I agree, great post, Keith!. And I agree that lubing the balance hole is a good idea. I apply McLube with a pipecleaner, usually with all the keys in key clamps. Takes a minute.
What we need to realize is that this is more complex than it seems at first glance, kind of like hammer/string interaction. The hammer almost always contacts the strings more than once (almost, because this may not be the case at the top treble), and knowing that can give us a better idea about voicing, and particularly about what we tend to call pre-voicing.
Similarly, a key rocking on a balance rail is not entirely straightforward. The center of the balance pin is rarely the actual fulcrum point. The center of rotation moves forward as the key goes down. How far forward depends on the condition of the punching on most pianos: if it is fairly soft and resilient, it will compress, and the fulcrum will stay close to the middle of the pin. Harder, denser felt will mean that the fulcrum will move forward, perhaps quite a bit. Smaller diameter punchings will mean less movement of the fulcrum point. )Less dense felt will compress over time, causing key level to be unstable, so there are tradeoffs).
If the fulcrum point moves, that means the key will rise on the pin as the front goes down. The feel of the result can be increasing resistance at the bottom of the key stroke, a spongy feel. This is exacerbated if the hole is cylindrical, and it can reach the point of being very noticeable. It can easily lead to the pianist feeling like the bottom of the keystroke has been reached, resulting in less actual dip than what was regulated, and reducing or eliminating aftertouch, somewhat like when a damper upstop rail is set too low. (With the action stack off, you can sometimes see and feel this with the keys being held above the punchings: you can push them down, but there is some resistance).
It is possible to make the fulcrum lie at the midpoint of the balance pin by gluing on a half card punching on the bottom of the key, as is sometimes done to change the key ratio. It could also be done with the Steinway style balance half round thingy, by cutting away material just forward of the center.
Concerning sizing the hole, parallel fluted reamers make this fairly easy and permanent, but there are caveats. My first exposure to those reamers was at the Steinway factory (Academy) maybe 15 years ago, and they used reamers .002" wider than the pin. This made for a fairly loose fit if you rotated them until the tool moved freely in the hole: the key would drop without resistance/friction. When I got home, I got one the size of the pin, which required a good bit of easing with the taper tool to reach the desired behavior. So I went to .001" over size, which requires just a bit of easing. Pianotek sells them .0015" over size, and this is a little looser than I'd like for long term. If you aren't too vigorous in turning it, the size will be about right. (Machining wood isn't as precise as machining metal or plastic. There are varying amounts of raised but uncut fibers on a microscopic level).
I have also done sizing using heat: a pin the size of the balance pin in question, inserted in a short length of 1/4" brass stock, used in a heat controlled soldering iron. This can be useful for old keys, where there are a lot of fibers sticking up. A little glue size followed by heat yields a better, more solid result.
Someone asked about RH effects. Yes, the hole gets smaller when the conditions are dry. All new pianos I have dealt with here have tight balance holes within 2 - 3 months (due to our low RH, 20% and below much of the year). OTOH, if the holes have been "eased" (wood compressed with the taper tool), this behavior will vary. The compressed fibers are likely to swell with higher RH, just as they will swell if you apply steam to remove or resize bushings. IOW, the fibers will return to their original configuration to a large degree. This is one reason I decided to ream all balance holes as standard procedure. In the process of steam sizing or rebushing, I have much less work to reestablish the balance hole configuration. (Actually, what with lubricated bushings and pins, and steam resizing of bushings, I have done maybe one rebushing job per year the past several years, on an inventory of 95 pianos, WAY down from 20 years ago).
In any case, it is worthwhile to bear this complexity in mind when dealing with keys, rather than just follow a given procedure. There are a lot of possible factors involved, and many ways to address them.
Regards,
Fred Sturm
"I am only interested in music that is better than it can be played." Schnabel