Short answer: match the washer’s inner diameter to the banjo bolt’s shaft, not to the fitting or the port. It’s the one measurement that actually determines fit, everything else about the washer is close to standard.
Why copper, and why crush
Copper is soft enough to deform permanently under the clamping load of the banjo bolt, filling in the microscopic imperfections between the fitting and the port on both sides. That’s the “crush”, it’s meant to yield once and seal, not spring back. Reusing a washer that’s already been crushed once is the single most common cause of a slow weep at a banjo fitting, because the metal has already taken its set and won’t deform a second time to fill new gaps.
Finding the right ID
Every banjo bolt has a listed thread size, M10, M8, 7/16, and so on, and the matching washer’s inner diameter is sized to fit over that shaft with a small amount of clearance, not to seal against the port itself. If you know your bolt size, the washer size follows directly; a 10mm-ID washer for an M10 bolt, and so on down the range BrakeQuip Products carries, from BQ1016 (10mm ID) to BQ3858 (3/8″ ID). If you don’t have the bolt off the vehicle yet, a set of calipers on the shaft (not the head, not the threads) reads the number you need in a few seconds.
Outer diameter and thickness aren’t a shopping decision
The only dimension that changes what fits and what doesn’t is the inner diameter. A crush washer’s outer diameter and thickness are sized by the manufacturer to work with that ID, wide and thick enough to seat flat under the bolt head and around the banjo fitting’s eye without you having to check either figure separately. This is why the short answer at the top holds: get the ID right and the washer fits, there’s no second measurement to cross-reference against the port or the fitting itself.
Same diameter, different thread, wrong bolt
The number stamped or listed for a banjo bolt is two things, not one: a diameter and a pitch, and it’s easy to get the diameter right and the pitch wrong. BrakeQuip’s own M10 banjo bolts come in three pitches, M10 x 1.0, M10 x 1.25 and M10 x 1.5, all the same nominal diameter and all needing the same 10mm washer, but not interchangeable with each other in the port they thread into. The inch-thread side has the same trap: 7/16 x 20 UNF and 7/16 x 24 UNS share a diameter and use the same washer, but a UNF bolt won’t thread cleanly into a UNS port. Getting the washer size right doesn’t confirm the bolt is right. Check the bolt against the caliper, master cylinder, or hose end it’s threading into. The old washer only confirms diameter, it says nothing about pitch.
Torque, and why it matters more than it looks
A banjo bolt doesn’t need to be tight the way a lug nut needs to be tight, it needs to be torqued to a specific figure so the copper crushes evenly without cracking the fitting or squeezing the washer thin enough to lose its seal. Most automotive banjo bolts in the sizes BrakeQuip stocks fall somewhere in the 12 to 25 ft-lb range, but the real number depends on the bolt diameter, the thread pitch, and what it’s threading into, so check the spec for the specific caliper, master cylinder, or hose end rather than using a single figure for every job. Under-torque and the washer never fully crushes, leaving a path for fluid to weep past. Over-torque and you can crack a caliper ear or deform the washer past the point where it seals, which shows up as the same slow leak from the opposite cause.
Always order two
A banjo fitting seals with one washer above the fitting’s eye and one below it, always in pairs, always fresh. It’s cheap insurance next to pulling a wheel back off to chase a leak that a $1 washer would have prevented.
The failure mode, in practice
Almost every banjo leak that isn’t a cracked fitting comes back to the washers, not the fitting itself. A reused washer that’s already taken its set is the most common cause. A torque that never fully crushed the washer is the second, and a wrong ID letting the washer sit off-center on the shaft instead of seated flat is the third. All three look identical from underneath the car, a slow weep at the fitting that gets worse under braking, and none of them show up on a visual check right after the job. The leak usually shows up a few days later, once the fluid’s had time to find the gap. If a banjo fitting is leaking on something that was just serviced, check the washers before you go looking at the fitting.
What we stock
The full range of copper crush washer sizes is in the Copper Washers category, sized to match BrakeQuip’s own banjo bolt lineup: 8mm, 10mm, 12mm, 15mm and 16mm on the metric side, 3/8″, 7/16″, 1/2″ and 9/16″ on the inch side.