The finished inserts have a channel cut into them, 4mm thick and 10mm deep. There is an equvalent channel routed into the main pin-block. When the epoxy is syringed into the joint, it forms a strong key. The force of the strings on the pin-block is such that it wants to tilt the insert forward. These keys aid in resisting this force. In addition there are
shallow keys, 1mm deep all around the inserts, aiding the flow of the glue which is pumped into the joints through a small hole drilled into the top of the inserts.