This manual covers C-125, C-145, and O-300 Series engines. These horizontally opposed, air-cooled, 6-cylinder engines are known for their robust engineering and reliability. The C-125 offers 125 horsepower from 125 cubic inches of displacement, while the C-145 provides 145 horsepower from 145 cubic inches. The O-300 series engines share the same engine type and are based on a 300 cubic inch displacement. These engines are designed for a variety of applications, providing dependable power. This document serves as an Illustrated Parts Catalog (IPC) and Parts Manual, presented in English. It is intended for reference in home, repair shop, or business environments. The manual is viewable on any computer and printable. It details the parts necessary for the maintenance and repair of the specified engine models. The owner is responsible for airworthiness, while the Pilot in Command holds responsibility for flight safety and operating limitations.