Back to back gauge size

[Simon D.]

Can anybody elucidate please?  On the US Railwire forum there is a discussion about back to back gauge for locos, and how they are often too narrow: https://www.therailwire.net/forum/index.php?topic=49460.0.  The British N Gauge brass gauges, eg from N Brass, are 7.45 mm wide, but the US NMRA standard  is 0.301 inches = 7.6454 mm according to Google.  www.nmra.org/sites/default/files/standards/sandrp/pdf/s-4.2_2019.01.04.pdf

The track is the same width, so is the variation because of different wheel sizes etc??

Many thanks

Simon

[ntpntpntp]

What back-to-back measurement you need depends on the measurements for the check-rails and guard rails etc. on your pointwork, which depends on the track system you're using?  To a lesser extent the thickness of wheel flanges and tyres can also have some bearing, as you need a compromise which will run through the pointwork without jamming on check rails and without falling into frog gaps.

To be honest, all you need to do is establish a wheelset with a back-to-back measurement which runs cleanly through your pointwork, and use that as a reference for other wheelsets.  Just holding the reference set up against another wheelset will show if there is a serious back-to-back difference.

That's all I've ever done in the four decades I've been running model trains, I've never needed a b2b gauge.