The _normal file uses the alpha channel to indicate shininess of the figure.

If you want to have a very shiny metal surface, make the alpha channel 'whiter'. The darker the colour on the alpha channel of the normal texture, the less shiny it should be. Never make th ealpha channel completely black ( 0,0,0 colour ) or it tends to remove it. I usually make it completely dark grey, and then lighten up areas of shiny metal. Make it completely white all over, and you get highly reflective results! Reverting to a stock _normal file would mean the thing ewas mostly dark. When you create a new texture file, the alpha channel is all white by default.