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).
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.
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.