The following sizes are relevant to the Dodd harp, but may
well be relevant to the Erard.
Of course, there was no standardisation of screw threads amongst different manufacturers, but I have found that there seems to be one or two favoured thread pitches.
The original sizes are, of course, all fractions of an inch, but I have found modern metric threads which work. The form of a modern thread is deeper than the old ones - if you look under a magnifier the thread looks like the thread on an old lemonade bottle top! In some cases it is necessary to drill out a slightly larger hole than the one recommended for the modern thead, which results in a flat-bottomed thread which fits the old one, otherwise the threads may bind. Sizes I found fitted on the Dodd semitone forks are:
* Note: There isn't to my knowledge a 1.8 by .5 tap - the trick is to use a 2mm by .45 taper tap and only go in a short way on the taper.
* Correction - I am informed that the Progress #18 thread fits the bill - 1.8mm with a pitch of .514mm - I am grateful to Andy Pugh for this information.
A word of warning! - Since the modern threads mentioned are not an exact match to the originals, it is easy to be caught out thinking that a semitone fork has seated on the shoulder of the operating shaft when, in fact it has just bound up on the thread. You then find that the adjustment moves after the instrument is played. A good way to check is to screw up and unscrew the fork a few times and see if the rest position moves. If it is too tight drill out the hole in the fork with a fractionally larger drill size.
2. Gilt work
The gilded work on old harps is always on top of a base of gesso, which is available ready made up. I think the original gilding was done with a bright reflective finish, which is done by water gilding. Oil gilding is easier, and I used it exclusively. Gilding work is best approached after reading a good book on it, but don't be put off - oil gilding is not that hard. But beware - new gilding looks very bright. I had no problem as everything had to be re-done, but you can cheat with gold-loaded wax polishes which can be used for restoration and blends in with old giltwork.
See "Practical Gilding" by Peter and Ann Mactaggart - available from Taranti.... useful addresses.
Making brass shims
Making soundboard pegs