I have been doing some tests lately on :
- making dowels using 2 methods
- best woods for dowels
I don't know if those will apply to any guitar I have, but I did them just for the sake of completeness and training. It all started when I saw my tech doing this to one neck heel hole of my UV70p, in which he used Indian Rosewood plug 8mm which he cut himself. The threads felt so good, tight, almost like screwing into metal (ok... not really but close) . So I thought of doing the same to the rest 3 holes, which probably never do, I will hand it to the same tech to do the job, but it got me into dowels, woods, etc.
I tried 2 methods for the dowels :
2) A variation of https://www.youtube.com/watch?v=CNp6Gen-PVU
and 4 woods :
1) Indian Rosewood
3) common kitchen cutting-board softwood
4) common Greek pine
Greek pine was apparently the wrong piece, not dried and subsequently very soft, not usable in my case. Cutting board's wood was also soft, and used this to install the plugs in. Beech is a lot like maple, very nice and friendly to work with, and also adequately hard. But hey, the neck has already maple, so we need to go to the next level. Enter Indian Rosewood! This wood is a hell to work with hand tools, like hand-saw. It smells bad, especially under stress or burning, but is hard a a rock.
Method 2 gave me more freedom in options, but I got my share of failed rosewood pieces. Method 1 almost always resulted in plugs slightly smaller than the intended diameter.
Used for re-drilling/tapping, Rosewood is so much harder than beech (or maple) (about 2 times), so for a 3mm interal dia, 5mm external dia wood screw, I'll drill the holes with a 3mm bit for beech but with a 3.5mm bit for rosewood.
I also used three different glues to secure the plugs into their test "host" wood. 2 CYA's and 1 PVA. Again PVA behaved like always, reliably, whereas 1 CYA (UHU) failed me.
So, Indian rosewood is smelly and hard to work but is so hard that it makes IMHO the ideal wood to accept neck screws.