This headset and bottom bracket bearings set includes seven bearing sizes to fit a wide range of bicycles. Sizes range from 40 32 mm to 21 mm, making them compatible with MTB, road bikes, children's bikes, and most common bottom brackets on the market. Material and construction Made from quality steel for durability and long service life. Bearings cover an inner/outer diameter range of 32 40 mm, with steel ball diameters from 4 mm to 8 mm, providing reliable performance. Easy installation The package contains 2 bearings, enabling quick replacement or upgrade of your bike's headset or bottom bracket. Simple swap: remove the old bearings and install the ones to restore function. Performance and benefits Smooth operation with precisionmade steel balls that reduce friction and support consistent, reliable rotation. Helps improve overall headset and bottom bracket performance, contributing to a more enjoyable ride. Use scenarios Replacing worn bearings on aging MTB or road bikes during routine maintenance. Upgrading the headset or bottom bracket on a family of bikes, including children's bicycles, with a single bearing set. Stocking a or mobile service with versatile bearings suitable for a variety of common frame standards and bottom brackets.