So, engineering types help me here too
However, a cup plug should not come out easily unless something is wrong. Size of the plug, size of the hole, extreme temps, etc.
I'll have to measure, which I will do today because I am prepping one of the 461s, but by driving it in, you are effectively compressing a cylinder (or a ring), that ring likely has some taper (although I am not sure of that, will check that today) but it also is pushing outward, because like a keystone on an arch, that extra material has no place to go when you push it in, so it is exerting force trying to keep it's size.
So you have friction, you have a ring in compression, and you have clearances designed for the planned heat of the materials. In some cases, as Ken has noted, guys also use adhesive or bonding agent on top of that. I think Ken mentioned JB Weld, but I have never done that
My guess is that when a plug comes out, it was a damaged hole, the wrong size plug, a damaged plug or in some odd cases an overheated hole. Remember, holes open with heat, when you drop a bearing on with heat with a shaft about the same size as the freeze plug, you only need 160-180 degrees and it slides right on. A bearing like that will hold an axle even when tossing the car around curves. Not saying a cup plug will grip like that, but the cup plug, although it has different levels of compression it also has less pressure exerted on it and should behave in a similar manner, and that part of the block doesn't likely get hot enough for the level of interference fit.
For me, the 15 thou difference in the 1 3/4 versus 1 49/64 is significant based on that interference. With steel plugs, likely not as much of an issue, but when you add a softer brass plug, the combination of less OD and less resistance to compression and distortion adds more variables.
For the small oil galley plugs, I think there is a reason when one pushes out. People beat the centers in, which changes the OD and makes them out of round, guys chisel the old ones out damaging the hole, sometimes people pound them into the step which likely distorts them, not to mention, little plugs generally just frustrate me anyway, so I drill every one for NPT. It's just so much easier. I didn't know about the fine thread plugs, might have to look into that, but never saw them used.
Last thing, diesel guys will understand this, but it applies to all engines. There is a shock wave from combustion and it can cause localized effects on inside surface of the cylinder wall, generally not to the extent of a very high compression diesel under load, but it is there. In fact, diesel coolant has to be designed and tested regularly to make sure it can handle it. Along with that pressure, a thin cylinder wall could have localized steam spots that can take off like bubbles, especially with straight water, and when bleeding a cooling system, an air bubble can do some damage, I have seen heater cores blown apart due to a big burp before the thermostat opens. . Depending on how these "burp" there can be spikes of pressure in a cooling system, but in a gasoline engine, they shouldn't be enough to blow a plug out either, unless there was another contributing factor
