Exhaust bungs / silencer plugs help to keep dirt and water from entering the muffler system while washing up after a day's riding. And by keeping one plugged in during long periods of non-use, it can also help to minimize internal corrosion.
Sizing:
FMF-012875: Small (eg, 2 Stroke mufflers)
FMF-011299: Large (eg, 4 Stroke mufflers)