I’ve found that BSW (Whitworth) work on most SU carb fittings, such as banjo bolts and float bowl lid bolts, on my 120.
I am not aware of any BSC threads on an XK120, apart from the thread on the tyre pump flex male thread, that screws into the tyre pump body female thread.
Where in particular is it found on the instruments.?
Yep headlight and indicator switches at least and probably the rest of the switches!
Are you sure Terry,
More likely BSB (Brass) than BSC (Cycle), but Lucas also used BA (British Association)
BSC was a special thread that origins were in the Motorcycle Industry, and then into Push-Bicycles as well.
Thus its use on Tyre Pumps that came from similar origins.
I suggest you obtain a copy of BS.811:1938 Cycle (B.S.C.) Threads (first published October 1938)
Apart from Tyre Pumps I am not aware of BSC being used anywhere else on SS or Jaguar Cars, apart from Ed Nantes advises its use on SS-Jaguar rocker-gear (I think, never checked first hand), and Ed knows what he is talking about.
Roger Payne
CANBERRA
Australia
rogerpayne@bigblue.net.au
Yes, the BSC ball adjuster screws on rocker arms is correct, though not applicable to XK engines of course.
I forgot to mention ANF on brake cylinders and backing plates and rear shock mounting bolts, and that the rear shock links have one end ANF and the other end BSF. You also find nuts with ANF thread but the hex size is BSF. The ANF fasteners are often colored red.
All part of the fun of this hobby. 
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Huh. And here I was thinking it’s part of the insanity of it all … 
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No wonder it takes so long to restore these things. Half of our time is used up either waiting for the proper hardware to arrive or limping back to the tool chest a third time.
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Wait: you got this down to only three return trips?
Yer a god.
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