Move the Helix up until its top goes a bit above the bolt body we built before. Change the Turns value to 15.
Under its Rendering rollout check both Enable in Viewport and Enable in Renderer and set the Thickness to about 3.5 units. Then convert it to an editable polygon and select the bottom end's cap polygon. Extrude it until it no longer intersects with the bolt body.
Select the bolt body and go to the Create Panel > Compound Objects > ProBoolean. Hit the Start Picking button and pick the Helix. Your bolt is finished. That was quick, wasn't it? :)
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