A search coil is attached to the shaft with a plastic bolt. But what will happen if the bolt is made of iron? Watch the video, a metal bolt on the Garrett ACE 250 detector coil.
If someone has the bolt broken in the field and nothing to replace it, perhaps, this is an alternative for him to continue hunting somehow. But I would try first to carve the bolt out of wood. It isn’t difficult: you should choose a branch of proper thickness, on one side, there must be a fixing thickening (for example, a knot), on the other side, several turns of a thread are cut with a knife.
