Hex socket head cap screws with an axial bore through the floor of the socket. The range covers vented cap screws for vacuum, hollow cap screws for fluid, wire and sensor feed-through, low-head hollow screws and through-bored socket set screws. Made M3 to M36 in stainless steel, 12.9 alloy steel, titanium and nickel alloys, with every socket gauged and every bore cleaned.
A hollow Allen bolt is a hex socket head cap screw with a hole drilled along its axis. The bore starts in the floor of the hex socket and runs out through the thread end. From the side it looks exactly like a standard DIN 912 / ISO 4762 Allen bolt, and it is tightened with the same Allen key.
Exotic Alloy makes hollow Allen bolts in Mumbai, India. The bore is drilled into socket head cap screws made to ISO 4762 and DIN 7984 dimensions, or the whole screw is machined from certified bar for exotic alloys and short runs. The range covers vented cap screws for high and ultra-high vacuum, hollow cap screws that carry gas, fluid, wire or a thermocouple through the joint, and lightweight titanium screws. We distribute across India and export to Oman, the UAE, Saudi Arabia, Qatar, Europe and the Americas.
A vented A2 or A4 screw can be drilled from a standard socket cap screw within days. Hollow 12.9 screws are drilled before heat treatment. Titanium, duplex, Inconel and Monel versions are turned from bar in 7 to 15 business days. Every size, length, bore and finish is made to your drawing.
Head and socket follow the standards below; the bore, length and finish follow your drawing.
All share the hex socket drive. They differ in head height, bore size and what the bore is for.
The standard vacuum fastener. The small bore lets gas trapped at the bottom of a blind tapped hole escape during pump-down. That prevents the slow “virtual leak” that limits high and ultra-high vacuum. Supplied in A2, A4 and titanium, cleaned and bagged.
A larger bore carries compressed air, coolant, a wire, an optical fibre or a thermocouple through the clamped parts. The socket head means no spanner clearance is needed. Bore edges are radiused so cables are not cut.
For thin parts and shallow counterbores. The lower head leaves a shallower socket and less metal under it, so we keep the bore small and check the head section before quoting.
Headless hex socket set screws (grub screws) with an axial bore. They act as threaded vents, adjustable orifices and wire guides, or as stops that still let air or fluid pass.
A banjo-type screw with a socket head, for compact hydraulic and lubrication manifolds where there is no room for the spanner a DIN 7643 hex banjo bolt needs.
Titanium already weighs about 56 % as much as steel for the same volume. A bore takes away more weight again. Used in motorsport, cycling, aerospace and drones, with the stress area calculated for every design.
| Point | Hollow Allen bolt (this page) | Hollow hex bolt |
|---|---|---|
| Drive | Allen key in the head socket | Spanner or socket on the hexagon |
| Clearance needed | Only above the head: fits counterbores and tight corners | Around the head for the spanner |
| Where the bore opens | Into the floor of the hex socket | Into the head face (through-bored) or blind (banjo) |
| Standard banjo design | None: socket-head banjo screws are made to drawing | DIN 7643, M8 × 1 to M45 × 1.5 |
| Typical grades | 12.9, A2-70, A4-70 / -80, titanium Gr. 5 | 8.8, A2 / A4, free-cutting steel, CW710R brass |
| Best for | Vacuum chambers, compact manifolds, flush heads, lightweight assemblies | Fuel, oil and hydraulic lines, external feed-throughs |
Head and socket dimensions are those of the standard screw. The third tab gives the vent and maximum bores we recommend for each size and how much of the solid stress area each leaves.
| Size | Pitch P | dk max plain / knurled | k max head height | s hex key | t min socket depth | w min socket floor | b thread length, ref. |
|---|---|---|---|---|---|---|---|
| M3 | 0.5 | 5.5 / 5.68 | 3 | 2.5 | 1.3 | 1.15 | 18 |
| M4 | 0.7 | 7 / 7.22 | 4 | 3 | 2 | 1.4 | 20 |
| M5 | 0.8 | 8.5 / 8.72 | 5 | 4 | 2.5 | 1.9 | 22 |
| M6 | 1 | 10 / 10.22 | 6 | 5 | 3 | 2.3 | 24 |
| M8 | 1.25 | 13 / 13.27 | 8 | 6 | 4 | 3.3 | 28 |
| M10 | 1.5 | 16 / 16.27 | 10 | 8 | 5 | 4 | 32 |
| M12 | 1.75 | 18 / 18.27 | 12 | 10 | 6 | 4.8 | 36 |
| M14 | 2 | 21 / 21.33 | 14 | 12 | 7 | 5.8 | 40 |
| M16 | 2 | 24 / 24.33 | 16 | 14 | 8 | 6.8 | 44 |
| M20 | 2.5 | 30 / 30.33 | 20 | 17 | 10 | 8.6 | 52 |
| M24 | 3 | 36 / 36.39 | 24 | 19 | 12 | 10.4 | 60 |
| M30 | 3.5 | 45 / 45.39 | 30 | 22 | 15.5 | 13.1 | 72 |
| M36 | 4 | 54 / 54.46 | 36 | 27 | 19 | 15.3 | 84 |
DIN EN ISO 4762:2004, product grade A, dimensions in mm. dk is shown for plain and knurled heads; s is the hex key size, t the minimum socket depth, w the minimum floor thickness under the socket and b the reference thread length of partially threaded screws. Shorter screws are threaded to the head. The standard covers M1.6 to M64; hollow versions are made from M3 to M36.
| Size | Pitch P | dk head dia. | k head height | s hex key | t socket depth | b thread, L ≤ 125 | Head height vs ISO 4762 |
|---|---|---|---|---|---|---|---|
| M3 | 0.5 | 5.5 | 2 | 2 | 1.5 | 12 | 67 % |
| M4 | 0.7 | 7 | 2.8 | 2.5 | 2.3 | 14 | 70 % |
| M5 | 0.8 | 8.5 | 3.5 | 3 | 2.7 | 16 | 70 % |
| M6 | 1 | 10 | 4 | 4 | 3 | 18 | 67 % |
| M8 | 1.25 | 13 | 5 | 5 | 3.8 | 22 | 62 % |
| M10 | 1.5 | 16 | 6 | 7 | 4.5 | 26 | 60 % |
| M12 | 1.75 | 18 | 7 | 8 | 5 | 30 | 58 % |
| M14 | 2 | 21 | 8 | 10 | 5.3 | 34 | 57 % |
| M16 | 2 | 24 | 9 | 12 | 5.5 | 38 | 56 % |
| M18 | 2.5 | 27 | 10 | 12 | 6.5 | 42 | — |
| M20 | 2.5 | 30 | 11 | 14 | 7.5 | 46 | 55 % |
| M22 | 2.5 | 33 | 12 | 14 | 8 | 50 | — |
| M24 | 3 | 36 | 13 | 17 | 8 | 54 | 54 % |
DIN 7984:2022, dimensions in mm. The low head is 54 to 70 % of the ISO 4762 head height and the key size is smaller (M10: 7 mm instead of 8 mm), so low-head hollow screws carry smaller bores.
| Size | s hex key | w min socket floor | d3 minor dia. | As solid mm² | Vent bore ≈ 0.3 d3 | Kept | Max. bore ≈ 0.5 d3 | Kept | Max. bore / s |
|---|---|---|---|---|---|---|---|---|---|
| M3 | 2.5 | 1.15 | 2.39 | 5.0 | 1 | 84 % | 1 | 84 % | 0.40 |
| M4 | 3 | 1.4 | 3.14 | 8.8 | 1 | 91 % | 1.5 | 80 % | 0.50 |
| M5 | 4 | 1.9 | 4.02 | 14.2 | 1 | 94 % | 2 | 78 % | 0.50 |
| M6 | 5 | 2.3 | 4.77 | 20.1 | 1 | 96 % | 2 | 84 % | 0.40 |
| M8 | 6 | 3.3 | 6.47 | 36.6 | 1.5 | 95 % | 3 | 81 % | 0.50 |
| M10 | 8 | 4 | 8.16 | 58.0 | 2 | 95 % | 4 | 78 % | 0.50 |
| M12 | 10 | 4.8 | 9.85 | 84.3 | 2.5 | 94 % | 4.5 | 81 % | 0.45 |
| M14 | 12 | 5.8 | 11.55 | 115.4 | 3 | 94 % | 5.5 | 79 % | 0.46 |
| M16 | 14 | 6.8 | 13.55 | 156.7 | 4 | 92 % | 6.5 | 79 % | 0.46 |
| M20 | 17 | 8.6 | 16.93 | 244.8 | 5 | 92 % | 8 | 79 % | 0.47 |
| M24 | 19 | 10.4 | 20.32 | 352.5 | 6 | 92 % | 10 | 78 % | 0.53 |
| M30 | 22 | 13.1 | 25.71 | 560.6 | 7.5 | 92 % | 12.5 | 78 % | 0.57 |
| M36 | 27 | 15.3 | 31.09 | 816.7 | 9 | 92 % | 15.5 | 77 % | 0.57 |
Calculated from ISO thread geometry: d3 = d − 1.2269 P, As = π/4 · ((d2 + d3)/2)², hollow As = As − π/4 · di². Bores are rounded down to 0.5 mm with a 1 mm minimum. The last column shows that even the maximum bore is at most 0.57 of the key size, so it always opens inside the socket floor.
Socket cap screws are usually chosen because they are strong: class 12.9 is the normal grade. A bore takes metal out of the threaded section that carries the load, so we calculate it for every design.
The proof loads and breaking loads in ISO 898-1 (8.8 to 12.9) and ISO 3506-1 (A2 / A4) are calculated for a solid screw. A hollow screw made from 12.9 material has 12.9 strength, but only over a smaller area, so it does not meet the solid-screw load tables.
We certify the material class to EN 10204 3.1 and state the hollow stress area, as in the bore table above. On request we pull finished hollow screws to failure and report the measured load. For the socket itself nothing changes, because the bore stays inside the key size and the socket depth t is untouched.
| Material / class | Standard | Rm min MPa | Rp0.2 min MPa | Hardness | Notes for hollow socket screws |
|---|---|---|---|---|---|
| Alloy steel, class 8.8 material | ISO 898-1 | 800 (≤ M16) / 830 | 640 / 660 | 22–34 HRC | Bore drilled before or after heat treatment |
| Alloy steel, class 10.9 material | ISO 898-1 | 1040 | 940 | 32–39 HRC | Bore drilled before heat treatment |
| Alloy steel, class 12.9 material | ISO 898-1 | 1220 | 1100 | 39–44 HRC | The usual grade of plain socket cap screws; bore drilled before hardening; electroplating only with baking, zinc flake preferred |
| Stainless A2-70 (304) | ISO 3506-1 | 700 | 450 | — | Vented vacuum screws; ISO 4762 lists class 70 up to M24 and class 50 above |
| Stainless A4-70 / A4-80 (316) | ISO 3506-1 | 700 / 800 | 450 / 600 | — | Chloride, chemical, marine and semiconductor service |
| Titanium Grade 5 (Ti-6Al-4V) | ASTM B348 | 895 | 828 | ≈ 36 HRC | Lightweight hollow screws for motorsport, aerospace and vacuum |
ISO 898-1 and ISO 3506-1 minimums for the material; titanium from ASTM B348. Multiply Rm by the hollow stress area to estimate the minimum breaking load in the thread, or use the calculator in section 09.
Pick the material for what passes through the bore and for the strength left after it. Every grade links to its datasheet.
303 for easy machining of long bores in non-critical service, Nitronic 60 where galling must be avoided, 254 SMO for seawater, A-286 for strength at temperature.
4140 / 4340-type steels give 12.9 strength in hollow screws for machinery and tooling. The bore is drilled before hardening, and the finish is black oxide and oil or zinc flake.
At this hardness the steel is too hard to drill well and is sensitive to hydrogen. So we bore before heat treatment, avoid acid pickling and electroplating unless the parts are baked, and recommend zinc flake for corrosion protection.
The lightweight hollow screw: strength between class 8.8 and 10.9 steel at about 56 % of the weight, made lighter again by the bore. Non-magnetic, and immune to seawater.
For vacuum and chemical service where strength matters less than low outgassing and corrosion resistance.
| Alloy | UNS | Condition supplied | Tensile min MPa | Yield 0.2 % min MPa | Elongation % | Why it is chosen for hex bolts |
|---|---|---|---|---|---|---|
| Inconel 625 | N06625 | Solution annealed (Grade 1) | 827 | 414 | 30 | Seawater, sour gas, cryogenic to 980 °C oxidation; subsea flange bolting |
| Inconel 718 | N07718 | Solution treated + aged (AMS 5662) | 1240 | 1035 | 12 | Highest strength to 650 °C; turbine and wellhead bolting |
| Monel 400 | N04400 | Hot-finished annealed | 517 | 207 | 35 | Seawater, hydrofluoric acid, no chloride SCC; marine and valve bolting |
| Monel K-500 | N05500 | Age hardened (AMS 4676) | 960 | 690 | 18 | Monel 400 corrosion resistance with three times the yield; pump shafts, subsea |
| Hastelloy C-276 | N10276 | Solution annealed (ASTM B574) | 690 | 283 | 40 | Wet chlorine, hot acids, mixed oxidising / reducing media; chemical reactors |
| Incoloy 825 | N08825 | Annealed (ASTM B425) | 586 | 241 | 30 | Sulphuric and phosphoric acid, sour gas (NACE MR0175); pickling plant |
| Duplex 2205 | S32205 | Solution annealed | 655 | 450 | 25 | Twice the yield of 316 with chloride SCC resistance; offshore topsides |
| Super Duplex 2507 | S32750 | Solution annealed | 750 | 550 | 15 | PREN > 40; seawater and desalination bolting |
| Titanium Grade 5 | R56400 | Annealed (ASTM B348) | 895 | 828 | 10 | Half the weight of steel, immune to seawater; aerospace and chemical |
| A-286 | S66286 | Solution treated + aged | — | — | — | Iron-base superalloy for 650 °C bolting; on request with certified values |
Bar minimums from the linked datasheets for the supply condition shown. Multiply the tensile strength by the hollow stress area to estimate the load a hollow screw can carry.
There are two routes. Standard stainless screws are bored after forming, while hardened grades and exotic alloys are bored before hardening or machined from bar.
Socket cap screw blanks or round bar received by heat against mill certificates; PMI on stainless, nickel alloys and titanium.
Cold-headed with the hex socket formed in the heading die for standard sizes, or turned and broached from bar for exotic alloys, large sizes and short runs.
The drill is centred by the socket itself and taken through the floor w and the shank, using peck or deep-hole drilling for long screws. Runout of bore to thread is checked on first-off parts.
Alloy steel bored first, then quenched and tempered to 8.8, 10.9 or 12.9; stainless used as formed or solution annealed; titanium annealed; Inconel 718 and Monel K-500 aged.
Threads are rolled on solid blanks or where the wall is thick enough, and cut where a large bore leaves a thin wall, so the tube does not collapse under the dies.
Bore exits at the socket floor and thread end are deburred, then the screw is flushed and ultrasonically cleaned. Vacuum screws are degreased and baked dry on request.
Socket gauged to ISO 23429, thread gauged, bore checked with pin gauges, hardness tested on heat-treated lots. Packed clean with the bores protected.
A worn or oversize socket makes the Allen key cam out and round the flats. That ruins the screw and often the part around it. Drilling through the socket floor must not disturb the socket, so we gauge it before and after boring with go and no-go hexagon gauges to ISO 23429, and confirm the socket depth.
The bore is kept smaller than the key size in every design, at most 0.57 × s in our standard sizes. The key still bears on the full length of all six flats.
Two things limit the finish on a hollow socket screw. High-strength grades are sensitive to hydrogen, and small bores cannot be electroplated reliably. See also our coated fasteners page.
| Finish | Specification | Inside the bore | Suitable for | Notes |
|---|---|---|---|---|
| Black oxide & oil | Thermal or chemical black, oiled | Partial | 10.9 / 12.9 alloy steel | The traditional socket cap screw finish; indoor and oiled service only |
| Zinc flake | ISO 10683 | Masked or not coated in small bores | 8.8 to 12.9 | No hydrogen embrittlement; the preferred corrosion finish for 12.9 |
| Zinc electroplate | ISO 4042 | Thin or absent | 8.8; 10.9 / 12.9 only with baking | Bake after plating to relieve hydrogen in high-strength screws |
| Electroless nickel | ASTM B733 / ISO 4527 | Uniform | Alloy steel needing an even coat inside the bore | No current, so no shadowing; adds wear resistance |
| Passivated | ASTM A967 / A380 | Full | A2, A4, duplex | Standard on stainless; restores the passive film after drilling |
| Silver plated thread | To drawing | — | Stainless vacuum screws | Dry lubricant that resists galling in stainless tapped holes under vacuum |
| Vacuum clean | Degrease, ultrasonic clean, bake on request | Oil-free | All vented screws | Packed in clean-room film |
Choose an ISO 4762 size, a bore and a material. The tool shows how much of the solid screw is left, the estimated minimum breaking load in the thread, and how the bore compares with the hex key size and the socket floor.
As = π/4 · ((d2 + d3)/2)² − π/4 · di², d2 = d − 0.6495 P, d3 = d − 1.2269 P. Estimated breaking load = As × Rm, for the threaded section only. Key size s, socket depth t and floor w from ISO 4762.
Use a hex key to ISO 2936, inserted to the full socket depth t. Worn keys and ball-end keys concentrate the load near the socket mouth and round the flats.
Take the preload of the solid screw for the material and friction, and reduce it in proportion to the stress area kept (the % above). Treat the result as a design figure to be confirmed.
Lubricate stainless-in-stainless threads with a vacuum-compatible lubricant or use silver-plated screws. Galling seizes a vented screw before it reaches its load.
Vented A2, A4 and titanium cap screws in vacuum chambers, deposition and etch tools, load locks, electron microscopes and analytical instruments.
Particle accelerators, fusion and cryogenic test rigs, and thermal-vacuum chambers, where virtual leaks and outgassing must be removed.
Hollow 12.9 screws that carry coolant or compressed air through fixtures, grippers and spindle assemblies, with a socket head for tight spaces.
Socket-head hollow screws with cross holes in compact manifolds and central lubrication systems where a hex banjo bolt will not fit.
Feed-through screws for thermocouples, fibre optics and wiring through housings and heat sinks, in stainless, brass or nickel alloys.
Titanium Grade 5 hollow socket screws for brakes, suspension, frames, drones and airframe fittings, where every gram counts.
| Check | Method | Applied to |
|---|---|---|
| Material identity | Mill certificate review; XRF PMI | Every heat; 100 % of stainless, nickel alloy and titanium lots |
| Hex socket | Go / no-go hexagon gauges to ISO 23429; depth t | Every lot, before and after boring |
| Bore diameter | Pin gauges | Every lot, sampled |
| Bore runout | Runout of bore to thread on a V-block or CMM | Long screws and large bores |
| Head & thread | Head diameter and height; go / no-go thread gauges | Every lot, to ISO 4762 or DIN 7984 limits |
| Hardness | Rockwell C, surface and core | Every heat-treated lot; decarburisation check on 12.9 |
| Burrs & cleanliness | Magnifier or borescope; flushing | Every lot; vacuum lots cleaned and bagged |
| Load test | Tensile test of finished hollow screws | On request, measured load reported |
| Leak / outgassing | Helium leak test of assemblies; RGA by customer | On request for vacuum parts |
Tell us the size, length, head type, bore, material, finish and quantity, or email your drawing to sales@exoticalloy.com. Our engineers respond within 2 to 4 business hours.