The NMRA only deals with a maximum length. For my Sn3 wheel sets from NWSL 5 years ago I got axel lengths from 0.995 to 1.025" for a Sn3 axel. I needed an axel length of 1.008-1.012". Even a single manufacture the axel length will vary, sometimes quite a bit depending on the day axels are made. If you use different manufactures wheel sets they could even drop out of the sideframes. Always check axel lengths before you upgrade so you know they will fit. Paul
