When a Brio toy train set was to be passed on to the next generation some of the track was found to have damaged couplers. We printed replacement parts to the grandfather's specifications.
Replacement coupler installed.
Sphere diam 12.0mm (.47") Sphere chop 8.5mm (.33") Stem diam 6.0mm (.24") Stem length 20.0mm (.79")
Three out of four of the dimensions are right on, but the cylinder diameter is measuring about 0.5mm short of 6mm. The part is absolutely serviceable as is, but if you wanted to tweak it, your variable rc was 3 before, bumping it to 3.25 should put it right on.
//scad.fed.wiki/assets/viewstl/viewstl.html?stl=http://scad.fed.wiki/assets/track-coupler/coupler.stl HEIGHT 370
Upon further coupler shaft measurements they have been shown to print with an oval cross section. We've found, but not resolved, similar aspect distortion printing cubes.