Julian Roberts wrote:with its surprising assertion that the P4 BB minimum is 17.57, surprising that is to anyone who thought 17.67 was the BB minimum?
The minimum back-to-back for P4
is 17.67mm, because it says so in the P4 standard: http://www.clag.org.uk/p4standards.html#table1
If you set your wheels to that, they will work fine on P4-compliant track. It has been proven over more than 40 years that the published P4 standards do work.
However, if I was building a layout using P4-compliant track, I would regard 17.57mm as the minimum back-to-back for my wheels because experience indicates that 0.1mm clearance over the maximum check span is adequate at all normal radii. (The maximum check span for P4 is given as 17.47mm -- same link).
If I had some wheels set to 17.57mm I wouldn't claim them to be complying with the P4
standard. If I wrote down or published such dimensions I would call it something else, such as P4-M
However, I don't believe many of my wheels would actually be set to 17.57mm because in setting them I would aim to get the back-to-flange dimension close to the minimum check gauge. That's 18.15mm, so subtracting a typical production flange thickness of 0.45mm, the dimension between the wheel backs would be about 18.15 - 0.45 = 17.7mm.
In practice I'm not building a P4 layout, so the back-to-back dimension I would use on one is a bit academic. But I have been closely involved in P4 layouts in the past, so I think I'm qualified to express an opinion.
We are now on the 7th page of this topic and seem to be going round in circles. I think it would be helpful if you were to re-state what is the actual problem you are trying to solve