Like most things in aviation, cleanliness is next to airworthiness. Here is a fuel selector setup from the 2-seaters (the 4-seaters except AG5B) look very similar. After taking the valve apart, clean everything well. Then onto the shaft put the metal detent, the spring and the washer (same order shown in the picture). The value cover (with screw threads) shown with the large outside o-ring MS29513-018 and the small inside shaft o-ring MS29513-011. Add some Parker fuel lube and put back together.