High-Performance Dual-Chamber Fluid Cooler The most effective hydraulic fluid & oil cooler available. Transmission fluid, engine oil and power steering fluid all have one major enemy – heat. Overheated fluids can cause costly damage from breakdown due to heat. The DUO-COOLER protects and prolongs life for all automatic transmissions, engines and power steering systems. Installation Requires two 1/4″ NPT hose connect fittings. HR-9800-12 (12″ long) Recommended for smaller engines, small-medium automatic transmissions & all power steering systems. HR-9800-18 (18″ long) Recommended for larger engines & large automatic transmissions.