Hollow Hex Bolts
Manufacturer
India · Mumbai
Anatomy of a Hollow Hex Bolt — External Hex Head, Axial Bore, Radial Port
A hollow hex bolt shares the identical internal fluid architecture as a hollow Allen bolt — an axial bore running the length of the shank, connected to one or more radial cross-drilled ports near the head, sealed by copper crush washers at the banjo fitting interface. The only difference is the drive feature on top: a hollow hex bolt has a standard external hexagon head, driven by an open-end spanner, ring spanner, or socket from the outside, rather than an internal Allen key engaging a socket recess.
This single difference in drive geometry changes where and how the bolt is used. A hex head can accept significantly higher applied torque without deformation or stripping, because the wrench engages the full outer hex geometry rather than the smaller internal socket recess — the same reason a standard hex bolt outperforms a socket cap screw at high torque. External hex drive also allows wrench access from the side in situations where there isn’t enough clearance above the bolt head for a socket wrench or Allen key to be inserted axially.
The trade-off is head size: a hex head bolt has a larger head diameter and height than an equivalent socket head, because the wrenching flats need to be on the outside of the head rather than recessed inside it. Where axial clearance above the bolt is tight but radial (side) clearance is available for a spanner, hollow hex bolts are the correct choice. Where the reverse is true — tight radial clearance but open access from above — hollow Allen (socket) bolts are preferred.
Hollow Hex Bolt vs Hollow Allen Bolt — When to Specify Each
Both types share the identical axial bore and radial port fluid architecture. The choice between them comes down entirely to clearance geometry and torque requirement at the installation point.
Hollow Hex Bolt (This Page)
External hexagon head engaged by an open-end spanner, ring spanner, or hex socket from outside the head. The full hex geometry accepts higher applied torque than an equivalent socket head without stripping the drive feature — important on larger bore sizes and higher-pressure hydraulic circuits where a substantial tightening torque is specified.
Hollow Allen Bolt (Socket Head)
Internal hex socket recess engaged by an Allen key or hex bit from directly above the bolt. The compact head diameter is the primary advantage — critical in confined engine bays, motorcycle brake calipers, and tight assembly spaces where a hex head’s wider wrenching flats would not clear surrounding components.
Why Hollow Hex Bolts — Torque, Access, and Serviceability
Higher Torque Capacity Than Socket Drive
Full hex geometry engages the wrench across six full flats rather than the smaller internal socket recess of an Allen-drive bolt. For higher-pressure hydraulic circuits and larger bore sizes (M16–M24) where the specified tightening torque is significant, hex drive resists cam-out and drive-feature stripping better than an equivalent socket head at the same torque level.
Side Access Where Axial Clearance Is Restricted
A spanner engages the hex head from the side, requiring only radial swing clearance rather than vertical space above the bolt to insert a hex key or socket driver. In machinery with components mounted directly above a hydraulic fitting, or in assembly sequences where the bolt must be tightened before an overhead component is installed, hex drive is often the only geometrically viable option.
Standard Tooling — No Hex Key Required
Every mechanic’s toolbox has open-end and ring spanners in every common size; a hex key set specific to socket head sizes is a separate tool requirement. For field service and general industrial maintenance where the technician may not carry a full hex key set, hollow hex bolts can be serviced with the same standard spanners used throughout the rest of the machine or vehicle.
Identical Fluid-Passage Function to Hollow Allen Bolts
The axial bore, radial port, and copper crush washer sealing system are dimensionally and functionally identical to hollow Allen bolts — the same bore diameter and port sizing per DIN 7643 apply regardless of which head drive is specified. This means the hydraulic performance (flow rate, pressure rating) is unaffected by the choice between hex and socket drive; the decision is purely about installation access and tooling.
Hollow Hex Bolt Material & Pressure Rating Options
Material selection depends on the fluid and pressure rating required — the same alloy logic as hollow Allen bolts applies identically here.
Hollow Hex Bolt Configurations & Related Hardware
Standard single-port hex-drive banjo bolts, extended dual-port variants, plus matching copper crush washers. M6–M24.
Standard Hollow Hex Bolt
External hex head, single radial port, axial bore. M6–M24, lengths to specification. For hydraulic fittings, industrial lubrication passages, and any banjo connection serviced with a standard spanner rather than a hex key. Bore and port diameter precision-gauged on every production lot.
Double Banjo Hex Bolt
Extended shank with two radial ports for dual hydraulic line junctions, hex drive throughout. Custom lengths per application drawing. For industrial hydraulic manifolds and dual-circuit fluid systems requiring higher installation torque than a socket-drive equivalent.
Copper Crush Washers
Soft annealed copper sealing washers to DIN 7603, sized to match each hollow hex bolt shank diameter — identical washer specification to the hollow Allen bolt range, since both bolt types use the same banjo fitting geometry. Sold matched in pairs; always specify new washers for every reassembly.
Custom Hollow Hex Bolts
Non-standard bore diameters, custom port positions, restrictor-orifice bolts with reduced bore for flow control, and drawing-specific configurations in any available material. Contact sales@akbarfasteners.in with bore diameter, port configuration, thread size, and length.
Hollow Hex Bolt Size Chart — M6 to M24
Thread size, standard axial bore diameter, and hex head across-flats per DIN 7643. Note the hex head is dimensionally larger than the equivalent socket head size for the same thread diameter.
| Thread Size | Std. Bore Dia. (mm) | Hex Across Flats (mm) | Typical Application |
|---|---|---|---|
| M6 | 3.0 | 10 | Small hydraulic lines |
| M8 | 4.0 | 13 | Compact hydraulic fittings |
| M10 | 5.0 | 17 | General hydraulic connections |
| M12 | 6.0 | 19 | Industrial hydraulic circuits |
| M14 | 7.0 | 22 | Heavy equipment hydraulics |
| M16 | 8.0 | 24 | High-flow hydraulic circuits |
| M20 | 10.0 | 30 | Industrial hydraulic manifolds |
| M24 | 12.0 | 36 | Large-bore fluid transfer |
Hollow Hex Bolts Application Areas
Industrial hydraulics, heavy equipment, agricultural machinery, and fluid systems requiring higher tightening torque or open-wrench serviceability.
Industrial Hydraulics
hydraulic press circuits
Heavy Equipment
higher-torque fittings
Agricultural Machinery
spanner-serviceable in field
Fuel Systems
higher-torque fuel connections
Lubrication Systems
bearing lubrication points
Commercial Vehicles
fleet-serviceable spanner drive
Marine Hydraulics
engine oil passage fittings
Process Plant Piping
higher-pressure process fluid
Hollow Hex Bolt Standards Coverage
Same ISO 9001:2015 quality system as our complete fastener range. Bore diameter and port position gauged on every production lot — identical dimensional QC to the hollow Allen bolt range.
Complete Documentation Package
Grade / Standard Stated
Tensile · Hardness
Diameter · Concentricity
Conformance
On Request
Related Bolt Types & Materials
Hollow hex bolts for higher torque and side-access serviceability. Hollow Allen bolts for compact head profile in confined spaces. Hex flange bolts for pure mechanical clamping. SS 316L for fuel and corrosive fluid service.
Hollow Allen Bolts
The compact-head alternative — identical axial bore and radial port fluid architecture, but with an internal hex socket drive for confined spaces where a wrench cannot swing but a hex key can be inserted from directly above. The dominant choice for motorcycle and automotive brake caliper connections.
View Hollow Allen Bolts →Hex Flange Bolts
For general mechanical clamping applications where fluid passage through the bolt is not required — the integral washer flange serves a purely mechanical joint function, contrasted with the hollow hex bolt’s fluid conduit function.
View Flange Bolts →SS 316L Fasteners
For hollow hex bolt applications in fuel systems, marine hydraulics, or corrosive fluid environments where carbon steel’s corrosion resistance is insufficient — SS 316L hollow hex bolts provide the same fluid-passage function with chloride and fuel-compatible corrosion resistance.
View SS 316L →Full Bolt Range
Complete bolt manufacturing capability at Akbar Fasteners — standard hex, flange, carriage, socket cap, hollow hex, hollow Allen, and all specialty head forms across carbon steel through exotic alloy, all from a single Mumbai facility.
View Bolt Range →Need Hollow Hex
Bolts?
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Export: +91 8696 32 4646 · Support: +91 800 9315 333 · export@akbarfasteners.in