The size of the piston needed for a 2-ton, 15 SEER AC condenser for the SE (Southeast) and SW (Southwest) regions can vary depending on the specific make and model of your air conditioning system. However, typically, for a 2-ton unit, the piston size required is often 0.071 inches. However, to determine the correct piston size, you’ll need to consult the manufacturer’s specifications or work with an HVAC professional.