Boiler End Plates

Should boiler end plates on a copper boiler be concave or convex? Is there a reason they go one way or the other?

smokebox of traction engine lying on plan

One reason for the convex endplates is that I intend to rivet the smokebox to the boiler of my miniature traction engine.

The rivets cannot go through the boiler surface forming a hole into the steam volume and a leak path.

On the other end of the boiler the connection is to the firebox and again I need to fix this through the surface of the boiler. Hence the end dome is again concave.

Hence we come back to the question as to whether the boiler end plates should be concave or convex? Or even does it matter?

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