for the alloy steel screws, but these are product figures, not joint ratings; for structural, lifting or safety joints use the fastener an engineer has specified.
Class 012.9 or 010.9? Hobson's article says most reduced loadability socket screws supplied to Europe are now class 010.9, that class 12.9 is much more prone to hydrogen embrittlement and stress corrosion cracking, and that before choosing 12.9 or 012.9 you should consider whether the extra strength is needed and the risks that come with it. It describes a customer whose 012.9 countersunk screws were failing soon after installation; changing to 010.9 stopped the failures. Choose 012.9 only where the extra strength is actually needed.
Which material: alloy steel, zinc, zinc flake or stainless?
- Plain alloy steel (012.9 and 010.9, plus imperial ASTM F835): no coating, supplied lightly oiled. For oiled machinery and dry indoor work.
- Zinc plated (012.9, 010.9 and imperial): a 5 micron zinc coating that Hobson recommends for indoor and non-corrosive environments.
- Zinc flake silver (012.9): a 10 micron coating that Hobson rates as high corrosion resistance with no risk of hydrogen embrittlement.
- 304 stainless (A2): metric ISO 10642 with hex socket, metric Torx (M3 to M8), and imperial UNC and UNF. Hobson says Torx drive increases the torque you can apply, prevents cam out and helps the screw last.
- 316 stainless (A4): metric ISO 10642 from M2 to M20 and a DIN 7991 M24 x 120mm. Molybdenum gives better resistance near the sea, but neither 304 nor 316 is made to sit in seawater, and even 316 can visibly corrode where salt spray is left to build up.
- Neptune and Bumax 88 high strength stainless: Neptune countersunk screws (A4-80) in M6 x 25mm and M12 x 30mm, and Bumax 88 from M3 to M16, which Bumax makes from 316L high molybdenum steel with strength properties that correspond to 8.8 carbon steel fasteners.
- Outdoors: Hobson's corrosion guide puts exposed outdoor fixings in stainless or hot dip galvanised (zinc plating only for enclosed, low corrosion areas). We carry no galvanised countersunk socket screws, so use 304 stainless outdoors and 316 near the coast or a pool.
Metric countersunk bolt sizes and lengths
Every metric countersunk socket screw we carry, by material, smallest to largest. M6 and M8 are the countersunk bolt sizes people search for most. Lengths are overall, measured from the top of the head, because the head sits flush. Select a row to open that size in the selector, then pick the length.
Metric countersunk socket screws, coarse thread. Alloy steel to DIN 7991, stainless to ISO 10642 (the 316 M24 is DIN 7991). Lengths in mm. Pack is the minimum order quantity. Bumax 88 is sold singly. Tap a number to show it in the selector above.
| Size | Material and class | Lengths (mm) | Pack |
|---|---|---|---|
| Plain alloy steel, class 012.9 | |||
| M3 | Plain alloy steel, class 012.9 | 6, 8, 10, 12, 16, 20, 25, 30 | 100 |
| M4 | Plain alloy steel, class 012.9 | 8, 10, 12, 16, 20, 25, 30, 35, 40 | 100 |
| M5 | Plain alloy steel, class 012.9 | 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50 | 100 |
| M6 | Plain alloy steel, class 012.9 | 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50 | 100 |
| M8 | Plain alloy steel, class 012.9 | 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 75, 80 | 100 |
| M10 | Plain alloy steel, class 012.9 | 12, 16, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 80, 90, 100 | 50 to 100 |
| M12 | Plain alloy steel, class 012.9 | 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 80, 90, 100 | 50 to 100 |
| M14 | Plain alloy steel, class 012.9 | 25, 30, 50 | 50 |
| M16 | Plain alloy steel, class 012.9 | 30, 35, 40, 45, 50, 55, 60, 65, 70, 80, 90, 100, 110, 120, 140, 160, 180, 200, 240 | 10 to 50 |
| M20 | Plain alloy steel, class 012.9 | 35, 40, 45, 50, 55, 60, 70, 75, 80, 90, 100, 120, 140, 150, 160, 180, 200, 240 | 5 to 25 |
| M24 | Plain alloy steel, class 012.9 | 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 180, 200 | 5 to 10 |
| Zinc plated, class 012.9 | |||
| M3 | Zinc plated, class 012.9 | 8, 10, 12, 30 | 100 |
| M4 | Zinc plated, class 012.9 | 8, 10, 12, 16, 20, 25, 30, 50 | 100 to 200 |
| M5 | Zinc plated, class 012.9 | 10, 12, 16, 20, 25, 30, 60 | 100 |
| M6 | Zinc plated, class 012.9 | 10, 12, 16, 20, 25, 30, 35, 40, 50, 75 | 100 |
| M8 | Zinc plated, class 012.9 | 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 65, 75, 90 | 100 |
| M10 | Zinc plated, class 012.9 | 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 80, 85 | 50 to 100 |
| M12 | Zinc plated, class 012.9 | 20, 25, 30, 35, 40, 45, 50, 55, 80 | 50 to 100 |
| M16 | Zinc plated, class 012.9 | 30, 40, 50, 55, 65, 120 | 25 to 50 |
| M20 | Zinc plated, class 012.9 | 35, 40, 45, 50, 55, 60, 70, 80, 90, 100, 120 | 10 to 25 |
| Zinc flake silver, class 012.9 | |||
| M3 | Zinc flake silver, class 012.9 | 10 | 100 |
| M4 | Zinc flake silver, class 012.9 | 12, 16, 20, 25 | 100 to 200 |
| M5 | Zinc flake silver, class 012.9 | 12, 16, 20, 25, 45, 50 | 100 |
| M6 | Zinc flake silver, class 012.9 | 12, 16, 20, 25, 40 | 100 |
| M8 | Zinc flake silver, class 012.9 | 16, 20, 25, 30, 35, 40, 45, 60, 75, 80, 100, 110 | 100 |
| M10 | Zinc flake silver, class 012.9 | 16, 20, 25, 30, 35, 40, 50, 70, 100 | 50 to 100 |
| M12 | Zinc flake silver, class 012.9 | 20, 25, 30, 35, 40, 45, 50, 55, 60, 80 | 50 to 100 |
| M16 | Zinc flake silver, class 012.9 | 30, 40, 45, 50, 60, 65, 70, 80, 90, 100 | 25 to 50 |
| M20 | Zinc flake silver, class 012.9 | 40, 45, 50, 70, 80, 100, 150 | 10 to 25 |
| Plain alloy steel, class 010.9 | |||
| M5 | Plain alloy steel, class 010.9 | 12, 20, 25 | 100 |
| M8 | Plain alloy steel, class 010.9 | 35, 40, 50, 70 | 100 |
| M10 | Plain alloy steel, class 010.9 | 20, 25, 40, 50 | 100 |
| M12 | Plain alloy steel, class 010.9 | 35, 40, 45, 50 | 50 to 100 |
| M16 | Plain alloy steel, class 010.9 | 50, 55, 60, 70, 80, 100, 110 | 25 |
| M20 | Plain alloy steel, class 010.9 | 40, 60, 80, 100, 120, 150, 160 | 10 to 25 |
| M24 | Plain alloy steel, class 010.9 | 60, 120 | 5 to 10 |
| Zinc plated, class 010.9 | |||
| M5 | Zinc plated, class 010.9 | 10, 12, 16, 20, 25, 30 | 100 |
| M6 | Zinc plated, class 010.9 | 12, 16, 20, 25, 30, 35, 40 | 100 |
| M8 | Zinc plated, class 010.9 | 16, 20, 25, 30, 35, 40, 50 | 100 |
| M10 | Zinc plated, class 010.9 | 20, 25, 30, 35, 40, 45, 50 | 100 |
| M12 | Zinc plated, class 010.9 | 25, 30, 35, 40, 45, 50 | 50 to 100 |
| M16 | Zinc plated, class 010.9 | 40, 50, 65 | 25 to 50 |
| M24 | Zinc plated, class 010.9 | 60 | 10 |
| 304 stainless, hex socket, A2 | |||
| M3 | 304 stainless, hex socket, A2 | 5, 6, 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50 | 100 |
| M4 | 304 stainless, hex socket, A2 | 6, 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60 | 100 |
| M5 | 304 stainless, hex socket, A2 | 6, 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 80 | 100 |
| M6 | 304 stainless, hex socket, A2 | 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 | 50 to 100 |
| M8 | 304 stainless, hex socket, A2 | 12, 16, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 75, 80, 90, 100, 120 | 50 to 100 |
| M10 | 304 stainless, hex socket, A2 | 12, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 120, 130, 150 | 25 to 50 |
| M12 | 304 stainless, hex socket, A2 | 25, 30, 35, 40, 45, 50, 60, 65, 70, 80, 90, 100, 120, 130, 150 | 25 |
| M16 | 304 stainless, hex socket, A2 | 25, 30, 40, 50, 60, 70, 80, 90, 100, 120, 150 | 10 to 25 |
| M20 | 304 stainless, hex socket, A2 | 40, 50, 60, 70, 80, 90, 100, 120, 150 | 10 to 25 |
| 304 stainless, Torx, A2 | |||
| M3 | 304 stainless, Torx, A2 | 6, 8, 10, 12, 16 | 100 |
| M4 | 304 stainless, Torx, A2 | 6, 8, 10, 12, 16, 20, 25 | 100 |
| M5 | 304 stainless, Torx, A2 | 8, 10, 12, 16, 20, 25 | 100 |
| M6 | 304 stainless, Torx, A2 | 8, 10, 12, 16, 20, 25, 30 | 100 |
| M8 | 304 stainless, Torx, A2 | 16, 20, 30, 35, 40 | 100 |
| 316 stainless, A4 | |||
| M2 | 316 stainless, A4 | 4, 5, 6, 8, 10, 12, 16, 20 | 100 |
| M2.5 | 316 stainless, A4 | 4, 5, 6, 8, 10, 12, 16, 20 | 100 |
| M3 | 316 stainless, A4 | 4, 5, 6, 8, 10, 12, 16, 20, 25, 30, 35, 40, 50 | 100 |
| M4 | 316 stainless, A4 | 6, 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80 | 100 |
| M5 | 316 stainless, A4 | 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60 | 100 |
| M6 | 316 stainless, A4 | 8, 10, 12, 16, 20, 25, 30, 35, 40, 50, 60, 70, 80 | 50 to 100 |
| M8 | 316 stainless, A4 | 16, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 75, 80, 90, 100 | 50 to 100 |
| M10 | 316 stainless, A4 | 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 | 50 |
| M12 | 316 stainless, A4 | 20, 25, 30, 35, 40, 45, 50, 60, 80, 90, 100 | 25 |
| M14 | 316 stainless, A4 | 40 | 25 |
| M16 | 316 stainless, A4 | 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 120 | 10 to 25 |
| M20 | 316 stainless, A4 | 40, 50, 60, 70, 80, 90, 100, 200 | 5 to 10 |
| M24 | 316 stainless, A4 | 120 | 5 |
| Neptune 316 stainless, A4-80 | |||
| M6 | Neptune 316 stainless, A4-80 | 25 | 100 |
| M12 | Neptune 316 stainless, A4-80 | 30 | 25 |
| Bumax 88 stainless (316L) | |||
| M3 | Bumax 88 stainless (316L) | 6, 8, 10, 12, 16, 20 | 1 |
| M4 | Bumax 88 stainless (316L) | 10, 12, 16, 20, 25, 30 | 1 |
| M5 | Bumax 88 stainless (316L) | 10, 12, 16, 20, 25, 30 | 1 |
| M6 | Bumax 88 stainless (316L) | 10, 12, 16, 20, 25, 30, 40, 50, 60 | 1 |
| M8 | Bumax 88 stainless (316L) | 16, 20, 25, 30, 35, 40, 50, 60, 70 | 1 |
| M10 | Bumax 88 stainless (316L) | 20, 25, 30, 35, 40, 50 | 1 |
| M12 | Bumax 88 stainless (316L) | 30, 40, 45, 50, 60, 70, 80 | 1 |
| M16 | Bumax 88 stainless (316L) | 40, 50, 60, 70, 80, 100 | 1 |
Imperial countersunk socket screws: UNC, UNF and BSW
Imperial countersunk socket screws are sized by gauge (#4 to #10) or inch diameter, with a UNC (coarse), UNF (fine) or BSW thread. The alloy steel screws are to ASTM F835 (UNC and UNF to ANSI B18.3, BSW to BS 2470); the 304 stainless ones are to ANSI B18.3.
Imperial countersunk socket screws, hex socket drive. Lengths in inches, overall. Pack is the minimum order quantity. Tap a number to show it in the selector above.
| Size | Thread | Material | Lengths | Pack |
|---|---|---|---|---|
| Plain alloy steel | ||||
| #4 | UNC | Plain alloy steel | 1/4", 3/8", 1/2", 5/8", 3/4" | 100 |
| #5 | UNC | Plain alloy steel | 1/4", 3/8", 1/2", 5/8", 3/4" | 100 |
| #6 | UNC | Plain alloy steel | 1/4", 3/8", 1/2", 5/8", 3/4" | 100 |
| #8 | UNC | Plain alloy steel | 3/8", 1/2", 5/8", 3/4", 1" | 100 |
| #10 | UNC | Plain alloy steel | 3/8", 1/2", 5/8", 3/4", 1", 1-1/4", 1-1/2" | 100 |
| 1/4 inch | UNC | Plain alloy steel | 1/2", 5/8", 3/4", 1", 1-1/4", 1-1/2", 1-3/4", 2" | 100 |
| 5/16 inch | UNC | Plain alloy steel | 1/2", 5/8", 3/4", 1", 1-1/4", 1-1/2", 2", 2-1/4", 2-1/2" | 100 |
| 3/8 inch | UNC | Plain alloy steel | 1/2", 3/4", 1", 1-1/4", 1-1/2", 1-3/4", 2", 2-1/2", 3" | 50 to 100 |
| 7/16 inch | UNC | Plain alloy steel | 1", 1-1/4" | 50 to 100 |
| 1/2 inch | UNC | Plain alloy steel | 3/4", 1", 1-1/4", 1-1/2", 1-3/4", 2", 2-1/2", 3" | 25 to 100 |
| 5/8 inch | UNC | Plain alloy steel | 1-1/4", 1-1/2", 1-3/4", 2", 2-1/2", 3", 3-1/2", 4" | 25 |
| 3/4 inch | UNC | Plain alloy steel | 1-1/2", 1-3/4", 2", 2-1/4", 2-1/2", 3", 3-1/2", 4" | 25 |
| 1 inch | UNC | Plain alloy steel | 2", 3", 4", 5", 6" | 10 |
| #10 | UNF | Plain alloy steel | 3/8", 1/2", 5/8", 3/4", 1", 1-1/4", 1-1/2" | 100 |
| 1/4 inch | UNF | Plain alloy steel | 3/8", 1/2", 5/8", 3/4", 1", 1-1/4", 1-1/2", 1-3/4", 2", 2-1/2" | 100 |
| 5/16 inch | UNF | Plain alloy steel | 1/2", 5/8", 3/4", 1", 1-1/4", 1-1/2", 2", 2-1/4", 2-1/2" | 100 |
| 3/8 inch | UNF | Plain alloy steel | 1/2", 3/4", 1", 1-1/4", 1-1/2", 1-3/4", 2", 2-1/2", 3" | 50 to 100 |
| 1/2 inch | UNF | Plain alloy steel | 3/4", 1", 1-1/4", 1-1/2", 1-3/4", 2", 2-1/2", 3" | 25 to 100 |
| 3/16 inch | BSW | Plain alloy steel | 3/8", 1/2", 5/8", 3/4", 1", 1-1/4", 1-1/2" | 200 |
| 1/4 inch | BSW | Plain alloy steel | 3/8", 1/2", 5/8", 3/4", 1", 1-1/4", 1-1/2", 1-3/4", 2", 2-1/2" | 100 to 200 |
| 5/16 inch | BSW | Plain alloy steel | 1/2", 5/8", 3/4", 1", 1-1/4", 1-1/2", 2", 2-1/4", 2-1/2" | 100 to 200 |
| 3/8 inch | BSW | Plain alloy steel | 1/2", 3/4", 1", 1-1/4", 1-1/2", 1-3/4", 2", 2-1/2", 3" | 50 to 200 |
| 1/2 inch | BSW | Plain alloy steel | 3/4", 1", 1-1/4", 1-1/2", 1-3/4", 2", 2-1/2", 3" | 25 to 100 |
| 5/8 inch | BSW | Plain alloy steel | 1-1/4", 1-1/2", 1-3/4", 2", 2-1/2", 3", 3-1/2", 4" | 25 to 50 |
| 3/4 inch | BSW | Plain alloy steel | 1-1/2", 1-3/4", 2", 2-1/2", 3", 3-1/2", 4" | 25 |
| Zinc plated alloy steel | ||||
| 3/8 inch | UNC | Zinc plated alloy steel | 1/2", 3/4", 1", 1-1/4", 1-1/2", 1-3/4", 2", 2-1/2" | 50 to 100 |
| 1/2 inch | UNC | Zinc plated alloy steel | 3/4", 1", 1-1/4", 1-1/2", 1-3/4", 2", 2-1/4", 2-1/2", 3" | 25 to 100 |
| 304 stainless | ||||
| #4 | UNC | 304 stainless | 1/4", 3/8" | 100 |
| #6 | UNC | 304 stainless | 3/8", 1/2" | 100 |
| #8 | UNC | 304 stainless | 3/4" | 100 |
| #10 | UNC | 304 stainless | 3/8", 1/2", 5/8", 3/4", 1" | 100 |
| 1/4 inch | UNC | 304 stainless | 3/8", 1/2", 5/8", 3/4", 1", 1-1/4", 1-1/2", 2" | 100 |
| 5/16 inch | UNC | 304 stainless | 3/4", 1" | 100 |
| 3/8 inch | UNC | 304 stainless | 2-3/4" | 50 |
| 1/4 inch | UNF | 304 stainless | 1/2", 5/8", 3/4" | 100 |
| 3/8 inch | UNF | 304 stainless | 3/4" | 50 |
Countersunk head sizes and Allen key sizes
The head diameter tells you how big the countersink must be, and the key size which Allen key or bit you need. An M6 countersunk bolt takes a 4mm Allen key and an M8 takes a 5mm key. Hobson notes the drive in a countersunk head is smaller than in a standard socket head cap screw, which limits the torque you can apply before the bit or the socket is damaged.
Head diameter, head height and hex key (across flats) in mm for DIN 7991 alloy steel countersunk screws, from Hobson's 010.9 and 012.9 torque sheets; M2 to M3 key sizes from Hobson's 316 sheet. Torx bit sizes are for the 304 Torx range (M3 to M8). Stainless ISO 10642 heads may differ slightly. Tap a number to show it in the selector above.
Countersunk bolt torque
Guide tightening torques from Hobson's DIN 7991 countersunk sheets, for metric coarse thread alloy steel screws in class 010.9 and 012.9, first tightening of a single assembly. Hobson works the clamp load at a bolt stress of 360MPa for 010.9 and 420MPa for 012.9 (lower than a standard cap screw, because the smaller drive limits the torque you can apply), with a k factor of 0.2 for plain, 0.14 for zinc plated and 0.22 for zinc flake. They are a guide only; critical joints need a proper tension method set by an engineer.
They do not apply to the stainless, Neptune or Bumax screws: Hobson publishes no countersunk torques for those. For imperial screws, Hobson's UNC torque sheet has a countersunk head column (for example 3/8 UNC: 29.2 lb-ft plain, 20.4 lb-ft zinc plated); ask us for the UNC or UNF sheet.
Guide tightening torque in Nm for metric alloy steel countersunk socket screws to DIN 7991, from Hobson's SC10 (class 010.9) and SCSO (class 012.9) sheets. We carry 010.9 in plain and zinc plated, and 012.9 in plain, zinc plated and zinc flake. Not for stainless, Neptune or Bumax screws. In M24 we carry 012.9 plain and 010.9 plain and zinc plated; the 012.9 zinc plated and zinc flake M24 values are shown for reference only. Tap a number to show it in the selector above.
Fitting countersunk bolts
- Countersink the hole to the right angle (90 degrees for metric up to M20, 82 degrees for the zinc plated UNC screws; check the head for other imperial screws and M24) and to the head diameter, so the whole cone of the head bears on the countersink.
- Choose the length from the top of the head: the screw's length includes the head.
- Use the right size Allen key or Torx bit, pushed fully into the socket, and do not exceed the torque for the class and finish.
- For stainless screws, slow the installation speed and lubricate the thread to avoid galling (threads seizing). Hobson also suggests using a nut of a different stainless grade, such as a 304 nut on a 316 screw.
Countersunk bolts in Adelaide
Allmat supplies countersunk socket screws, cap screws, button heads, nuts and driver bits to fabricators, machinists, shopfitters and trade across Adelaide and South Australia. Add the screws to a quote with the nuts and bits that go with them, and we will price the lot.
Countersunk bolts for the job
Countersunk socket screws in alloy steel and stainless, with the nuts and driver bits to go with them, are available to order from Allmat for fabricators, machinists, mechanics, shopfitters, builders and trade across Adelaide and South Australia. Order through Allmat, we pick and pack it, then click and collect from the Allmat yard or have it delivered to site.
Goes with
Questions
Are countersunk bolts as strong as normal bolts?
No. Hobson classes countersunk socket screws as reduced loadability: the head has a smaller stress area than the thread, tensile strength is 80% of the property class, and the head is marked with a leading 0 (010.9, 012.9). Hobson does not recommend them for safety-critical connections; use a standard socket head cap screw, or the full strength fastener the engineer specifies, there.
What angle is a countersunk bolt head?
Hobson's metric DIN 7991 sheet gives 92/90 degrees up to M20, so use a 90 degree countersink for metric screws up to M20. Its sheet for the zinc plated UNC countersunk screws gives an 82 degree taper for those; for BSW, UNF and stainless imperial screws, and for M24, check the head angle before you countersink.
What size Allen key fits an M6 countersunk bolt?
A 4mm hex key. M4 takes 2.5mm, M5 3mm, M8 5mm, M10 6mm, M12 8mm and M16 10mm, from Hobson's DIN 7991 sheets. The 304 Torx screws take T10 (M3) to T45 (M8).
What torque for an M8 countersunk bolt?
As a guide from Hobson's DIN 7991 sheets: 21.1Nm plain or 14.8Nm zinc plated in class 010.9, and 24.6Nm plain, 17.2Nm zinc plated or 27.1Nm zinc flake in class 012.9, first tightening. Hobson gives no countersunk torques for stainless.
Do you have stainless countersunk bolts?
Yes: 304 stainless with hex socket from M3 to M20 and Torx from M3 to M8, 316 stainless from M2 to M24, Neptune A4-80 and Bumax 88, plus imperial 304 in UNC and UNF. Use 316 near the coast or a pool, and rinse salt off.
How is a countersunk bolt measured?
By thread size and overall length, including the head, because the head sits flush in the countersink. An M8 x 30 is an M8 thread, 30mm long from the top of the head to the tip.
Where can I buy countersunk bolts in Adelaide?
Order through Allmat. Order ahead by phone on 1300 03 03 63 or send your list for a quote, then click and collect from 25 Kapara Road, Gillman, or have them delivered to site.
Socket Countersunk Screw: sizes and finishes we supply
915 lines by type and finish. Pick one above for its product code and pack sizes.
| Head | Finish | Sizes | Lengths | Lines |
|---|---|---|---|---|
| Countersunk | 316 Stainless | M2, M2.5, M3, M4, M5, M6, M8, M10, M12, M14, M16, M20, M24 | 4 to 200 (23 lengths) | 186 |
| Countersunk | Steel | 3/16, 1/4, 5/16, 3/8, 7/16, 1/2, 5/8, 3/4, 1, #4, #5, #6, #8, #10 | 1/4 to 6 inch (17 lengths) | 185 |
| Countersunk | Steel | M3, M4, M5, M6, M8, M10, M12, M14, M16, M20, M24 | 6 to 240 (29 lengths) | 167 |
| Countersunk | 304 Stainless | M3, M4, M5, M6, M8, M10, M12, M16, M20 | 5 to 150 (24 lengths) | 154 |
| Countersunk | Zinc Plated | M3, M4, M5, M6, M8, M10, M12, M16, M20, M24 | 8 to 120 (21 lengths) | 117 |
| Countersunk | Zinc Flake Silver | M3, M4, M5, M6, M8, M10, M12, M16, M20 | 10 to 150 (20 lengths) | 64 |
| Countersunk | 304 Stainless | 1/4, 5/16, 3/8, #4, #6, #8, #10 | 1/4 to 2-3/4 inch (10 lengths) | 25 |
| Countersunk | Zinc Plated | 3/8, 1/2 | 1/2 to 3 inch (10 lengths) | 17 |
Data sheets and documents
The documents for the product you choose are listed with the selector above. Here is every Socket Countersunk Screw document from Hobson Engineering.
All Socket Countersunk Screw documents (28)
Data sheets and technical data (13)
- A4-70 vs BUMAX 88 in Ammonium Nitrate Environment
- BUMAX 88 and BUMAX 109
- BUMAX Grades
- BUMAX Definitions
- Breaking Strength
- Product Finish Comparison
- Stainless Steel - Chemical Properties - Prevent Galling
- SOKO Metric Recommended Tightening Torque Class 12.9
- SOKO UNC Recommended Tightening Torque
- SOKO UNF Recommended Tightening Torque
- SOKO Metric Recommended Tightening Torque Class 10.9
- Tightening Torques SC10 Technical Document
- Tightening Torques SCSO Technical Document
Did You Know flyers (6)
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