Hex bolts with an axial bore: fully through-bored hollow bolts, DIN 7643 banjo bolts with cross holes, vented screws for vacuum and feed-through bolts for wires, sensors and lubricant. Made from M6 to M48 in stainless steel, brass, duplex, nickel alloys and titanium. Every bore is deburred, cleaned and inspected before dispatch.
A hollow hex bolt is a hex head bolt with a hole drilled along its axis. Outside, it fits and tightens like any hex bolt. Inside, the bore carries something the joint needs: fluid to a port, trapped gas out of a blind hole, a wire or thermocouple through a wall, or grease to a bearing.
Exotic Alloy machines hollow hex bolts in Mumbai, India, from certified hexagon and round bar. The range covers fully through-bored bolts, DIN 7643 hollow screws (banjo bolts) for DIN 7642 ring fittings, vented hex screws for vacuum, and lubrication-feed and cable feed-through bolts. The bore is drilled, deep-hole drilled or gun drilled to suit its length. Every cross hole is deburred where it breaks into the bore. We stock and supply across India and export to Qatar, the UAE, Saudi Arabia, Oman, Europe and the Americas.
Hollow bolts are rarely identical from one application to the next, so we make them to your drawing. You set the thread and pitch, hexagon, length, bore, cross holes, material and finish. DIN 7643 banjo bolts in free-cutting steel and CW710R brass follow the standard exactly. Stainless, duplex, Inconel, Monel, Hastelloy and titanium versions are typically turned around in 7 to 15 business days.
The range we machine. DIN 7643 banjo bolts follow the standard; everything else is made to your drawing.
The hex head is the same; the bore decides the job. Pick the type by what has to pass through the bolt.
The bore runs from the head face to the thread end, so the bolt is open at both ends. Used to pass gas, fluid, a wire, an optical fibre or a thermocouple through a clamped joint, and to save weight in titanium and alloy steel bolts.
The bore starts at the thread end and stops inside the shank, where 4 cross holes (2 or 3 on the smallest size) open into the annulus of a ring fitting. Used on fuel, oil, hydraulic and brake lines, where the pipe leaves the port at an angle that can be set freely.
A small axial bore lets the gas trapped under a screw in a blind tapped hole escape during pump-down. That removes a common “virtual leak” in high and ultra-high vacuum chambers. Supplied in 304, 316 or titanium, cleaned and bagged for vacuum.
The bore carries grease or oil from a nipple in the head to a cross hole at the bearing, pin or bush the bolt passes through. Common on pivot pins, linkages and conveyor rollers.
A smooth, radiused bore takes a cable, a thermocouple sheath or a capillary through a panel, housing or flange while the bolt holds the parts together. Potting or a gland can be added at the head.
A longer shank with two sets of cross holes lets two ring fittings be stacked on one port. Inch (UNF) and BSP banjo bolts for imported equipment are made to drawing.
| Type | Bore | Cross holes | Standard | Sealing | Typical sizes | Main use |
|---|---|---|---|---|---|---|
| Through-bored hollow bolt | Head to tip, open both ends | None | Head to ISO 4017 / DIN 933; bore to drawing | Not required, or gland / potting | M6 – M48 | Pass-through, weight saving |
| Banjo bolt | Blind, from thread end | 4 (2 or 3 on M8×1) | DIN 7643 | Two DIN 7603 rings | M8×1 – M45×1.5 | Fuel, oil, hydraulic, brake lines |
| Vented screw | Small, head to tip | None | Head to DIN 933 / ISO 4017; vent to drawing | None (vacuum side) | M3 – M12 typical | High and UHV vacuum |
| Lubrication-feed bolt | From tapped head to cross hole | 1 or 2 at the bearing | Grease nipple DIN 71412; bolt to drawing | Nipple thread | M10 – M30 | Pins, pivots, linkages |
| Feed-through bolt | Head to tip, radiused | None | To drawing | Gland or potting | M8 – M48 | Cables, thermocouples, capillaries |
The banjo bolt table is transcribed from DIN 7643:2005-07, Table 1. The through-bored table gives example bores for each coarse size and the stress area left after each, so a drawing can start from a sensible bore.
| Pipe OD mm | Thread d1 | s across flats | m head height | l13 length, short ±0.2 | c1 thread, short min | l14 length, long (S) ±0.2 | c2 thread, long min | d30 axial bore H14 | d27 cross hole, 4-hole H14 | d28 cross hole, 3-hole H14 | d29 shank h12 | r5 underhead radius |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 4 and 5 | M8 × 1 | 12 | 5 | 17 | 8.5 | — | — | 4 | — | 3 | 8 | 0.2 |
| 6 | M10 × 1 | 14 | 6 | 19 | 8.5 | 23 | 12.5 | 5.5 | 2.8 | 3.5 | 10 | 0.2 |
| 8 | M12 × 1.5 | 17 | 6 | 24 | 11 | 28 | 15 | 7 | 3.5 | 4.5 | 12 | 0.2 |
| 10 | M14 × 1.5 | 19 | 6 | 26 | 11 | 32 | 17 | 9 | 4.5 | 5.5 | 14 | 0.2 |
| 12 | M16 × 1.5 | 22 | 6 | 28 | 11 | 36 | 19 | 11 | 5.5 | 7 | 16 | 0.2 |
| 15 | M18 × 1.5 | 24 | 6 | 32 | 11 | 42 | 21 | 13 | 7 | 8.5 | 18 | 0.2 |
| 18 | M22 × 1.5 | 27 | 7 | 39 | 13 | 50 | 24 | 16 | 9 | 11 | 22 | 0.4 |
| 22 | M26 × 1.5 | 32 | 7 | 45 | 13 | 60 | 28 | 20 | 11 | 13.5 | 26 | 0.4 |
| 28 | M30 × 1.5 | 36 | 7 | 51 | 13 | 68 | 30 | 24 | 13 | 16 | 30 | 0.4 |
| 35 | M38 × 1.5 | 46 | 8 | 61 | 15 | — | — | 32 | 16 | 20 | 38 | 0.6 |
| 42 | M45 × 1.5 | 55 | 9 | 69 | 15 | — | — | 39 | 20 | 24 | 45 | 0.6 |
All dimensions in mm. Sizes are named by pipe outside diameter. The short thread version is the default; the long version is ordered with “S” and is not made for M8, M38 or M45. From 6 mm pipe the standard design has 4 cross holes of diameter d27; 3 holes of d28 are supplied by agreement. The 4 mm and 5 mm size has 2 or 3 holes at the maker's choice. The standard also requires that the thread run-out must not reach the cross hole.
| Size | Pitch mm | d3 minor dia. mm | As solid mm² | Small bore ≈ 0.3 d3, mm | As mm² | Retained | Large bore ≈ 0.5 d3, mm | As mm² | Retained | Wall at root, large bore mm |
|---|---|---|---|---|---|---|---|---|---|---|
| M6 | 1 | 4.77 | 20.1 | 1 | 19.3 | 96 % | 2 | 17.0 | 84 % | 1.39 |
| M8 | 1.25 | 6.47 | 36.6 | 1.5 | 34.8 | 95 % | 3 | 29.5 | 81 % | 1.73 |
| M10 | 1.5 | 8.16 | 58.0 | 2 | 54.8 | 95 % | 4 | 45.4 | 78 % | 2.08 |
| M12 | 1.75 | 9.85 | 84.3 | 2.5 | 79.4 | 94 % | 4.5 | 68.4 | 81 % | 2.68 |
| M14 | 2 | 11.55 | 115.4 | 3 | 108.4 | 94 % | 5.5 | 91.7 | 79 % | 3.02 |
| M16 | 2 | 13.55 | 156.7 | 4 | 144.1 | 92 % | 6.5 | 123.5 | 79 % | 3.52 |
| M18 | 2.5 | 14.93 | 192.5 | 4 | 179.9 | 93 % | 7 | 154.0 | 80 % | 3.97 |
| M20 | 2.5 | 16.93 | 244.8 | 5 | 225.2 | 92 % | 8 | 194.5 | 79 % | 4.47 |
| M22 | 2.5 | 18.93 | 303.4 | 5.5 | 279.6 | 92 % | 9 | 239.8 | 79 % | 4.97 |
| M24 | 3 | 20.32 | 352.5 | 6 | 324.2 | 92 % | 10 | 274.0 | 78 % | 5.16 |
| M27 | 3 | 23.32 | 459.4 | 6.5 | 426.2 | 93 % | 11.5 | 355.5 | 77 % | 5.91 |
| M30 | 3.5 | 25.71 | 560.6 | 7.5 | 516.4 | 92 % | 12.5 | 437.9 | 78 % | 6.60 |
| M33 | 3.5 | 28.71 | 693.6 | 8.5 | 636.8 | 92 % | 14 | 539.6 | 78 % | 7.35 |
| M36 | 4 | 31.09 | 816.7 | 9 | 753.1 | 92 % | 15.5 | 628.0 | 77 % | 7.80 |
| M39 | 4 | 34.09 | 975.8 | 10 | 897.2 | 92 % | 17 | 748.8 | 77 % | 8.55 |
| M42 | 4.5 | 36.48 | 1120.9 | 10.5 | 1034.3 | 92 % | 18 | 866.4 | 77 % | 9.24 |
| M45 | 4.5 | 39.48 | 1306.0 | 11.5 | 1202.1 | 92 % | 19.5 | 1007.4 | 77 % | 9.99 |
| M48 | 5 | 41.87 | 1473.1 | 12.5 | 1350.4 | 92 % | 20.5 | 1143.1 | 78 % | 10.68 |
Calculated from ISO thread geometry: d3 = d − 1.2269 P and As = π/4 · ((d2 + d3)/2)². The hollow stress area is As − π/4 · di². Bores are rounded down to the nearest 0.5 mm. The hex head and length follow ISO 4017 / DIN 933 unless the drawing says otherwise; see the hex bolt dimension table.
This is the most important section for anyone specifying a hollow bolt. A bore removes metal from the one place that carries the load, the threaded stress area. The figures below are calculated from thread geometry, not estimated.
| Thread | As solid mm² | Axial bore d30 mm | As with bore mm² | Threaded section retained | Wall at thread root (d3 − d30) / 2, mm | Bore flow area mm² |
|---|---|---|---|---|---|---|
| M8 × 1 | 39.2 | 4 | 26.6 | 68 % | 1.39 | 12.6 |
| M10 × 1 | 64.5 | 5.5 | 40.7 | 63 % | 1.64 | 23.8 |
| M12 × 1.5 | 88.1 | 7 | 49.6 | 56 % | 1.58 | 38.5 |
| M14 × 1.5 | 124.5 | 9 | 60.9 | 49 % | 1.58 | 63.6 |
| M16 × 1.5 | 167.2 | 11 | 72.2 | 43 % | 1.58 | 95.0 |
| M18 × 1.5 | 216.2 | 13 | 83.5 | 39 % | 1.58 | 132.7 |
| M22 × 1.5 | 333.1 | 16 | 132.0 | 40 % | 2.08 | 201.1 |
| M26 × 1.5 | 475.0 | 20 | 160.9 | 34 % | 2.08 | 314.2 |
| M30 × 1.5 | 642.1 | 24 | 189.7 | 30 % | 2.08 | 452.4 |
| M38 × 1.5 | 1051.7 | 32 | 247.4 | 24 % | 2.08 | 804.2 |
| M45 × 1.5 | 1492.5 | 39 | 297.9 | 20 % | 2.08 | 1194.6 |
Stress areas are for the fine threads of DIN 7643 and the d30 bore of Table 1. The cross holes reduce the section again locally in the plain shank, so the shank and the thread are both checked on special designs.
ISO 898-1 and ISO 3506-1 give proof loads and breaking loads for solid bolts, calculated on the full stress area. A hollow bolt made from class 8.8 or A4-80 material has the strength of that material, but only over a smaller area, so it cannot honestly be marked or certified as an 8.8 or A4-80 bolt on load.
We certify the material to EN 10204 3.1 and state the property class of the material. Where a load rating matters we calculate the reduced stress area, as in the tables above and the calculator below. On request we pull finished hollow bolts to failure and report the actual load.
| Material / class | Standard | Rm min MPa | Rp0.2 min MPa | Typical use in hollow bolts |
|---|---|---|---|---|
| 11SMnPb30+C (1.0718) | DIN EN 10277-3 | size-dependent, per EN 10277-3 | — | DIN 7643 regular material: free-cutting steel for banjo bolts, zinc plated |
| CuZn35Ni3Mn2AlPb (CW710R) | DIN 3859-1 | per supply condition | — | DIN 7643 alternative material: high-tensile brass banjo bolts for fuel, oil and marine lines |
| Steel, class 8.8 material | ISO 898-1 (material values) | 800 (≤ M16) / 830 | 640 / 660 | Hydraulic and machine hollow bolts; certified as material, not as a solid-bolt proof load |
| Steel, class 10.9 material | ISO 898-1 (material values) | 1040 | 940 | High-load banjo and feed-through bolts where the bore is small |
| Stainless A2-70 (304) | ISO 3506-1 (material values) | 700 | 450 | Vented vacuum screws, food and water lines, general corrosion |
| Stainless A4-70 / A4-80 (316) | ISO 3506-1 (material values) | 700 / 800 | 450 / 600 | Chloride service, marine, chemical and semiconductor vacuum |
The fluid or atmosphere in the bore usually decides the material, because the inside of a small bore cannot be coated reliably. Every grade below links to its datasheet on this site.
303 machines best for long bores in non-critical service; 904L for acids; Nitronic 60 where the bolt must not gall; A-286 for hot sections.
Cold-drawn leaded free-cutting steel to EN 10277-3. It drills and cross-drills cleanly at speed, which is why DIN 7643 makes it the default. Zinc plated for hydraulic and fuel lines.
Brass banjo bolts suit fuel, lubricating oil, compressed air and marine fresh-water lines. They need no coating inside the bore and machine to a smooth finish.
For seawater lines, non-sparking service and electrical feed-throughs where conductivity matters.
| 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. Multiply the tensile strength by the hollow stress area from section 05 or the calculator to estimate the load a hollow bolt can carry.
A hollow bolt is a machined part first and a bolt second. The bore, the cross holes and the cleanliness of both decide whether it works.
Hexagon bar for standard heads, round bar for large or special heads, received by heat against mill certificates; PMI on stainless, nickel alloys and titanium.
Head, shank, underhead radius and thread diameter turned on CNC lathes and sliding-head machines; heads forged first for large diameters.
Twist drilling for short bores, peck or deep-hole drilling for longer ones, and gun drilling for long, straight, concentric bores. Concentricity of bore to thread is checked on first-off parts.
Banjo cross holes drilled to the d27 / d28 diameter on a live-tooled lathe or indexing fixture, positioned so the thread run-out stays clear of the holes as DIN 7643 requires.
The edge where a cross hole breaks into the bore is deburred by brush, thermal or electrochemical deburring. A loose burr in a fuel or hydraulic line can block an injector or score a valve.
Threads are rolled where the wall can take the rolling force. Where the bore leaves a thin wall they are cut instead, so the tube does not collapse under the dies.
Ultrasonic cleaning and flushing of the bore, then plating or passivation, bore and cross-hole inspection, and clean packing with the bores capped.
When a cross hole is drilled into an axial bore, the drill pushes a burr into the bore at the point where it breaks through. From outside the bolt looks perfect. In service the burr breaks off and travels down the line into a filter, an injector or a valve seat. In a vacuum bolt it traps the very gas the vent is meant to release.
We deburr every intersection and inspect it with a borescope or magnifier, then flush the bore. For automotive and hydraulic customers, component cleanliness can be documented to ISO 16232 on request.
The usual question for a coating is how well it protects the outside. For a hollow bolt the harder question is whether it reaches inside the bore. See also our coated fasteners page.
| Finish | Specification | Outside | Inside a small bore | Use on hollow bolts |
|---|---|---|---|---|
| Zinc electroplate, passivated | ISO 4042, Fe/Zn8c 2C (DIN 7643 regular) | Good, about 8 µm | Thin or absent: electroplate throws poorly into deep, narrow holes | Steel banjo bolts on oil, fuel and hydraulic lines, where the fluid itself protects the bore |
| Electroless nickel | ASTM B733 / ISO 4527 | Uniform | Uniform wherever the solution flows: no current, so no shadowing | Steel hollow bolts that need protection inside the bore, and wear-resistant banjo bolts |
| Passivation (stainless) | ASTM A967 / A380 | Full | Full, the acid reaches the bore | Standard on all stainless hollow bolts; restores the passive film after drilling |
| Zinc flake | ISO 10683 | Very good | Can block small bores; bores masked or not coated | Through-bored structural hollow bolts with large bores only |
| Bright / self colour | As machined, cleaned | — | — | Brass, bronze, nickel alloy and titanium bolts, which need no coating |
| Vacuum clean | Degrease, ultrasonic clean, bake on request | Oil-free | Oil-free | Vented screws for high and UHV vacuum, packed in clean-room film |
Choose a thread, a bore and a material to see how much of the solid bolt is left, the wall at the thread root, an estimated minimum breaking load in the thread, and the flow area of the bore. DIN 7643 sizes load their standard bore automatically.
As = π/4 · ((d2 + d3)/2)² − π/4 · di², with d2 = d − 0.6495 P and d3 = d − 1.2269 P. Estimated breaking load = As × Rm. This is an estimate for the threaded section only; cross holes, the head and the mating thread are checked separately.
Tighten to the value given by the sealing ring or equipment manufacturer for that ring and size. Use new sealing rings each time the joint is opened; copper and aluminium rings work-harden.
Tighten as an ordinary screw of the material, reduced in proportion to the stress area kept. Use a vacuum-compatible lubricant or silver-plated threads on stainless to prevent galling.
Scale the preload of the solid bolt by the stress area kept (the % above). Check thread engagement and the head separately, and treat the result as a design figure to be confirmed by test.
DIN 7643 banjo bolts on fuel pumps, filters, injectors, turbocharger oil feeds and gearboxes, in zinc-plated steel or CW710R brass.
Banjo bolts and double banjo bolts on hydraulic power units, brake calipers and master cylinders, where the line has to leave the port at any angle.
Vented 304 / 316 and titanium screws for vacuum chambers, deposition and analytical instruments, particle accelerators and space simulation.
Feed-through bolts for thermocouples, pressure capillaries and cables through flanges, enclosures and furnace walls, in 316, Inconel or Monel.
Lubrication-feed bolts through pivot pins and linkages on cranes, excavators, agricultural and packaging machinery, fed from a grease nipple in the head.
Titanium Grade 5 and alloy steel hollow bolts to save weight, and brass or bronze banjo bolts for marine fuel and cooling systems.
| Check | Method | Applied to |
|---|---|---|
| Material identity | Mill certificate review; XRF PMI on finished parts | Every heat; 100 % of stainless, nickel alloy and titanium lots |
| Bore diameter & depth | Pin gauges, bore gauge, depth gauge | Every lot, sampled; first-off and last-off parts |
| Bore concentricity | Runout of bore to thread on a V-block or CMM | Long and gun-drilled bores, every lot |
| Cross holes | Diameter and position by pin gauge and optical comparator | All banjo designs; DIN 7643 run-out rule checked |
| Burrs & cleanliness | Borescope or magnifier; flushing; ISO 16232 extraction on request | Every banjo, fluid and vacuum lot |
| Thread & hexagon | Go / no-go thread gauges; across flats, head height, length | Every lot |
| Hardness | Rockwell or Vickers | Heat-treated and cold-drawn material lots |
| Load test | Tensile test of finished hollow bolts to failure | On request, with the actual breaking load reported |
| Pressure / leak test | Hydrostatic or helium leak test of the assembly | On request, for banjo assemblies and vacuum parts |
Tell us the type, thread, length, bore and cross holes, material, finish and quantity, or email your drawing to sales@exoticalloy.com. Our engineers respond within 2 to 4 business hours.