What is the significance of shaft play in a turbo? in other words why do people emphasize "zero shaft play"?

 

The shaft play is an indication of the bearing condition in the turbo. If there is normal shaft play, the turbo will function properly. If there is too much shaft play, there is excessive gap in the bearing to the shaft which will result in the compressor and turbine wheels moving laterally more than they're supposed to. This results in oil passing by the compressor and into the charge piping. In bad cases, the compressor wheel can contact the housing and start chewing off parts of the aluminum housing.