Just to add to greywether’s folklore – not that I’m encouraging vandalism but it won’t work just any hour of the day.
Into it [the Toothie Stane, the large outer portal stone] have been driven at least two dozen nails or screws. Local legend explains these by saying that it used to be the custom for anyone suffering from toothache to seek a cure by driving a nail into the stone at midnight!
I am indebted to Mr S. R. Skilling [for an account] of a rather similar superstition from SW. England, to the effect that an aching tooth could be cured by hammering a nail into the trunk of an oak tree.
The excavation of the chambered cairn at Brackley, Kintyre, Argyle. JG Scott, in PSAS 89 (1955/6).