A.top of the piston and bottom side of head, with the piston at BDC
B.piston and head, including the space around the piston to the top of the upper ring and under the valves, with the piston at TDC
C.bottom of the piston and bottom side of the head at TDC, regardless of upper ring location and valve placement
D.top of the piston and bottom side of the head at TDC as compared to that which exists between the top of the piston and bottom side of head at BDC