There should be a metal tag with a model number on it. It is a five digit number. The last three refer to cfm and output. 10 is the 10" stroke pump and is the biggest. 09 is the 9" and 08 is the 8", then R for right, or L for left. If it doesn't have the tag, you can pull the pully and look at the crankshaft. I can't remember the differences thought, but someone on this board knows I'm sure.