Get a clear plastic 360 degree protractor and bolt it on the crankshaft. Clamp a length of soft wire to the exhaust port using one of the muffler screws as a pointer so you can look through the front of the protractor and see the tip of the wire against the scale. Put a piece of paper on the piston and turn the crank until the paper is just nipped by the piston at the top of the exhaust port (or use Mk1 Eyeballs ) and check the protractor reading. Now turn the crank in the other direction until the paper nips again and check the reading. The total exhaust opening will be the difference between the two readings and will most likely be somewhere around 140-150 degrees. Doing it this way you don't have to find top or bottom of the stroke. The transfer ports are a bit harder but by looking across the top of the piston you can get quite an accurate reading by seeing when the piston just reaches the top of the ports. The transfers will be somewhere in the area of 120 degrees total.