Honda OEM radiator cap, 19045-MZ3-405, for GL1200 and the GL1500. Picture shows two views of cap but is sold as each. Cap measures about 2 inches in diameter.
Maintenance schedule: Honda advises inspecting the radiator and coolant core every 8000 miles or 12800 km. Replace your coolant every 2 years or 24,000 miles - whichever comes first.