The John Deere 644B is a heavy-duty wheel loader engineered for robust performance in demanding applications. It features a turbocharged 6-cylinder diesel engine with a displacement of 531 cubic inches (8.7 L), delivering a gross power of 160 hp (119 kW) and net power of 145 hp (108 kW) at 2200 rpm, with peak torque of 432 lb-ft (586 Nm) at 1500 rpm. The loader is equipped with a power shift planetary transmission offering 4 forward and 2 reverse gears, enabling a max forward speed of 25.8 mph (40 kph) and reverse speed of 10.2 mph (16 kph). The hydraulic system includes an independent engine-driven gear pump supplying 58.2 gallons per minute (220 L/min) at a relief valve setting of 2250 psi, ensuring strong bucket and steering control. Key bucket specifications include a breakout force of 24,120 lbs (10,941 kg), a heaped capacity of 2.5 cubic yards (2 m³), struck capacity of 2.1 cubic yards (2 m³), and a bucket width of 8.8 ft (2 m). Dump clearance at maximum lift reaches 9.4 ft (3 m) with a hinge pin height of 12 ft (4 m), providing excellent material handling reach and lift height. The machine’s dimensions include a wheelbase of 8.7 ft (2.65 m), overall length with the bucket on the ground of 21.1 ft (6.4 m), and a height to the top of the cab of 10.7 ft (3.3 m). Operating weight is approximately 28,100 lbs (12,746 kg), supported by 14.5-25 and 20.5-25 tires, and a turning radius of 15.5 ft (4.7 m). Additional features include a 12 V electrical system with a 42 A alternator, a 56-gallon (212 L) fuel tank, and substantial fluid capacities for engine oil, hydraulics, cooling, axles, and transmission to support extended operation. Rear axle oscillation is 22°, enhancing stability on rough terrain. The design balances power, hydraulic efficiency, and maneuverability for versatile use in construction, loading, and material handling tasks.