ORIGINAL: rhklenke
I'm definately not an engine expert, but this is my understanding. I also believe that the timing of the engine running in reverse with the housing rotated is not exactly the same as when its running forward with the housing upright, but its close enough that it runs OK.
Yep, I had an OS40FSR that I ran "backwards" on many occasions by rotating the front of the crankcase -- it put out great power and the timing difference was not obvious in the way it ran.
One thing to be aware of however, is that there may be a little extra wear on the piston/rod until the engine settles into its new direction of rotation. The side-loads on the piston-rod are shifted 180 degrees when the direction of rotation is reversed. Once again however, I found no perceptable problems resulted from this.
I just wish more modern engines had a bolt-on front section to the crankcase so we could avoid being stuck with a relatively small selection of pusher props.