I am now absolutely stumped by this. If I set the measurement 'C' on the diagram to the required 200mm +-1mm I do not get any clutch action, even with the set screw ('4') wound right in. Only at about 190 mm does the clutch work. Checked the diaphragm spring against a good one and it's fine. Anyway, the clutch had not been slipping. Tried a completely different clutch, same story. Other than getting quite quick at whipping out the clutch (third time), I've made no progress. How can you lose 10mm there? Has the inside of the nylon piston collapsed? Should I just ignore it and get on with it?