In a grand action, the position of the jack relative to the knuckle can be adjusted, but as far as I can see, the only way to do a similar "adjustment" in a normal upright action is to change the thickness of the butt felt.

While working on my project piano, replacing butt leathers and backcheck felts, I noticed that some of the butt felts have already been replaced with felt that is slightly thinner, and even amongst the original ones there are various states of wear and compression. This results in some jacks resting further underneath the butt than others. It seems to me these notes, with the thinnest felts, have a greater propensity to bobble in soft play, all other things being as equally regulated as possible.

Intuitively, I would think that the thickness of the felt needs to be chosen somewhere in-between the extremes of
too thin = too much friction at let-off
too thick = jack doesn't carry the butt reliably but slips out before let-off.

Can anyone tell me whether there are more precise guidelines for choosing the thickness of those felt pads, and what they are?

Look in a section of the piano where there is little or no wear and just duplicate that. Factories were pretty good at getting this correct.

The only area of this piano's action that isn't worn is notes 86, 87 and 88. (Because it's an 85 note piano. )

But I take your point. Duplicate what's left of the factory default, that's what I'll try to do then. The original felts have all been pressed by the jack springs into a slight wedge shape (more compressed at the butt, less at the lower end), so I'll probably use the thickness at the lower end as being "original".

Thanks!

Generally speaking, the jack should be perpendicular to the curved hammer butt skin. Technically, it would be perpendicular to a tangent drawn where the back edge of the jack contacts the skin.

Mark:

Consistency is probably the important thing. If a standard thickness works well, replace them all.

Thank you both!

(High accuracy and consistency is probably a bit of a moot point in my project piano, because the butt skins are quite strongly indented, and some more than others. And I wish to stop somewhere short of rebuilding the whole action...)

Mark:

I know what you mean. I call it the little bird and big worm problem. Where do you try to bite it off, or do you have to swallow the whole thing?

Can you not measure the part of the leather outside of the indention to find the original thickness?

Yes, sure I can do that. But the jack doesn't know about the original thickness, because it works on the compacted part of the leather. All I was trying to say is this: I'm wary of replacing one more item, and find the action mushroom into having me replace yet another.

At that rate, I'd need to do a string, pinblock and soundboard-bridge job.