6mm long: enough to go through the sleepers and standard underlay thickness. Not too long as to avoid fatigue. Self tapping: will create their own thread into the wood. No pre-drilling required unless screwing into hard wood. Blackened: will not show in the middle of sleepers. Size: correct size to fit through holes in Peco and Hornby track. Each bag contains approximately 100 screws: this is enough for more than 16 yards of track if using 6 screws per length. Recommended to be used with Golden Valley Hobbies screwdriver part number GVDRIVER. Do not overtightened as to not damage the track.