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I've just removed fork from my Marin Four Corners and to my surprise the upper part of headset is not IS with sealed cartridge as advertised, but ZS with loose ball bearing. As the effect I don't know the brand and model.

So I have ZS upper headset with loose ball bearing and I would like to replace with cartridge bearing. As for the specification all I have is what I can measure -- like inner depth of the cup, diameter of the the bearing, its thickness.

How to find a match? Should I look only for bearing, or new cup plus bearing? If the former should I take bearing as thick as cup depth, or the same thickness as the original bearing? And what about the angles (like 45x45 or 36x45), which one I should pick?

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If you want to replace it and the lower is fine, all you need to do get is get a new complete ZS upper. There is no replacing loose balls with cartridge.

Insertion depth vs headtube bore depth is a measurement that can come into play on some bigger hit type bikes and the headsets for them, but the rest of the time you can ignore it unless you notice yourself holding a cup that's obviously much longer than the old one. Usually the machined depth in the head tube is comfortably longer than needed.

There can be variance in stack height between the new part and the old one. Stack height in this context means if you were to take the bike right now and measure the distance between the bottom of the stem and the top of the head tube, then subtract the total thickness of all the spacers, the remainder is the stack height of the upper. If the stack height is different with the new upper, you may have to redo your spacer stack. Usually this is simple to resolve, other than in cases where there's no spacer stack to tweak the dimensions on.

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  • Thank you very much. To complete "the picture", the height of cup measurement is up to bottom surface of the "collar" (so it would be insertion depth) or to top surface of the collar (so it would be total height of the cup)? Commented Feb 11, 2023 at 19:05
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    @greenoldman There are two measurements that can be relevant. 1. The end of the pressed of the cup to the lip or collar that stops against the frame, which most would call "insertion depth," and which doesn't usually matter other than on gravity bikes. 2. The stack height of the upper, whose measurement begins at the same lip (the exact plane where the insertion depth measurement ends), and goes until the plane of the where the upper meets the first spacer (or the stem if there are no spacers). Commented Feb 12, 2023 at 6:27

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