Sierra International 18-0550 Plug Off Cap The Sierra 18-0550 Plug Off Cap is interchangeable with the following: GLM 22280, Evinrude/Johnson 909830, 3852476, Mallory 9-42721, OMC 909830, 3852476. Each Sierra product meets or exceeds the original equipment part it replaces. Sierra Plug Off Cap ID: 1-1/4" or 31.8mm Sierra part number: 18-0550 Interchangeable with the following: GLM 22280, Evinrude/Johnson 909830, 3852476, Mallory 9-42721, OMC 909830, 3852476