Originally Posted by
airosen
If it's this, you just need an 8 mm allen.
Or a 7mm allen (!) if it happens to be one of a couple '80s Campagnolo cranks w/self-extracting bolts.
Originally Posted by
airosen
If it's this, you need a socket wrench.
Typically 14mm, but some (Campagnolo in particular) will require a thin-wall 15mm socket, a pipe socket, or Campagnolo's own spoon-shaped remover to clear the remover threads.
Pipe socket:
-Kurt