I pulled my cassette yesterday to clean it and the bottom spacer was difficult to get off because the freehub body has become nicked up from the cogs. It’s a problem when you generate massive wattages like I do. Can I replace just that part or should I file off the little nicks? It’s a Shimano 10sp.
Both… you can clean the freeub with a small file carefully to remove the knicks and burrs. It should also be replaceable, but I don’t know who makes your hub. You might also look into getting either a steel freehub OR a cassette with all the gears integrated onto a single spider (force is distributed) versus single stacked gears.
I got you beat though… when I first started riding a single speed mtb I revolved a gear completely around a free hub body (and destroyed the freehub). It was the perfect storm of crappy/worn components and me laying some torque down coming into a switchback.
It’s a lightweight aluminum carrier so it’s soft metal. Gently file away the burrs and put it back in to use. It will probably give you many good years yet.
One tip…when putting your hub back on, see if you can pre-load the cogs against the surface of the cassette splines as you tighten down the lock ring. Most, if not all of that damage could have been from the first time you loaded each of those gears and rotated it into impact with the splines.