The Receiver Pressure Gauge O-Ring (4x1.5mm) is Part Nº 40 in Huben's official spare parts diagrams for the K1 — appearing in both Diagram 2 (Receiver/Main Body) and Diagram 6 (Filling Port), reflecting its dual role in sealing your air rifle's pressure system. This small but essential component keeps the connection between the receiver and pressure gauge airtight, preventing leaks that can affect fill accuracy and shot consistency. Over time, O-rings naturally wear from repeated fills and pressure cycles, so replacing them with this genuine Huben part ensures a precise fit and reliable seal — helping your K1 maintain consistent power output and stay in top working condition. Using original spare parts like this one is the simplest way to protect your air rifle's performance and avoid unnecessary air loss or pressure gauge inaccuracies.