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My headset (onza mongo 3) has got to the point where it's both rough to turn and has play in no matter how I adjust it. (Unless I tighten to the point where you can't turn it, in which case it's arguably not rough to turn, but there may be other issues with that solution).
Should I replace just the bearings or the entire headset? Do headsets even wear out, or just the bearings?
If just the bearings, does anybody know which bearings I need to buy? Google seems no use so I have no way to find out before stripping it down, by which time I'd like to have the new bearings ready...
TBH it depends on the bearings/headset- decent headsets from cane creek and FSA can be about the same price as a pair of bearings, and some bearings are a pain to track down too. So really it should just be replace the bearings but sometimes it just works out better to do the whole thing.
Don't know company ONZA and how they are in customer service.
Maybe send them your request:
https://onza.com/pages/contact-us
Best: if ONZA supplies you new bearings - or a new headset.
If the ONZA headset is not available at all any more you might have to pick a whole new headset from FSA or similar brand (example from KCNC https://www.yumpu.com/en/document/view/54526047/kcnc2016 ).
To track down a new headset is a bit of an art...
The height of the new assembly / different headset might vary a bit in the end - means you might have to order a 1 1/8 spacer kit as well to fine tune the clamping height (will cost 5 bucks or so).
Hopefully the manufacturer of your bike cut the fork tube long enough and put spacer in to have some options in height.
If so: new headset from different company is low risk.
If not: best to get the original headset or better - the bearings!