Diagram showing the use of terms on this site

PROTOTYPICAL DIAMETERS varied with wear, commonly up to 1½" was allowed off the tyre thickness (=3" off the diameter) from new.

At 10 mm/ft that allows your wheels to be up to 1.8 mm diameter under nominal size.

The diagram is based on an early design. A large amount of lip or overlap gives the impression of a thick tread. In this example the overlap allows for a rivet or bolt to pass right through the tyre lip and the wheel centre from the front to the back (as in a GER Claud). Other designs such as those using Gibson rings show different amounts of overlap - and a different apparent tread for the same actual tyre tread.

CRANK PIN THROW
In this catalogue the pin throw is its distance from the wheel centre - not the diameter of its travel. Where possible I have measured direct from a preserved locomotive or from works drawings. Otherwise for outside cylinder locos I have halved the cylinder travel. There are a few exceptions such as in the GN and LBSC Atlantics where the pin is stepped out between the coupling and the connecting rods giving a position 1" less than would be expected.