Valve - Front
The piston is half way up on the exhaust stroke. When the piston reaches
TDC with the ports fully open, the disk will begin to cut off the exhaust.
The valve runs clockwise.
Looking down on top of the head the scotch yoke and upper portion of
the piston are visible, with the crank webs beneath. When the small hole
in the centre lines up with the oil hole in the centre of the sliding block,
the engine is at TDC.