I must admit, from the outset, I assume Al has run a pencil in the nut notches etc before narrowing it down to the machine heads.

Interestingly, two guitars I have use the same tusq nut but only one of them needs regular pencil lubrication. Why? I can only assume the zero fret, which I am a fan of, puts less pressure on the nut as it becomes the fulcrum point.

Elementary physics regarding ratios and numbers of turns of string, I tend to be surprised if the machine heads actually cause problems in most cases. I would try to look at other places where the strings can move afterwards, such as nut, saddle and pin slots.

(Just for aesthetic rather than techncial reasons, I try to put three turns of string on the machine head post, but one guitar, with slotted head, needs more than that if I use coated strings or they do slip slightly.)

I have seen where a friend couldn't keep his guitar in tune but loved it, he put a nut clamp on, borrowed from an old Yamaha Pacifica electric. I asked if it made any difference to the tone, but he said not. (I suppose after the hassle of fitting it, you woudn't admit it didn't quite work.....)