To help me sort out engine placement in the frame, I bought a left-side crankcase half from an original Aermacchi 350 engine off Ebay. Since I will not not be able to run the swingarm bolt through the pivot boss cast into the back of the Ducati engine, I was worried that the front sprocket would be too far away from the swingarm pivot, and mess with chain tension. My goal is to locate the countershaft sprocket as close as practical to its original position. After sizing everything up, I am confident that the distance won’t be any greater than originally designed by Aermacchi.
The second photo shows the Aermacchi crankcase half sitting on top of the Ducati crankcases. The countershaft holes are aligned in this image. Other than the Ducati crankcases extending further toward the front of the bike, they are of remarkably similar dimensions. What you can’t see is that the Ducati is wider. Because of this, the new engine will have to sit slightly higher in the frame (1″–1-1/2″) so that the side covers clear the lower frame rails.