These are a more aggressive curvature flyweight than 4 stroke models from the factory. These ones have a curvature more similar to the 8DN-20 type curvature, and can be used for tuning either 2 or 4 stroke models. This more aggressive curvature is often used at much less grams than the factory (flatter) curvature- a favorite for quick revving drag applications. Available in 53, 55.5, 57.5, 60, 62,64, 66, 68 gram.