On all but one of the Scotts that have been thru’ my hands, those lugs have been 18mm metric. The one exception is an early (1928) TT bike that has 3/4″ Whitworth threads @10tpi. Ironically, when the bike was new it was sold to Holland, and fitted with a sidecar for the Dutch TT as Assen. You would think that metric threads would have been far more appropriate !
Brian
