I have heard that vertical engine don't last as long because gravity pulling down on the pistons wears the cylinder and piston out faster. No, I'm not sure I believe this. Vertical is the better design for most ZTR's and I see the trend going in that direction. Does any one have hard evidence that one type is better than the other? Or is there no difference at all? Of course with a Horizontal engines you get into all kinds of problems like the mule drive.