What do you mean by merging ? Was it just compound part creation ?
No, it was Merged in FE Model window.
If I use “Create Compound Part” in Geometry window then it wont allow me to hex mesh the individual parts of the bolt-head-nut.
This feature only assigns the elements from multiple separate parts to one common part without actually changing the mesh: Merge parts in FE models - #5 by Matej
Hex meshing on compound parts is only possible with the transfinite algorithm.
Yes, for that you would need volume partitions done in such a way that those condition are met. It’s not always feasible but often worth giving a try for simpler geometries.
if i can remember properly, single bolt is a specific case. The mesh need to be perfectly symmetric, boundary condition and loads also. When these are not fullfiled then bolt will rotate around its own axes lead to convergence problems or hard to achieves.
@synt This makes perfect sense. I did not consider the possibility of the bolt rotating about it’s own axis. I think the best alternative is to use only half the model and use symmetric BC like you have shown.
If you want to do it manually, you can use the Exploded view.
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Of course, the Exploded View 
Why did I not think of that !?
Thank you.
