Hollow Allen Bolts
Manufacturer
India · Mumbai
Anatomy of a Banjo Bolt — Axial Bore, Radial Ports, Sealed by Copper Washers
A hollow Allen bolt (also called a banjo bolt, hollow bolt, or drilled bolt) is a hex socket head cap screw with a precision-drilled axial bore running the length of the shank, connected to one or more radial cross-drilled ports near the head. This combination of an axial bore and radial ports allows fluid entering from the head end (or from a side port) to flow through the bolt body and exit at a different point — turning the fastener itself into a fluid routing component, not just a clamping element.
The classic application is the hydraulic banjo fitting: a banjo-shaped fitting (a flattened ring with a hose barb) is clamped to a component (a caliper, master cylinder, or hydraulic valve body) by passing the hollow bolt through the banjo’s central hole and threading it into the component. Fluid enters the banjo fitting from the hose, flows through the banjo body, into the bolt’s radial port, down the axial bore, and out through the bolt’s tip into the component — completing the hydraulic circuit entirely through the fastener.
Sealing is achieved with copper crush washers — one on each side of the banjo fitting, between the fitting and the bolt head, and between the fitting and the component face. Copper is specified because it is soft enough to cold-flow and conform to minor surface imperfections under bolt clamping torque, creating a reliable static seal without a separate O-ring or gasket. New copper washers are required at every disassembly — reused copper washers have already work-hardened from the first crush and will not reseal reliably.
Why Hollow Allen Bolts — The Fastener That Is Also a Fluid Fitting
Fastener and Fluid Connector in One Component
A hollow Allen bolt eliminates the need for a separate banjo fitting bolt and a standard clamping bolt at the same location — the fluid connection and mechanical retention happen through the same drilled fastener. This is precisely why brake caliper hydraulic connections, which need both a secure mechanical joint and a sealed fluid path in a very confined space, universally use hollow Allen bolts rather than a threaded hose fitting plus a separate retaining bolt.
Compact Fluid Routing in Confined Spaces
Banjo bolt connections allow the hose or fitting to swivel around the bolt axis before final tightening — critical when routing a brake hose or hydraulic line in a tight wheel well or engine bay where a fixed threaded fitting orientation would not clear surrounding components. The banjo fitting can be rotated to the optimal hose routing angle, then the hollow bolt is torqued to final spec, locking the orientation permanently.
Precision Bore for Consistent Flow and Pressure Rating
The axial bore diameter and radial port sizing are precision-drilled to a specified tolerance because the bolt itself becomes a flow-restricting orifice in the hydraulic circuit — undersized or off-centre drilling changes the effective flow area and can affect brake pedal feel or hydraulic response time. Akbar Fasteners gauges bore diameter and port alignment on every production lot to ensure consistent flow characteristics across a production batch.
Serviceable — Designed for Repeated Disassembly with New Washers
Unlike a permanently brazed or welded fluid connection, a hollow Allen bolt banjo joint is designed to be opened and resealed repeatedly during the service life of the equipment — brake hose replacement, caliper service, and hydraulic line inspection all require opening this joint. The design intent is that new copper crush washers are fitted at every reassembly, making the joint serviceable indefinitely rather than a one-time permanent connection.
Single-Port vs Double-Port Radial Drilling
The number and position of radial cross-drilled ports determines whether the bolt serves a single hydraulic line or splits/combines flow from two lines at one connection point.
Single-Port Banjo Bolt
One radial cross-drilled port connects to the axial bore, creating a single fluid path from the banjo fitting through the bolt to the component. This is the standard configuration for most brake caliper connections, single hydraulic hose fittings, and lubrication points with one supply line.
Double-Banjo / Dual-Port Bolt
A longer hollow bolt with two banjo fittings stacked on the shank, each with its own radial port into a shared or partitioned axial bore — used where a single hydraulic supply must split to feed two downstream components, or where two brake lines (front-to-rear split circuits) need to connect at one caliper mounting point.
Hollow Allen Bolt Material & Pressure Rating Options
Material selection depends on the fluid (brake fluid, fuel, oil) and the pressure rating required. Carbon steel is standard for automotive brake and hydraulic service; stainless for fuel and corrosive fluid; brass for low-pressure lubrication.
Hollow Allen Bolt Configurations & Related Hardware
Standard single-port and double-port banjo bolts, plus the copper crush washers and banjo fittings that complete the assembly. M6–M24.
Standard Banjo Bolt
Single radial port, axial bore, hex socket head. M6–M24, lengths to specification. The standard hollow Allen bolt for brake caliper connections, single hydraulic hose fittings, and single-point lubrication passages. Bore and port diameter precision-gauged on every lot for consistent flow characteristics.
Double Banjo Bolt
Extended shank length with two radial ports for dual hydraulic line junctions. Custom lengths per application drawing. Used for dual-circuit brake system junction points and hydraulic manifold splitter connections requiring two separate hose terminations at a single mounting point.
Copper Crush Washers
Soft annealed copper sealing washers to DIN 7603, sized to match each hollow Allen bolt shank diameter. Sold matched in pairs per bolt for banjo fitting sealing. Copper’s softness allows cold-flow conformity to surface imperfections under clamping load — always specify new washers for every reassembly.
Custom Hollow Allen Bolts
Non-standard bore diameters, custom port positions and quantities, 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 Allen Bolt Size Chart — M6 to M24
Thread size, standard axial bore diameter, and socket size per DIN 7643. Custom bore diameters available for specific flow requirements.
| Thread Size | Std. Bore Dia. (mm) | Socket Size (mm) | Typical Application |
|---|---|---|---|
| M6 | 3.0 | 5 | Small hydraulic lines |
| M8 | 4.0 | 6 | Motorcycle brake calipers |
| M10 | 5.0 | 8 | Automotive brake calipers |
| M12 | 6.0 | 10 | Master cylinder connections |
| M14 | 7.0 | 12 | Heavy vehicle brake systems |
| M16 | 8.0 | 14 | High-flow hydraulic circuits |
| M20 | 10.0 | 17 | Industrial hydraulic manifolds |
| M24 | 12.0 | 19 | Large-bore fluid transfer |
Hollow Allen Bolts Application Areas
Automotive brake systems, hydraulic machinery, fuel systems, and lubrication circuits — wherever a fastener must also carry fluid through its own body.
Automotive Brakes
master cylinder fittings
Motorcycle / Two-Wheeler
hydraulic clutch fittings
Industrial Hydraulics
hydraulic press circuits
Fuel Systems
fuel rail connections
Lubrication Systems
bearing lubrication points
Agricultural Equipment
implement fluid connections
Marine Hydraulics
engine oil passage fittings
Racing / Performance
custom bore restrictor bolts
Hollow Allen Bolt Standards Coverage
Same ISO 9001:2015 quality system as our complete fastener range. Bore diameter and port position gauged on every production lot — critical for consistent hydraulic flow characteristics.
Complete Documentation Package
Grade / Standard Stated
Tensile · Hardness
Diameter · Concentricity
Conformance
On Request
Related Bolt Types & Materials
Hollow Allen bolts for fluid-passage applications. Standard socket cap screws for general clamping. Hex flange bolts for integral washer convenience. SS 316L for fuel and corrosive fluid service.
Full Bolt Range
Complete bolt manufacturing capability at Akbar Fasteners — standard hex, flange, carriage, socket cap, hollow Allen, and all specialty head forms across carbon steel through exotic alloy. All from a single Mumbai facility.
View Bolt Range →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 Allen bolt’s fluid conduit function.
View Flange Bolts →SS 316L Fasteners
For hollow Allen bolt applications in fuel systems, marine hydraulics, or corrosive fluid environments where carbon steel’s corrosion resistance is insufficient — SS 316L hollow bolts provide the same fluid-passage function with chloride and fuel-compatible corrosion resistance.
View SS 316L →Brass Fasteners
For low-pressure lubrication and general oil passage applications where brass’s machinability and non-sparking properties are preferred over steel — brass hollow bolts for grease fittings and low-pressure lubrication circuits.
View Brass Range →Need Hollow Allen
Bolts / Banjo Bolts?
Bore diameter · port configuration · material · thread size · length. Akbar Fasteners Pvt. Ltd. responds within 24 hours.
Export: +91 8696 32 4646 · Support: +91 800 9315 333 · export@akbarfasteners.in