It depends on the type of metal used in the castings. Bolts to hold an exhaust pipe to the outlet of a manifold is fine but in aluminum heads or blocks bolts can tear out the threads easily when tightening a manifold to a head,in some cast iron this will also happen.I recently worked on a 1938 Bantam for a friend.The previous owner took out the studs that hold the head to the block and used bolts because he liked the look better .I had to put heli coils in 4 of the bolt holes because they were stripped out. With studs the torque is on the end with the nut mostly and with bolts it's on the block.In situations with heat the metal weakens in the threads so you are better off with studs .