What I have just tried is boring the holes to exactly the diameter of the
modern ultra-round 4mm pins, then whacking the pins with a hammer to take
them out of round - slightly flattened ovals - by what turns out to be 0.2mm
or so. They are lovely to turn.
I think the problem with modern pins is that they are too round and that
they have too close a fit in their holes.
I expect that this solution is too much hard work for an instrument that's
already strung though - first rule of harpsichord tinkering: if it works,
leave it alone, no matter how much you think it could be improved.
A little bit of candle wax will help as well. Stroke the pin along a candle
to deposit a little bit of wax on it and put it back in: the tick-tick-tick
how does one un-stick modern tuning pins mounted in a relatively new walnut