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Shouldn't the striking distance be the same for all hammers? My farthest base hammer (#1), with essentially no wear, has a bore of 2.15", and my treble bore is 2.35". The trebles may be questionable for wear, but not by 2 tenths of an inch. Is this typical? I am about to order a few treble hammers to replace a few worn trebles.

The strike distance should indeed be roughly the same .... but the distance between bass and treble hammers to the string is different .... being overstrung !

You have me wondering how you are going to manage getting a few new treble hammers though .... they only come in complete sets

Cynthia, I am unsure of your question, depends i the piano is cross strung or no, but you cannot "really" order a few treble hammers.
If they are really worn you could glue a little leather on the last high treble hammers. not an extraordinary result but you will be less prone to break strings.

the techs where doing so some 50-80 years ago.

Some of the US suppliers used to sell partial sets of treble hammers, usually the top twenty.

OK, I forgot about the cross over of the bass strings over the tenor strings. That explains the difference.

What is the danger in replacing these hammers? There have already been a number of these high treble hammers replaced. The wear on the highest trebles is much more than on the rest of the hammers, or maybe there just wasn't much felt there in the first place. It appears that a few of the highest trebles were lacquered unlike the rest of the hammers and are practically solid plastic of some sort. Acetone/lacquer thinner had no effect.

These strings will be eventually replaced, but one new challenge at a time.

International Piano Supply will sell a few hammers, but this depends on their having the right size, so we'll see. I could live with just one new hammer I suppose. Also, I see that in my treble hammers, the hammers appear to be raked downward slightly. The angle between the hammer centerline and the shank is about 88 degrees. Is this typical?

OK, found my answer here, https://www.pianoworld.com/forum/ubbthreads.php/topics/2086727/Hammer%20Rake%20Angle%20(upright).html

You mean, aside from explosion or releasing poisonous gases? (I hate it when that happens!) The biggest danger is that you will do it poorly. The pitfalls that you can run into depend on how you do it.

Collodium was used on the top end most likely.

Usually, string height - center pin height = bore distance. I measure center pin height for note #1 and #88, then measure the string heights for the first and last note of each section (plus the first and last wound tenor strings, if present). I also measure bore distance on the hammers... usually they are the same, or close enough... it is normal for string heights to vary a few mm throughout the piano. Using two methods lowers the chance of making mistakes.

it is normal for string heights to vary a few mm throughout the piano.
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Very true, especially where the agraffe transitions to capo and from there to the treble - some pianos worse than others.
The question for Cynthia is how to cope with this.
Keep a straight hammer line? Keep a consistent bore distance? Keep a consistent hammer blow distance? Keep a consistent key dip? You cannot have it all.

up to 3 mm the hammers can have a standard bore.