Replacing an old Shimano Freehub is expensive if you have to buy a new wheel with it or sometimes impossible if it is now obsolete. Sometimes it just gets a little sticky as the grease dries up which is especially apparent in winter as the cold weather stiffens up the old, dried-out grease and the hub begins to slip. This can be inconvenient or even dangerous if it happens on a busy road.
There is a cheaper and less wasteful way to fix the hub and make it as good as new and that is to service it yourself. You will need some special tools but they are not too expensive and anyway, they're cheaper than a new wheel and hub.
This video explains how to remove, split the hub, clean and degrease everything then reassemble and refit.
You just have to be careful with the loose ball bearings and put them back in the same side of the hub and axle as they came from.
Here's the tool kit I used which is available on US amazon.com: https://amzn.to/3QlDWQj
Here's the tool kit I bought that can be found on UK amazon.co.uk: https://amzn.to/3qk8FDq
Here you can get from 10mm to 19mm Allen key wrench for the freehub retainer bolt, check which size you need as they vary: US Amazon.com - https://amzn.to/3N3kqqw
10mm to 19mm Allen Key UK amazon.co.uk: https://amzn.to/43gO01I
Should you find that your freehub is too far gone for refurbishment, here's a link to a bunch of Shimano Freehubs if you need to replace it on US amazon.com: https://amzn.to/3vZDZut
(Check the freehub model carefully as they are very different and varied or could even be obsolete).
Shimano Freehubs on UK amazon.co.uk:
https://www.amazon.co.uk/s?k=shimano+freehub+body&crid=3GIGSG61MGCF&sprefix=shimano+freehub%2Caps%2C122&ref=nb_sb_ss_ts-doa-p_2_15
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