The Allis-Chalmers D-21, D-21 Series II, Two-Ten (210), and Two-Twenty (220) represent robust agricultural tractors from the mid to late 1960s through the 1970s. Engineered by the Allis-Chalmers Manufacturing Company, a prominent American producer of agricultural and industrial machinery, these models were designed for higher horsepower utility tasks. They feature diesel engines optimized for high torque at low RPM, suitable for demanding farm work, and typically utilize gear-drive transmissions with multiple speed options. While lacking modern electronic systems, their strength lies in their mechanical and hydraulic prowess, including power lift and power steering systems, and robust construction for reliable operation. This comprehensive English-language service and shop repair manual provides in-depth coverage for the Allis-Chalmers D-21 series tractors. It is meticulously organized to assist tractor owners, mechanics, and service technicians in maintaining and repairing these machines. The manual details complete engine overhauls, fuel system diagnostics, cooling system maintenance, electrical system troubleshooting, and repairs for the transmission, clutch, brakes, steering, and differential. Furthermore, it includes extensive information on assembly and disassembly procedures, routine maintenance schedules, lubrication points, fluid specifications, and troubleshooting common wear points and electrical issues, ensuring the longevity and optimal performance of your Allis-Chalmers tractor.