When I was working underground from 1950 to 1965 wooden pit props were bought in cut to length to suit the nominal seam height and then delivered to the colliery. The colliery therefore held stocks of props to suit the height of each of its working seams. The props were transported on to the coal face and then cut to length to suit the seam height, less the thickness of the top 'plank' or steel 'corrugated strap' which was sometimes used. The length was generally measured using the method described by Dave Summers and was made so that the prop was a tight driven fit in the seam. I cannot comment about the girth of the props but I presume that there was some sort of specification which the purchasing people used at that time.
Other lengths of wooden prop were used for work in roadways etc.
Alan.
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