Lennox 97M78 Replacement Piston. Lennox goal is to provide contractors with high-quality HVAC solutions and replacement parts. This system includes changes and inventory updates to ensure peak performance. Lennox has a large line up parts and products for high-efficiency packaged rooftop units, split systems, HVAC controls, furnaces and indoor air quality products. This Lennox 97M78 Replacement Piston is designed for a R410A 4 Ton Air Conditioning Unit.