Originally Posted by
MilitantPotato
I'm not positive, but I'm fairly sure you can use a 9sp cog on an 8 speed cassette, so long as you've individual cogs and spacers. I found a few 9 speed 28T cogs floating round for $20.
No.
Campagnolo made the inside diameter of the splines smaller when they moved to 9 speeds and started using alloy freehub bodies across the line. It's why Campagnolo freehubs don't get notched like Shimano alloy, but comes at the expense of cog interchangeability (like the 10 speed only freehub configuration Shimano abandoned for that reason).
You must first swap a 9/10 speed freehub body onto an 8 speed hub (I put Record titanium 9 speed freehubs on both my 1996 Chorus hubs) but it needs to be compatible. All oversized alloy axle freehubs will not interchange (Chorus/Record from 1999 on, Daytona/Centaur 2000-2006). The cartridge bearing hubs have 12mm over-sized axles and I don't think their freehubs interchange with the cup-and-cone hubs with 10mm axles. Athena/Veloce loose bearing have a different axle and some people say that must move too if you're transplanting to a Chorus/Record hub or vice-versa.
Buying a used compatible hub, swapping, and selling the left overs as 8 speed would work.