why does my hot water tap not turn as fully as it did?

Most tap valves consist of a hexagonal 'male' shaft which goes up and down inside a 'female' body on a short screw thread. The washer is on a round piece at the bottom of the male. If the washer is too thin, the tap has been closed too tightly or the washer seat has been ground down too far they go out of alignment. Isolate the water, remove the head and valve, separate the parts, clean, re-washer, lubricate (vaseline), and reassemble (fiddly) and refit.

Dave Kee (Garden & Property Maintenance) Wilts
January 2011