I checked the piston housing and did not notice any damage or wear. However, the piston is badly damaged and I will put it new along with piston seals, rotating rings (?), All first speed friction discs, seperator plates and disc springs.
i measured the other disc / plate stacks. Carefully cleaned the discs before measuring. Forward stack- 40.1 mm, Reverse 40.2 mm, 3rd gear 24.3 mm, 2nd gear 15.9mm. Forward stack had one slightly bluish points on disc plate. Think of replacing this from 3rd gear.
My colleague and I could not agree on whether it is possible that these discs have been changed before. i don't want to believe they could last 15k engine hours and wear so little.
We figured out how to save on expensive gearbox costs by placing a gear pressure gauge in the cab. Or the pressure should be measured regularly for preventive purposes. As far as i know there are no sensor for each gear, only main pressure sensor.