Hammers used to be advertised and sold as 12 lb. or 14 lb., etc.  I'm
no engineer, but it seems this was a very inaccurate method for
estimating what the weight of a hammer will be after its glued, and
with the various woods they used for moldings.

Especially since it's your first time replacing hammers, I recommend
getting them from a place that specializes in action parts, like Brooks
Ltd.  I would request that they bore, taper, and shape the hammers for
you.

Brooks has quality hammers (Abel, Knight, and more).  I don't believe
you should shell out for Steinway or Renner hammers in  this instance.
Brooks does a good job selecting the ideal hammer, and their work
(boring, tapering, shaping) is accurate.  I find the big supply houses
don't do as good a job selecting hammers, boring, tapering, or shaping.
Brooks does charge more than most supply houses.

If you change the hammers, also change the shanks, and don't forget to
"travel" them.

Bill  Maguire