Most of the suggestions above are for black engines, ie: textured parts. The black in question is the powdercoat on the Rocker boxes, side cam chest cover etc. These are totally different than the black (or silver, early & gray, later CVO engines cases, cyl heads) The powdercoat is very hard, same stuff as the frame is coated with. Clean and treat if like the frame, do not use harsh stuff like abrasives, chemicals etc. I would personally stay away from straight S100, althought diluted would probably be O.K. Plain ole car wash soap & water with micro fiber sponge. Be careful with reg paint detailers on engine parts, some of them were never designed for high heat cycles the engine will force on them and they will fail and could fail chemically damage the surface (extreme situation, but possible). Contact Meguairs or Mothers and ask them what they recommend on powder coat if you need to brighten it vs opionions.