How can grand hammers be installed if the old ones are missing? I got a piano already disassembled, and there are no guide hammers, just a box of shanks.... It is a Steinway .

First position the Key Frame and then start taking measurements.

The standard distance for Steinway hammers is 5-1/8" from the center pin. The angles will have to be worked out.

Position key frame, dry fit sample hammers and determine strike point aurally.

You might try - after positioning the action, be certain the hammer on note 88 is shaped good and be certain it is hard with lacquer or key-top so that it will make a good sound, position the hammer on the shank a small distance from the capo bar - maybe 1/16" - and move it in or out till it sounds best. Mark the shank on 88 and cut the other shanks the same length and you will be close enough.
Depending on the model Steinway you can refine the strike line as you go down so don't trim the shanks till you know what you want.

How far you can move the top hammer is constrained by the belly rail. The top hammer probably should be farther in than the action can be moved to give ideal sound in pretty much every piano I see, so I recommend that when the action is set so the keys are positioned so the white key fronts are decently behind the keyslip, and the black keys are decently ahead of the fallboard, and the hammer just clears the belly rail. You can make adjustments by moving the action in or out.

But again, as I said, the standard distance from the center pin to the midline of the hammer on a Steinway grand is 5-1/8" and adjustments can be made from that.

most factory make a best tone strike for the treble, but then the 1st bass is just adjusted so the back of the sharps is the same distance from key cover than note 88

You cannot decide the strike line with the hammer gluing distance, quite the opposite in fact, as the string's plane is not level, having the heads straight will make the strike line vary a little (noy on a Yamaha indeed)

a neater way is to use an elastic thread, installed under the strings; but it is common that the strike line can be considered straight only up to note 49, and tweaked above in the last 2 sections for tone reasons, it may depend of the piano and the model)

Basic formula imply a fixed strike ratio up to A49, but real world show different approaches, all playing around the 1/7 to 1/9 , but particularely on small models the ratio can even be higher so the piano have enough partials present;

ALso common is a point just near of 1/8, then the small moves of the action in the cavity produce more differences that if the strike where 1/8 from the start.

Anyway, I find more astute to verify my strike line on the models, then the reamer is used to play with rake in some parts, so the line is respected (not really important for the line, in the basses, but rake is changing the dynamic of tone.

Also, just to facilitate things, look at the 0.5 to 3° strings plane and think how you will get your hammer 90° from that

check in Reblitz or in availeable documentation ... the schools about that may vary from continent to continent and from factory to factory, On a Steinway you are advantaged with the supple stack , hence the one size only heads used.

THanks Kamin This will be an adventure

THe reamer (helicoidal reamer is the best) is the tool of choice... I often see hammers that where glued without reaming or with not enough reaming. As the wood swell as soon as it is moistened (even with cyano) you are very fat with a hammer you cannot position correctly. Even when reamed, some shanks are conical and some can have a bow, then you have to move back and forth strong the hammer to move it..

A precise fit without glue is what allows the gluing to be done clean.

ALso the reamer allow you to move the rake where necessary, as the front of the hammer can move a little (more than the back)

Artistic angle, dealing with the hammer orientation while traveling up is then a question of training .
The original parts from Yamaha are excellent in that aspect, no much papering no much shank burning, a pleasure to work with...

Steinway can supply you with all the angles and distances from centre pin etc with regard to their models.
When I was working with a Steinway Tech, we had angle jigs and tools to determine all this. All available from Steinway.