The crankcase of a motorcycle is different from other aluminium parts of the vehicle. It usually comes with a smooth, shiny finish, unlike the matt, gravely finish that other parts have. This means that cleaning and polishing it is not only easy but also satisfying because you will have a beautiful, shiny crankcase when you are finished. You need to clean the crankcase prior to polishing it, but you can cheat by running the part through a dishwashing cycle!
Wipe down the crankcase with a cloth and mild soap to remove as much dirt as you can.
Run the crankcase through a normal cycle on your dishwasher and let it dry thoroughly.
Apply a small amount of metal polish to a microfiber cloth. Rub the polish onto the crankcase in small concentric circles and work your way all around.
Add more polish to the cloth as you use it up, until the surface of the case is fully covered with polish.
Buff the polish to a shine with a clean portion of the microfiber cloth until the crankcase is clean and shiny.
Polish your crankcase regularly to ease the work you will have to do each time.
Test your dishwashing detergent on a spare aluminium part to make sure it doesn't damage the finish.