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Some do and some don't. These things are very small and it is difficult to maintain exact dimensions to the degree of accuracy required... also because the coefficient of expansion exceeds the dimentional accuracy required due to the miniaturization. There is an article in Live Steam magazine this month that describes making the valves and the author says that he can make two to the most exacting dimensions and get them to work well, but if he exchanges parts between the two, one or both will cease to operate properly. Even attempting to duplicate a part, usually the plunger that is what is lifted, to the samed 'mic'd' dimensions will not necessarily make a working pop valve. The suggestion to change the ball is made often as just another ball may be of the needed dimension to make it work better (though it could also make it worse!). I have had one go off with such a pleasing POP and just as quickly shut off, so I know they can work very nicely, but most often they dribble a bit, then leak some more, then vent steam and then either shut off abruptly, or do the reverse of the start up... or vice-versa, go off sharply and vent too much steam and dribble shut. Taking the ball out and putting it back can affect the operation, where just making adjustments do nothing of value... I assume it is that the ball may not be perfectly round and the removal and reinsertion just makes some different dimension in the correct location change things.
 
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