100% CroMoly steelProprietary dimensions and spline for use only with Surly Mr. Whirly cranks3 types:Short; to fit 68 and 73mm BB shells :BBSUWA68Long; to fit 100mm BB shells e.g. Pugsley with 17.5mm offset: BBSUWA10XL; to fit 100mm BB shells but for Moonlander with 28mm Offset, spindle length 171mm: BBSUWA71CroMoly.