It's actually a who, not a what (wink, wink). Crank scrapers wipe the excess oil off the cranks throws as they pass by the block opening, reducing windage. The Ishihara-Johnson units are simple, effective, and fit between the oil pan and the block. They are a starting point, and to be more effective, they need to be customized for each application: I spent a few hours trial fitting and grinding mine to fit my engine to optimize the gaps between the crank throws and my scrapers. Probably not essential for a street motor, but I can be anal sometimes............they make a second tier product that has a teflon wiper that "clearances" itself for a much closer fit than I wanted or probably needed. I have attached their website link for your enjoyment.