A recent discussion prompted me to start checking my back to back gauges collected over the years and I discovered that all 3 are different with
1 x 7.3 mm
1 x 7.39 mm
1 x 7.65 mm
and the question has to be which one is correct?
A search turned up that MOROP says 7.4 mm to 7.6 mm and NMRA says 7.6 mm to 7.75 mm which means that the 7.65 mm is nearly in the middle for all standards and just over the MOROP top end, the 7.39 mm just meets the MOROP standard and the 7.3 mm meets no standards! I guess I will stick with the 7.65 mm, though I will need to use the 7.39 mm for locos as that has a slot in the centre for the drive gear.
The 'standards' are here, sorry about the MOROP being in German but the diagram and tables are easy enough to read.
https://www.nmra.org/sites/default/files/standards/sandrp/pdf/s-4.2_2019.01.04.pdf (https://www.nmra.org/sites/default/files/standards/sandrp/pdf/s-4.2_2019.01.04.pdf)
https://www.morop.eu/downloads/nem/de/nem310_d.pdf (https://www.morop.eu/downloads/nem/de/nem310_d.pdf)
Hope they are useful - just shows how standard 'standards' can be
Except for a couple of old Arnold chassis all my limited rolling stock have been regauged to NMRA standards. I have also regauged my Minitrains locos.
There is more to this than just the back to back measurement.
It is also all about the thickness of the flange on the wheels and there has been a wide range of these from various manufactures over the years. So setting a fixed back to back for everything only works if all the items have the same flange thickness.
I use a 7.4mm back to back gauge as a guide, but I also keep a Peco point handy to test each item.
It does help us that the tolerances in the Peco 009 points are quite large and allow for quite a bit of variance with a 1mm flange gap. (1.2mm in places.)
You may well find differences in the back to back clearances in a single Peco 12 inch 009 point made before 2010 of between 6.8mm to 7.2mm, but the gauge is not constant either this can vary from 9.2mm down to 8.8mm all in the same point!
The points made from 2010 are better.
In short setting a 'standard' as with many other measurements in 009 is not in my opinion the best way forward. It is far better to just ensure that everything runs ok through the points that you have in use rather than trying to stick to a set 'standard'.
Obviously if all the manufactures of locos, rolling stock, wheels and track had all used the same standard since the mid 1960's then having 'standards' for 009 would be a good, but the simple fact is virtually none of the manufactures use the same standard and in 009 we tend to use a very wide variety of manufactures items!