Here’s a scan of the P:160 specs taken from the Model Railway Study Group paper. The table on the left is to .354″ gauge and is uncorrected. On the right is my sketch for gauge making corrected to .353″ gauge. By reducing the gauge by 1 thousandth of an inch you have to alter the check gauge, between checks,and the back to back by a comparable amount. In checking my B2B gauge it appears I went with .332″. For a more complete look at the MRSG paper here is a link to the Scalefour Society historical page: http://www.scalefour.org/history/p4manual.html
I wonder if the difference in gauge is due to a difference in definition or due to conversion first to metric and then back to imperial. Both appeared to have played factors in the MRSG definition of P87. In the end, a thousandth of an inch doesn’t make enough difference in P87 to be worth the amount o worry we spent on it.
Got me! I thought it was from reducing straight off of the P4 spec but that gives .3529″… So it must have happened elsewhere, oh well. That said I do need it to be a given number and .353″ works as well as any in addition to being the right one. For one thing I need practice hitting my marks and more importantly micrometers don’t read 3 or maybe 4, they read one or the other and so it is of some importance that I attempt to work to a given number. When I’m less diligent errors much greater than the odd thou creep in without asking. There is more than one roller gauge in my collection moonlighting as the end of an airtank or as a switch machine component.