Let's speak in terms as if you are sitting in the cockpit looking forward.....to help keep right and left correct.

The firewall of most planes will have a little bit of RIGHT thrust built in. What this means is that the firewall is angled to the right. We do this because of the forces that the engine creates causes the airplane to want to pull to the left if mounted to a firewall with no right thrust.

Now, because of the thrust angle built into the firewall, if you mount the engine in the center it will not exit the cowl (in reallity, you want to be centered to the centerline of the fuse) in the center. The angle of the firewall tilts the engine to the right and therfore has to be moved to the left to get the prop washer back to center.

Look at the picture... In the drawings of the planes, the one on the left is incorrect because the engine is centered on the firewall, the one on the right is correct because it is offset to the left which centers the prop shaft on the centerline where it exits the cowl.


Hope this helps