What is considered a "single stroke" of a cross compound air pump? Is it when the steam side piston goes up once and then down once?<br>Or is a single stroke when the steam side goes up and down and the air side goes up and down?<br><br>

A single stroke is just that. It's when the piston rod completes a one way trip from top to bottom, or bottom to top. The number of strokes equals the number of exhausts you hear comming out of the stack. It doesn't matter if the compressor is a single stage, or a cross compound.<br>