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Broke my heart to cut out this decorative plug.. but in the interest of research. Cast plugs don't come out easily like steel nipples. So I chose to tap the inside to 1". I think I can braze the flowered plug cut off back onto a 1" steel plug.

So feeding the bottom at 4 gpm, return from top same side. 45 minute run time.
Delta T started at 12, closed to 8 at 45 minute run.

BTU output at start, cold radiator 25.8K BTU, 500 X 4 X 12.6= 25,200 BTU/ hr

BTU output at 45 minute 19.4

So the BTU meter reading and the hydronic formula, in italics, pretty much square.
Also, no question the connections on the same side, top and bottom does allow the radiator to heat just fine. It differs from the bottom to bottom flow path by the heating profile, a small amount.

Getting toasty in here, I'll run it to thermal equilibrium, supply and return temperature stabilize.

@Steamhead
OK, I read through your case study again. I think I answered my question about unequal distribution, and how you fixed it.
I was having a problem visualizing the two sided boiler. So one side had lower flow and was getting too hot? You increased the flow to the hot side, and that resolved the problem?

No, the opposite. One side had way more flow, and that was where it was overheating and leaking- right near the return connection.

As a car guy, I'm sure you understand "dwell" in old-school breaker-point ignition. You need a certain amount of dwell to build up the magnetic field in the ignition coil, so when the points open and the field collapses, there will be enough energy in the "collapse" to generate a spark.

Same concept here. If the water is racing through the boiler fast enough, it won't stay in the boiler long enough to pick up the heat. Not enough dwell, as it were. In this case, the wildly oversized circ and the unequal distribution combine to produce this result.

@hot_rod , we'd have to locate some design info that was used to design gravity systems. I'm sure it's out there.

Oh, and having had that experience, I've been able to solve some other systems' problems. One that comes to mind was a converted gravity system with ~700 square feet of radiation. The old B&G 100 circ had been replaced with a Taco 0010, which has almost exactly the same performance curve. The lady complained that the bedroom was too hot, so I helped her turn the shutoff valve clockwise. But the room got hotter!

I'm sure you all see what the problem was now- reducing the flow made the water distribute through all of the radiator, which increased its output.

I replaced the 0010 with a 007, opened the valve, and the problem went away.

Remember, when dealing with a gravity conversion, you have to think like the Dead Men did, and try to mimic the flow they designed the system for.