I recently installed a new belt on a Zanussi 520TD which might be similar.
1) Remove the top cover (2 hex screws at the back)
2) Fish out the old belt - use a wire coathanger made into a hook and fish around under the sides of the drum)
3) Remove the bearing screws from the rear of the machine - you might need to remove the cover over the electrics first)
4) Work the new belt through the hole making sure that it is looped over the drum spindle. Wiggle it around to ensure it is looped round the drum.
5) Hook the belt over the motor spindle and there you go!
Note: this is a bit fiddly, but it makes sure you don't disturb the front of the drum where the heating elements are.
Hope this helps - Prospero
Prospero, August 2004
