Accurate measurements beyond norm when new

I’ve just had a pair of valves made for my BSA L25 which the maker tells me are dimensioned ‘to an accuracy of 20 microns.’ They fit very well and I’m sure a micron is a tiny measurement, way beyond the accuracy needed for my Beesa. But what is a micron? I bought the little vintage 350cc bike in the UK on a working trip about six years ago.

Lloyd Cutler, email, USA.

You assume right, Lloyd, this is a high degree of accuracy and I can’t imagine BSA’s Small Heath, Birmingham factory achieved such in 1925 when your machine was built.

1 micron = 0.001mm or 0.00003937in

1 inch = 25400 microns

In some English speaking countries, the micron is known as the micrometer or micrometre.