The metal parts. As obvious as that seems, I may well be missing the point of your question.
Most ignition wires have the core crimped to the terminal. Most ignition wires are of a suppressor type, so that is the only form of attachment available.
With 'real wire' sets and the core being metal, strip the wire to expose the core, clean/tin the wire, tin/pool the BACKside of the terminal. Place the tinned core wire through the hole in the terminal, bend it back to be at the tinned spot, tag it to the solder pool on the terminal. Then crimp the sheath of the wire in the terminal sleeve. There's your strain relief.
No corrosion, nothing coming loose.