It is a bit trickey, you have to remove the blade assembly. I bought belt from espares took a week to receive. Remove the old belt which must have been worn out. Put the new belt only half on the big wheel and push the belt into the motor's recess, this is the trickey bit once the motor side is engaged push the remaining belt in place on the big wheel. Make sure the belt is evenly pushed on the big wheel, and rotate with hand, it should rotate evenly. Rplace the blde unit and test run on an even surface and it should run smoothly. If belt not evenly placed then it might slip away. I looked at this site and did not find the answer so I tried the way I described above which worked. The belt cost around 6 quid and it takes about 30 min to change. The Flymo dealer charge 25 quid for this.
cheers best of luck.
Bhim, April 2007
