Banjo Bolt Sizes Explained: Thread, Pitch, Length and Bore
A banjo bolt is one of the few fasteners where getting the size slightly wrong does not just make assembly awkward. It leaks. The bolt seals a pressurised fluid path, so thread diameter, thread pitch, length and bore all have to match the port and the fitting before the joint will hold.
Most wrong orders come down to one of three things: reading the diameter but not the pitch, assuming the length figure means something it does not, or matching the bolt and forgetting the sealing washers. This guide covers the sizes in common use, how each dimension is measured and what to check before you place an order or send a specification to a manufacturer.
How a Banjo Bolt Size Is Written
A metric banjo bolt is described by three figures, usually in this order:
| Figure | What it describes | Why it matters |
|---|---|---|
| M10 | Nominal thread diameter in millimetres | Sets the port it will enter. The easiest dimension to measure and the least likely to be wrong. |
| 1.25 | Thread pitch, the distance between threads in millimetres | Two bolts of the same diameter and different pitch look almost identical. This is the most common cause of a wrong part. |
| 24 | Length under the head in millimetres | Sets how far the bolt reaches into the port and where the cross drilling sits relative to the fitting eye. |
So M10 x 1.25 x 24 is a 10mm thread, 1.25mm pitch, 24mm long under the head. Catalogues often shorten this to M10 x 24 and leave the pitch out, which is where problems start. If a listing does not state the pitch, treat the size as incomplete.
Imperial banjo bolts follow the same logic with threads per inch in place of pitch, so 7/16-20 is a 7/16 inch thread at 20 threads per inch.
Common Banjo Bolt Sizes and Where They Are Used
The sizes below cover the majority of requests. They reflect common practice across brake, fuel and hydraulic systems rather than a single standard, so always confirm against the port or the part being replaced.
| Thread | Pitches commonly seen | Typical use |
|---|---|---|
| M8 | 1.0, 1.25 | Smaller brake and clutch lines, motorcycle applications, low volume fuel feeds |
| M10 | 1.0, 1.25, 1.5 | The most widely used size. Brake calipers, master cylinders, turbo oil feeds, fuel rails |
| M11 | 1.25 | Less common, seen on some European vehicles |
| M12 | 1.0, 1.5 | Larger calipers, ABS units, higher flow oil and fuel connections |
| M14 | 1.5 | Hydraulic and fuel systems where flow requirement rises |
| M16 | 1.5 | Industrial hydraulic connections |
| M18 | 1.5 | Larger industrial hydraulic and lubrication systems |
| Imperial | 3/8-24, 7/16-20, 7/16-24, 1/2-20 UNF | Common on North American and older British systems |
Coler supplies metric and imperial banjo bolts and banjo screws in single and double configurations, and manufactures to drawing where a standard size does not fit the application.
Thread Pitch Causes More Wrong Orders Than Diameter
M10 x 1.0 and M10 x 1.25 share the same major diameter. Side by side they are difficult to tell apart, and the wrong one will start into a port by hand for two or three turns before it binds. That is long enough for someone to assume it is cross threaded rather than the wrong part, and long enough to damage the port if it is forced.
Three ways to confirm pitch before you order:
- Use a thread pitch gauge. Fastest and most reliable.
- Run a nut of known pitch onto the bolt by hand. If it runs the full thread with no resistance, that is the pitch. If it binds within a couple of turns, try the other.
- Count threads over a measured distance rather than eyeballing the whole bolt, particularly on the imperial side where 7/16-20 and 7/16-24 look similar at a glance.
If you are replacing a bolt rather than specifying a new system, measure the old one. Assuming the pitch from the vehicle or machine make is where most returns come from.
What the Length Figure Actually Covers
Length on a banjo bolt is measured from under the head to the end of the thread, not the thread length alone and not the overall height including the head.
Length matters in both directions. Too long and the bolt can bottom out in a blind port before the washers are compressed, which gives a joint that feels tight and still leaks. Too short and there is not enough thread engagement in the port to hold pressure safely. It also positions the cross drilled holes relative to the fitting eye, so a bolt of the right thread and wrong length can seal and still restrict or block flow.
When replacing a bolt, measure the original under the head rather than working from the part it threads into.
Single, Double and Stacked Banjo Bolts
A single banjo bolt carries one fitting and needs two sealing washers, one either side of the eye. A double banjo bolt carries two fittings on the same port and needs three washers, so it is longer under the head and has a second set of cross drillings.
If you are converting a single port to two lines, the thread and pitch stay the same but the length changes. Ordering a double banjo bolt at the single bolt length is a common mistake and leaves the upper fitting without enough thread engagement.
Bore Diameter and Cross Drilling
The bore is what makes a banjo bolt a fluid carrier rather than a fastener, and it is the dimension most often left off a specification. Under DIN 7643, which covers hollow screws for ring type banjo fittings, hollow screws for pipe outside diameters of 4mm to 5mm may be supplied with two or three cross drilled holes, while those for 6mm and above are supplied with three or four.
A larger bore does not automatically improve flow. If the port or the line upstream is the restriction, increasing the bolt bore changes nothing except wall thickness, and thinner walls on a pressurised fastener is not a trade worth making by accident. Specify the bore against the flow requirement of the system, not against the largest that will physically fit.
Coler holds bore tolerance on its CNC machining line so flow stays consistent across a batch, which matters on production assemblies where a variable restriction shows up as inconsistent system behaviour rather than an obvious fault.
Sealing Washers Are Part of the Size
A banjo bolt does not seal on the thread. It seals on the washers, which means the washer specification belongs in the order alongside the bolt.
- Inner diameter matches the bolt thread diameter, so an M10 bolt takes an M10 washer.
- Material affects how the joint seals. Copper is the common choice on brake systems, aluminium on lighter duty applications and steel where the joint sees higher loads.
- Crush washers deform to form the seal, so they are a single use item on most applications. Reusing them is a frequent cause of a slow leak after a rebuild.
Coler supplies matching metric and imperial sealing washers in copper, aluminium and steel alongside its banjo bolt range.
Materials, Grades and Finishes
Material choice follows the fluid, the pressure and the environment rather than the size. Steel is the default for pressurised systems. Stainless suits corrosive or washdown environments. Brass and aluminium appear on lower pressure and weight sensitive applications.
For corrosion protection on steel, the usual routes are zinc plating, GeoMet 500 and phosphating, with anodising available on aluminium parts. The practical way to specify this is to give the salt spray hours the part has to reach and let the manufacturer match a finish to it, rather than naming a coating and discovering later that it does not meet the requirement.
Banjo bolts sit alongside the wider automotive parts range, so the fitting, the washers and the clamp that holds the line can come on one purchase order.
How to Identify a Banjo Bolt You Already Have
If you are working from an existing part rather than a drawing, this sequence gets you to a complete specification:
- Measure the major diameter across the threads with a vernier caliper. This gives you the M number or the imperial fraction.
- Check the pitch with a thread gauge, or by running a nut of known pitch. Do not skip this step even if the diameter seems obvious.
- Measure the length from under the head to the end of the thread.
- Count the cross drilled holes and measure the bore through the centre.
- Note the head drive and the washer seat, including any step under the head that centres the washer.
- Photograph the bolt against a rule, and photograph the port it came out of.
Those six items are enough for a manufacturer to quote without further questions.
What to Send for an Accurate Banjo Bolt Quote
For a quote that comes back right first time, send:
- Thread diameter and pitch, or the imperial thread and threads per inch
- Length under the head
- Bore diameter and number of cross drillings
- Single or double configuration
- Material and any finish or salt spray requirement
- Working pressure and fluid type
- Sealing washer requirement, including material
- Initial order quantity and expected annual volume
- A drawing or a sample where one exists
If any of those are still open, say so. It is easier for a manufacturer to advise on an undecided dimension than to requote a part built to an assumption. The same principle applies across any component, and our guide on what to send for a fast, accurate quote covers it in more detail.
FAQs
What is the most common banjo bolt size?
M10 is the most widely used thread diameter across brake and fuel systems, most often in 1.0mm or 1.25mm pitch. That does not make it a safe default. Confirm the pitch against the port before ordering.
Is M10 x 1.0 the same as M10 x 1.25?
No. They share a 10mm major diameter and have different thread pitches, so they are not interchangeable. The wrong pitch will thread in partway by hand before binding, and forcing it damages the port.
Can I use a longer banjo bolt than the original?
Only if the port is deep enough and the cross drilling still lines up with the fitting eye. In a blind port, a longer bolt can bottom out before the washers are compressed, which produces a joint that feels tight and leaks under pressure.
Do I need new sealing washers every time?
On crush washer joints, yes in most cases. The washer forms the seal by deforming, so a reused washer has already taken its set and is a common source of a slow leak after reassembly.
Can banjo bolts be made in non standard sizes?
Yes. Thread, length, bore, number of cross drillings and head profile can all be produced to drawing where no standard size fits. This is routine on retrofit work and on machinery where the original part is obsolete.
Specify All Four Dimensions, Not Two
A banjo bolt is only correctly specified when thread diameter, thread pitch, length under the head and bore are all defined, with the sealing washers named alongside them. Two out of four gets you a part that threads in and leaks.
If you are working from an old bolt, measure it. If you are specifying a new system, define the flow requirement before the bore. If the size you need does not appear in a catalogue, it is almost certainly still manufacturable.
Send Coler the thread, pitch, length and bore, or send the bolt itself and a photograph of the port, and the team will confirm the specification and quote against it. Where the size you need does not exist as a standard part, Coler will quote on a drawing or a sample rather than a catalogue number, in metric or imperial, single or double, with matching sealing washers.