1. De-activate all sub-object selection. In Command Panel, go to Create>Geometry. Create a cylinder in Top viewport. Make sure its Radius=85, Height=60 amd Sides=6. Create this cylinder at the same center of tube. Then, move this cylinder to the top area of tube (look at image below). Convert cylinder to Editable Poly. Go to Modify tab. Activate Polygon selection. Select polygons at top and bottom cap, then delete them.
2. Change to Edge selection. Select 2 edges at the side of cylinder (look at image below). In Edit Edges rollout, click Settings button right next to Connect. In opened dialog box, enter Segments=6. Click OK. Six vertical edges will be created. Repeat this process to all other sides of cylinder.
3. Now, let's modify cylinder. Activate Vertex selection. Select 6 vertices at each corner of cylinder (select in top most area only). To select more than one vertext, hold Ctrl while selecting. In Soft Selection rollout, activate Use Soft Selection. Use Falloff=46, Pinch=0.6. Then in Left or Front viewport, move these vertices down a little bit. When finished, de-activate Soft Selection.
4. De-activate all sub-object selection. Make sure one object is selected. In Edit Geometry rollout, click Attach button, then click other object in viewport. Here, you combined 2 objects as one.
Now, we need to close holes. Activate Border selection. Hold Ctrl and select 2 borders at top area of bolt. Then, in Edit Borders rollout, click Bridge button. One hole is now closed.
5. Using Bridge, close another hole at the bottom part of bolt head. Then, select border at the most bottom part of bolt. Click Cap button to close this hole.
6. Now, we are going to close hole on top of bolt head. Select border. Activate Select And Uniform Scale Tool. Hold Shift and scale down this border. Use Cap to close the hole. Then, using Select And Move Tool, in Left or Front viewport, move this border down a little bit..
7. Now, let's move to the final modification. Change selection to Edge. Select all spiral edges. You can easily select all spiral edges, only by selecting one edge and click Loop in Selection rollout. Be careful, sometimes you have to manually select or de-select edges in circled area. Rotate view in Perspective viewport to make sure the only spiral edges are selected.
8. In Edit Edges rollout, click Settings button right next to Extrude. In opened dialog box, enter Extrusion Height=10 and Extrusion Base Width=6. Click OK.
3. Last step, click Settings button right next to Chamfer. In opened dialog box, enter Chamfer Amount=4. Click OK.
3. De-activate all sub-object selection. Your bolt model is finished. Save. You'll need this when you are going to create nut.