Stringers. Packing-Blocks and Friends
The following is from A treatise on wooden trestle bridges By Wolcott Cronk Foster
“Stringers.—A stringer should be placed immediately beneath each rail, and in order to guard against defective timber it ought to be “split” or composed of two or more pieces.
“These pieces should be separated from each other by either cast-iron washers or spools, or wooden packing-blocks, or both. A considerable difference exists in the present practice as to the amount of separation. It varies all the way from nothing to 13 in. From 1-1/2 in. to 2 in. is a very good distance.”
MM Version 2
The spools vary widely as can be seen in the drawing above. Starting with No. 71 I ended up with this design. It has a 2.224 mm diameter (in O scale this is 4.2″) with a height of 1.587 mm (in O scale this is 3″) with a shaft-hole 0.66 mm/0.026″ – which allows us to use a Quilting Pin for the shafting (in O scale this is 1-1/4″).