The main difference is that BH90 is a high-performance hose with a smaller inner diameter (ID) and stiffer wall for better power & less flex (more on/off feel), while BH59 is a standard hose with a larger ID and softer wall for more modulation (gradual feel), but they share the same external diameter and olive, so only the internal barb (insert) is different and incompatible, requiring specific BH90 (silver) or BH59 (gold/brass) barbs for their respective hoses.

BH90 is mostly for 105, Ultegra, Dura Ace

BH59 is mostly for older MTB brakes

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are brake hoses BH59 or BH90 compatible?

Yes, Shimano BH59 and BH90 brake hoses are cross-compatible in terms of fitting many levers and calipers, but you must use the correct connecting insert (barb) for the specific hose you are using. The outer diameter and olive are the same for both, but the internal diameter is different.