Key stock, also known as keyway stock or key bar, is a small metal component used in machinery and mechanical systems to transmit torque between rotating shafts and components, ensuring a secure connection and preventing slippage. Key stock is often used to connect components like gears, pulleys, sprockets and couplings to shafts, providing a positive driving force without relying solely on friction.

Key stock features a standardized shape, usually rectangular or square in cross-section, and is precisely sized to fit into matching keyways or keyslots that are milled or machined into both the shaft and the component. The keyway provides a recessed groove into which the key stock fits, creating a tight and secure coupling that prevents relative movement between the components.