Now heat the component lead. Then line it up so it is perpendicular to the crack and equally spaced on both sides of the crack.

Melt it evenly and gently into the gear across the crack. Try to only press it in 1-2mm, so most of it is beyond the edge of the gear, but don't push it in too far that it touches the shaft.

Let it cool for 3-5 seconds and then move. Moving beforehand may weaken the bond between the metal and plastic.

Necessary note: This is an art and may take time to get right, I highly suggest practicing with other plastic pieces you care less about first. Plan your attempts first. Its not quite a do or die thing, but you will start to render the plastic useless if you do not properly insert the lead.