This is a 40-series Integrated IS52 Bottom headset that has a Crown Race diameter of 39.79mm and a bottom stack height of 1mm. Features: Strong seal, can cross with other 40 series bearings and headset parts Bearings are black oxide sealed Durable steel crown race with low-friction face-seals Specifications: SHIS: IS52/39 Upper/lower: Lower Stack: 1mm Compatibility: 1-1/8" straight steerer tubes to tapered 52mm 1.5" IS bottom head-tubes Color: Black Weight: 43g