The most reasonably priced component used on the Cr125r 1994 (r) European Direct Sales / Cmf Expansion Chamber (1) is the Washer,plain, 6mm.
A sample from the products for the Expansion Chamber (1) include: Bolt,flange,sh 6x18, Bolt,flange,sh,6x25, Bolt Flange 6x40, Glass Wool.
On this parts fiche the Washer,plain, 6mm is the most specified component.