I felt I had to check the engine again to make sure I remembered correctly.
Below are two images in order to illustrate the issue with the short driving pin for the disc. As you can see (I hope) the crank pin is only protruding very little past the conrod, and the crank pin is also chamfered at its end. This gives very little material that can grab the disc, which in turn is made from a very soft material.
I think the design would have needed a new dedicated crank with a longer crank pin, together with a brass disc or a brass counter surface on the backplate, in order to work well.
