Richard Syms
I have always used 16.5mm back-to-back on Gibson wheels, using an L-shaped gauge which I think came from EMGS Stores many moons ago. I use a GW models wheel press to quarter and assemble them, but I do find that some of the axles supplied by Gibson are slightly overlong, which can create problems. I always start by placing a wheel on each side of the bb gauge, without an axle in place, and measuring the outside dimension over the wheel bosses with a caliper. I then check the axle lengths with the caliper, and, if necessary, file the ends down to ensure that they won’t protrude from the bosses when assembled.