Yamaha - Oil Tank Cap Assembly Oil System 6E5-21770-03-00. Overview. This is one of those parts you don't think about until it starts causing a mess or letting air into the system. A good oil tank cap needs to seal tight and keep everything where it belongs, especially on 2-stroke setups where oil flow matters. I've seen cracked or loose caps lead to leaks and even affect how oil feeds into the motor. Swapping it out is a simple fix that keeps your oil system clean and working the way it should.. Key Features. Secure Sealing Design - Keeps the oil tank properly sealed to prevent leaks and contamination.. Integrated Vent Function - Allows proper airflow to support consistent oil delivery.. Durable Construction - Built to handle vibration, exposure, and long-term marine use.. OEM Fitment - Threads and seals correctly with Yamaha oil tanks for reliable installation.. Engine & Boat Compatibility. Designed for Yamaha 2-stroke outboard engines with remote or onboard oil tank systems. Commonl