Replacement Carbon Steel bearings for use with our PF30 / BB386 30mm threaded and press-fit bottom brackets. We have selected to use carbon steel over standard chrome steel bearings for their superior hardness and increased load capacity, offering a higher life cycle which makes them an ideal choice for mountain and road bike bottom brackets, with the 2 rubber seals acting as a barrier to help lock out dirt and ingress, also keeping the lubrication trapped inside.