[Mr V]

The general rule of thumb when tapping is that if the 2 materials are the same you need the thickness of the material that is being tapped to be 1x the diameter of the bolt that it will be used with. Coarse/fine thread it doesn't matter.

Now if you are using a hard/strong bolt into a hole tapped into a soft/weaker material, say a steel bolt into a piece that is made of aluminum the thickness of the tapped material needs to be 2x the diameter of the bolt. Again it doesn't matter if they are coarse of fine threads and there is no rule that says you need X threads.

Both of those cases are based on achieving a strength the same or substantially similar to what would be obtained by using a nut. If you don't need the ultimate strength of the bolt then you can get away with tapping into thinner material but you'll need to use a lower torque to avoid stripping the tapped material.

If you look at bolts and their corresponding standard nut (not nylocks, or jam nuts) you'll find that the height of the nut is equal to the diameter of the bolt.

Things get more complicated when you are going through a tube. The problem is that when you go through both walls of a tube you usually end up with a situation where the tube starts to crush before you reach the ultimate clamping strength of the fastener. Again it does depend on whether you need the ultimate strength of the fastener being used. But if you go through both walls of a tube then you will need to use a lower torque when tightening the fastener in some cases.


I would look at how to make that connection without a bracket and without the need for a spacer, ie move one of the components if possible so that you could attach the two pieces directly. Another option that I would consider is the use of two pieces of angle and rivets to attach them. It would again be best without a spacer but it would increase the number of fasteners and distribute the load over a greater area. Net weight should be lower and the strength should be greater, with high strength rivets.