[mfisher1971]

Pulling apart my rear cassette and freehub today for a little maintenance and I notice this:

Any ideas/thoughts what might be the cause?

 

[Bos]

Alloy freehub shell and steel cassette cogs cutting into the shell during pedaling. It's very common, generally a light rub with a fine toothed file will remove burrs to allow easy reinstall of Cassette.

 

[92gli]

Don't worry about it. If anyone here tells you to worry about it, don't worry about it.

 

[HollyBoni]

Probably a loose cassette.

On my dads MTB some brilliant mechanic installed a SRAM 9 speed cassette with one cog missing, meaning you could not tighten the cassette at all. He used the bike like this for years. It rattled like hell...
His freehub looked similar to this. I installed a new cassette, didn't change the freehub. No problems at all.

 

[LyNx]

As the others have said, that is from the individual cogs on a loose cassette, most likely judging from the lack of it below the 4th cog, an SLX. As said, a fine file will take the edges right off and if you want to mitigate this in the future get a higher level cassette which has more cogs pinned to alu carriers like an XT and/or see if you can get a steel freehub for your hub.

 

[mfisher1971]

Thanks for the info. I'll be shopping for steel.

 

[92gli]

Thanks for the info. I'll be shopping for steel.

Can you just paypal me the money instead? Cause you'll be pissing it away. I'll spend it wisely. On beer.