The International Harvester 1086 Tractor, produced approximately between 1976 and 1981, stands as a cornerstone of agricultural productivity, recognized for its robust power, durability, and versatility. As part of the successful '86 Series,' this model offered significant advancements over its predecessors, often featuring a comfort cab with enhanced ergonomics for extended operation and a standard 540/1000 RPM Power Take-Off (PTO) with independent PTO options. Powered by a range of International Harvester diesel engines, including D-358, D-414, DT-414, and DT-436 variants, and equipped with a 16-speed Power Transfer (PT) or Torque Amplifier (TA) transmission, the 1086 was engineered for demanding farm tasks, delivering approximately 86-110 PTO horsepower and solid torque figures. Its hydrostatic power steering and hydraulic wet disc brakes further enhanced operational ease and safety, solidifying its reputation as a reliable workhorse. This comprehensive Workshop Service Manual (WSM) provides essential information for the service, repair, and maintenance of the International Harvester 1086 Tractor. Covering a wide array of systems including the engine, powertrain, hydraulics, electrical, chassis, and operator station, this manual details procedures from routine maintenance tasks such as lubrication and fluid changes to complex component overhauls and diagnostics. It includes specific guidance on troubleshooting common issues, detailed technical specifications, torque values, clearances, and wiring diagrams, ensuring that owners, mechanics, and technicians have the necessary resources to maintain the tractor's performance and longevity. The manual is presented in English and is designed to be fully printable.