against its locating pin and eventually cause the pin to loosen and come adrift. It should be obvious that this last difficulty will be most pronounced when the port window is not perfectly symmetrical, as any departure from symmetry will result in the ring being displaced to one side as it is pushed back into its groove.
Careful craftsmanship will prevent this asymmetrical displacement of the ring; it will not, of itself, forestall other problems associated with very wide exhaust port windows. In my opinion, one should never simply and arbitrarily widen a port out to the 70-percent limit. Instead, the safe and sensible approach is to begin at 62-percent, with a shape that is as nearly an ellipse as is possible. Quite obviously, sharp limitations are going to be imposed by the shape of the existing port window; the idea is to provide the most generous radii permitted by the basic shape with which one must begin. Obviously, too, this reshaping of the exhaust port window will be easier if you have opted for increasing the exhaust timing, as that will give you room to work above the existing port. Then, having established the initial shape, you will have to inspect the rings and the edges of the port window for evidence of scuffing or snagging. Seldom will there be any problem around the lower edge of the port, as the piston slows considerably near the bottom of its stroke. Most of any scuffing that appears will be around the comers of the port; outright snagging will make its presence known in the appearance of scratches leading upward from the center of the port window.