Porting is both art and science much like painting.

I trace the port profile on a piece of cardboard placed inside of the cylinder to assist in maintaining the profile of the port while cutting.

To determine the duration for a change prior to cutting I use a degree wheel and a feeler gage. On a cylinder that I am looking at now 1mm (.039”) represents 12 degrees of exhaust duration and 8 degrees of intake duration. Read the degree wheel angle at exact port closing and then with the feeler gage in the port and of course multiply by two for duration.

Aluminum cylinders can be easily cut with a tile cutter and Dremel tool. After cutting the sharp edges should be broken. I use die files and then Emory cloth. When finished there should be no sharp edges.

Recently I measured a G-62 that had a finger port that was 3/32 high by 1/4 wide on the exhaust. The benefit of finger porting as opposed to simply changing the duration has been forgotten. It is probably are related to an attempt to achieve longer power pulse. On motorcycles we sometimes cut the top of the piston to change exhaust timing.

Bill