Mechanics and Fixing Any Questions Answered

Bottom bracket bearing 6805 25x37x7 Campag UT, use OEM bearings if you ever need to
Bottom bracket bearing 6805 D24 24x37x7 HT2/GXP etc., use a complete BB, it's cheaper after allowing for labour
Bottom bracket bearing 6806 30x42x7
Bottom bracket bearing 6807 35x47x7 Not common
Bottom bracket bearing 19x28x5 Use complete cartridge BB
Wheel bearing 6000 10x26x8
Wheel bearing 608 8x22x7 Only used on weight weenie front hubs, avoid
Wheel bearing 63803 D28 17x28x7
Wheel bearing 6800 10x19x5 Only used on weight weenie front hubs, avoid
Wheel bearing 6801 12x21x5
Wheel bearing 6802 15x24x5
Wheel bearing 6803 17x26x5
Wheel bearing 6804 20x32x7
Wheel bearing 6900 10x22x6
Wheel bearing 6901 12x24x6
Wheel bearing 6902 D26 15x26x7
Wheel bearing 6902 15x28x7
Wheel bearing 6903 17x30x7
Wheel bearing 6903 D18 18x30x7 Not common
Wheel bearing 63802 15x24x7
Wheel bearing 6902 D16 16x28x7 Not common
Wheel bearing 6902 15x28x7
Wheel bearing 6002 15x32x9

they've left headsets off completely! What are the most common sizes?

41.0×36°×45° ACB
41.8×45°×45° ACB

For anything else, just buy the headset.