40204-ZY3-000 Extension Case Grom - Overview. The 40204-ZY3-000 Extension Case Grom is a critical rubber component used within the extension case system of Honda marine engines. Its primary role is to provide a protective seal that safeguards engine internals from water intrusion and potential environmental damage, ensuring reliable and efficient operation. Precisely engineered for Honda marine engine systems, it ensures seamless integration with existing parts, maintaining optimal performance a