Flat mount brake boss gauge

These are gauges to check whether or not a bike's flat mount brake mount is close to specification.

Mount the part in a dropout with a 12mm thru axle like it is a wheel, press it flat against the dropout, and rotate it so the little nubs fit in the brake mount holes (yes, you need to remove your brake calipers first). The nubs should fit in the caliper mount holes and the flat areas around the nubs fit flat against the mount area. If the nubs do not fit, then the frame or fork (or tool) is out of spec, though the brakes might still work fine. If the flats aren't... well, flat, then the rotors may rub or wear unevenly and the flats may need to be faced by a professional.

No pictures since this is a work in progress and I don't even have flat mount brakes.

Support is needed for either of the parts, choose the build direction that makes sense to you. The thru axle holes are generously over size to accommodate filament printers that undersize holes.