A few years back, I went into Clerkenwell Screws, passed the man behind the counter a bolt from a second hand table and asked if he had any nuts for it. Almost immediately he came back with the comment, “oh yes, that’s a British Standard Whitworth” went in the back and came back with a handful of nuts to allow me to re-attach my table legs to my table.
I tried the same today with an unknown thread that was almost M5 (slightly too small and but apparently the right pitch 0.8mm). The response was “that’s slightly too small to be M5” and my threaded item was passed back to me unidentified.
However, given that I knew the item was from the 1960s there was a suspicion that it was a BA thread, looking up the values I found that the outside diameter for a 2BA was 4.7mm and the pitch 0.81 so there is a strong suspicion that, that’s what it is. The plan is to retap the item to be M5 instead using the new metric taps I’d just bought from Clerkenwell screws a set of 3 for just under £7.
I’ve just discovered too late that I can get cheaper HSS taps from UKDrills on Ebay….