This manual covers the Volvo B21 engine, a foundational inline-4 cylinder internal combustion engine known for its robust construction and reliability, forming the basis of Volvo's early "Redblock" engine family. While specific production years vary across its applications, the B21 engine is characterized by its 2.1-liter displacement, achieved through an 88.1 mm bore and 80.0 mm stroke. Depending on the specific variant, such as the B21A or the turbocharged B21ET, power output typically ranges from approximately 90 to over 130 horsepower. Fuel delivery systems vary, including carbureted setups and Bosch K-Jetronic fuel injection for turbo models. This engine has been a cornerstone in various Volvo vehicles, valued for its enduring performance and engineering integrity. This comprehensive English-language service manual provides detailed information essential for the maintenance and repair of the Volvo B21 engine. It encompasses a wide range of topics, including engine specifications, lubrication requirements such as oil type and capacity, filter recommendations, and cooling system details. Procedures for ignition system maintenance and timing belt replacement intervals are also addressed, though specific details may refer to individual vehicle applications. The manual is designed to assist owners, technicians, and mechanics in understanding the engine's operation, performing routine servicing, and diagnosing potential issues, ensuring the continued optimal performance and longevity of the B21 engine.