The center part of the clutch shaft is square, and both clutch drive disks slide on that square. The clutch fork in the center pushes either right or left to engage the forward or reverse clutch. The fork and the pedal mechanism have very limited movement, so the clearance of the two clutches when not driving has to be minimal or there is not enough pedal movement to fully engage one or both clutches. The clearance is set by a shim stack in the center of the shaft, which sets the distance between the two drive disks when neither of them is being pushed sideways by the fork. That means as the clutch friction material wears down, the pedal moves further until there is not sufficient movement of the mechanism to achieve full engagement. At that point you have to remove the whole clutch shaft complete with both clutches and the side bearings that support the shaft in the two chassis rails. The whole mechanism has to be stripped on the bench, and the center shim stack changed to reduce the clutch clearance. Then the assembled shaft has to be reinstalled in the mower. It is a fair amount of work, and if you are using the machine for a job that involves anything but straight cruising with the clutch fully engaged, it might have to be done fairly often.
