Guide
What Should a Banjo Bolt Drawing Specify Beyond Thread Size?
A banjo bolt drawing is the part drawing for a hollow screw that passes through a ring-type fitting so fluid can flow from a line, through a cross hole and the bore of the bolt, into a hydraulic or lubrication component. Beyond the thread size it has to fix the bore and cross-hole geometry, the two sealing faces, the material and the surface protection, because those, and not the thread, decide whether the joint seals and stays clean. This guide is about industrial hydraulic and lubrication lines.
By Saola Manufacturing engineering team · Published · Updated
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What does DIN 7643 cover, and what does it leave to the drawing?
DIN 7643 standardizes hollow screws for ring-type banjos and gives their dimensions, materials, surface protection and designation, but it does not describe your sealing washers, cleanliness or inspection requirements. Those still belong on the drawing or in the purchase order.
The public summary of DIN 7643 (05/2007 edition) says it applies to hollow bolts used with ring fittings per DIN 7642, that it covers eight sizes from M10 x 1 to M30 x 1.5, and that this edition replaced the August 1988 one. Its table fixes the main dimensions for each size, and the standard text is the place to read them, available from the publisher, so the drawing should reference the standard and its edition and not copy values from a catalog.
Referencing the standard keeps the drawing short, but it does not finish the job. A standard part is defined for general use; your system may need a different length, a smaller bore for a restrictor, a second cross hole, or a surface finish the standard does not call for. Each departure should be drawn explicitly and labeled as a deviation from the standard.
Which features beyond thread size does a banjo bolt drawing need to state?
State the thread and its tolerance class, the overall and thread lengths, the head and wrench flats, the bore, the number and position of cross holes, both sealing faces, the material and the surface protection. The table lists what each feature needs and why.
A banjo bolt looks simple, but nearly every dimension affects flow or sealing. Wikipedia’s banjo fitting article (2026) describes the construction: the bolt passes through the center of the union, with face seals on both sides creating the fluid path between the union and the bolt, so there are two sealing faces to control and a flow path through the bolt itself.
| Feature | What to put on the drawing | Why it matters |
|---|---|---|
| Thread | Size and pitch, tolerance class, thread length or undercut | Engagement and fit in the receiving port |
| Head | Wrench flats, height, seat face under the head | Torque transfer and the upper sealing face |
| Bore | Diameter and depth, blind or through, finish | Flow area and where chips and burrs collect |
| Cross holes | Number, diameter, angular position relative to a datum, edge condition | Alignment with the ring port and flow restriction |
| Sealing faces | Flatness, surface texture, squareness to the thread axis | Seal on both sides of the ring |
| Material | Standard designation and condition | Strength, corrosion and machinability |
| Surface protection | Coating specification or a clear statement that none is applied | Corrosion and torque-tension behavior |
| Cleanliness and marking | Acceptance basis, packaging, marking method | Particles in a hydraulic circuit and traceability |
Why do the bore and cross holes need their own notes?
Because the intersection of the cross hole with the bore is where burrs form and where they do the most harm: a loose burr travels into a valve or bearing, and a standing burr cuts the flow. A thread callout says nothing about either.
State how many cross holes there are, their diameter, and their angular position relative to a datum such as a wrench flat, because the hole has to line up with the port of the ring fitting after the bolt is torqued. Say what edge condition is required where the cross hole meets the bore, using wording such as a defined edge break and removal of loose burrs, instead of a bare general note; the break sharp edges drawing note guide and the deburring methods guide cover the wording and the options.
Intersecting holes are also hard to inspect, so decide on the acceptance method at the RFQ stage: a visual check with magnification, a borescope, or a cleanliness test of the finished part. A tolerance that nobody can measure is not a requirement.
How should the two sealing faces be specified?
Specify each sealing face by flatness, surface texture and squareness to the thread axis, and state which sealing washer the joint is designed around. A seat that is not square to the thread tilts the washer and leaks even when the thread is perfect.
The faces under the bolt head and against the ring seat the sealing washers. DIN 7603 (2001 edition) is the general standard for sealing rings for fittings and safety plugs, with a table pairing ring size to thread; its public summary does not mention banjo fittings or DIN 7643, so confirm with the system designer which ring and which ring material the joint is designed around instead of assuming one.
Put the squareness requirement as a geometric tolerance relative to the thread axis per ASME Y14.5-2018, and give the surface texture as Ra on the seat; the difference between Ra and Rz callouts is explained in the Ra vs Rz guide. Leave the washer itself off the bolt drawing unless you buy it from the same supplier, and name it in the purchase order or assembly drawing.
What material and surface protection should be called out?
Call the material by its standard designation and condition, and state the surface protection explicitly, including a plain statement if none is wanted. DIN 7643 addresses both material and surface protection, but its details sit in the standard text.
For industrial lines the usual material choices are a free-machining steel with a protective coating, or stainless steel where corrosion or media demand it. Free-machining grades cut cleanly and hold small cross holes well, while stainless raises cutting forces and galling risk on the thread; the trade-offs are covered in the stainless steel thread galling guide. State the grade by designation so the buyer and the machine shop mean the same bar.
Plating is applied after machining and, at Saola, through a partner plant that Saola manages, not at the lead CNC plant. The drawing should therefore name the coating specification and thickness basis, mask any area where plating would affect the sealing faces or the bore, and allow for the coating in the thread tolerance. The ASTM B633 zinc plating guide explains how a zinc coating is specified.
What should the RFQ and first-article inspection cover?
Ask for the standard and edition the supplier is quoting, how burrs at the cross holes are removed and checked, what packaging protects the sealing faces, and a first-article report that includes the geometric characteristics, not only the thread.
The RFQ should state the DIN 7643 edition, any deviations, the annual quantity and release pattern, and the cleanliness basis. Ask the supplier to name the deburring method for the bore and cross holes and the way it is verified.
For first articles, ask for the thread gauge result, the head and length dimensions, the bore and cross-hole data, and squareness and flatness of both sealing faces. Leak performance belongs to the assembled joint and its washer, so it is tested at system level, not on the bare bolt. The package contents are listed in the machining RFQ guide.
Can a CNC shop turn and cross drill a banjo bolt from bar?
Yes: a banjo bolt is a bar-fed turned part with a drilled bore and a cross hole, which is routine CNC turning and milling work, and the real risk is holding the sealing faces and keeping the cross hole clean.
At Saola the turning and drilling would run at the lead CNC plant, described on the CNC turning capability page and the CNC milling capability page, with any plating arranged through a partner plant that Saola manages. Send the drawing with the standard edition, the sealing washer, the coating note and the annual quantity.
Frequently asked
Is a banjo bolt the same as a hollow screw?
In industrial terms, yes: a banjo bolt is a hollow screw with a cross hole, used with a ring-type fitting. DIN 7643 is titled as dimensions for hollow screws for ring-type banjos.
Does DIN 7643 include the sealing washer?
The public summary does not say so. DIN 7603 is a general sealing ring standard for fittings and plugs, and its summary does not mention banjo fittings, so confirm the ring with the system designer.
Can I order a custom banjo bolt to a different length?
Yes, as a deviation from the standard, drawn and labeled as such. State the changed dimension and the reason so the supplier does not default to the standard value.
How do I stop burrs from reaching the system?
Specify the edge condition at the cross hole and bore intersection, require a named deburring method, and agree how it is checked, such as magnified visual inspection or a cleanliness test.
Which sizes does DIN 7643 cover?
Eight sizes, from M10 x 1 to M30 x 1.5, according to the public summary of the 05/2007 edition. Larger or non-standard threads are custom designs.
Sources
- DIN 7643, Standard dimensions for hollow screws for ring-type banjos, fasten.it summary of the 05/2007 edition (supersedes 08/1988)
- DIN 7603, Sealing rings for fittings and safety plugs, fasten.it summary of the 2001 edition
- Banjo fitting, Wikipedia, page read 2026