Yeh, I know it goes at the front, but instructions say 5mm righthand offset of centre, when looking at the front of the plane. However, that seems to be with the engine mounted horizontally with the exhaust port facing up, cylinder head to the right.

I'm fitting an OS52fs and have it mounted horizontally but with the exhaust port facing down and cylinder head to the left.i.e the other way around.

So, my question is, does the 5mm offset have to be to the left of centre?

You are measuring off the wrong item. As instructions state, the off set pertains to centerline of crankshaft. Thus would not matter actual position or rotational mount of motor, or brand. Just offset the whole motor based upon the dimensions off the centerline of the crankshaft.



Wm.

Probably your plane has a few degrees of right thrust built in and the left offset is to get the crankshaft centered in the cowling. It is a fairly common practice.