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Will thermal shock hurt a boiler and how much thermal shock? Who knows?

We were told in school that you could drain and refill a steel boiler with cold water but don't do it with CI. That's good enough for me.

I also have had the experience that @retiredguy mentioned about draining a boiler that the steam is connected to another operating boiler.

You will only do that one time believe me. The idle boiler is cold and the steam rushing in condenses and causes hammer. 1700 times a volume reduction it is a noise you never forget. It is a lot more powerful than you would think.

But as far as thermal shock why take the chance?

It will not help the boiler, it will cause stress to the cast iron.

We had a CI boiler Smith 2500 that the lwco failed. My coworker showed up and the boiler was red hot and glowing. He went back the next day after it had cooled and the boiler held water. He installed a new LWCO and fired it up and made steam. No leaks so we let it run.

A few days later it puked water all over.

@paddys , post some pics of your radiators. I'll bet there's a proper place for a steam vent on them, just have to drill and tap it.

The part above the water line in a steam boiler could be and likely is significantly above ~212 f since it won't be able to transfer heat to the steam gas nearly as efficiently as it can transfer to the liquid water. I wonder is surging and thermal shock and maybe particularly not resistant to thermal shock construction is how the problematic burnahms run in to problems.

1 radiator in particular, 2nd Floor Bedroom 2(the smallest radiator in the system), gurgles like crazy. Sounds like boiling water in the wall and at the radiator itself. This starts once it heats up. The last rad in the system, 1st floor entry, eventually does the same thing but it takes a bit longer. Working on installing the Gorton #2 and hopefully that will alleviate this problem.

It probably won't help. You likely have a sagging steam supply pipe leading to that radiator that is holding some condensate after the heating cycle ends. Ideally, all the pipes are pitched in a way that all the condensate drains back to the boiler. When the next call for heat starts, the steam has to heat up all this sitting water and will cause knocking, or gurgling, or both.

The numbers at the bottom of this image below are your "net square feet of steam" which is equivalent to EDR, which is the total square feet of exposed radiator surface in your home. The short answer is your home is tiny and you should have used the 75k boiler because its "sq ft of steam" value more closely matches your actual radiation of 230. Unfortunately it's very very common to have an oversized boiler. Yours is a lot less oversized than some we see around here. It will work, it just may so-called "short cycle" a little more than you'd like. You can minimize it by not turning down your thermostat at night or when you are away from the house for less than a day.

I don't claim to know the reason, but it is common to get corrosion where the return water, make up water and hot boiler water meet.

I have seen this many times

Not really. The gas valve locked open as a result of the electrical event keeps fuel going. That same electrical event on an oil burner would stop the fuel from pumping from the tank to the combustion chamber.