[p2bigman]

can you use a 10 speed cog and swith it over to an 8 speed

 

[LordOpie]

too vague.

Are you asking if you can take two cogs off of a 10spd cassette? Are you asking if you can use one set of shifters to control two different speed cassettes?

regardless, why are you doing whatever it is you're donig?

 

[p2bigman]

i ment the other way around sorry

 

[Wumpus]

To go from 8sp to 10 speed requires a new right shifter(rear). I've heard that the 10 speed cogs use a differently splined freehub(so you could have to get a new rear hub) and I have no idea if an 8sp derailer is going to be precise enough to shift on a 10 speed cassette(so you will probably need a new derailer)

Edit: and you will need a new chain.

 

[LordOpie]

In addition to what Wumpus said...

If you're riding an 8spd bike, you've probably had it for a while and probably have ridden it a fair amount... if that is the case, then the teeth on the front rings might be worn down significantly enough that you might need to replace them.

And if you're doing that, check the bottom bracket while you're at it. It might need replacing.