This 3" Model Boiler is suitable for single cylinder model steam engines of up to 3/4"(19mm) bore and stroke, twin cylinders engines of up to 5/8"bore(16mm) and 5/8"stroke (16mm)

This 4" Model Boiler is suitable for single cylinder model steam engines of up to 1"(25.4mm) bore and stroke, twin cylinder engines of up to 3/4"bore(19mm) and 3/4"stroke(19mm)