I know that screws/bolts above 1/4" are UNF or to a lesser extent UNC
What are the smaller screw sizes?
Are they UNF, Whitworth, or BA ?

 

Generally UNF. About the only place you find BA threads are on the Dunlop brakes and SU fuel parts (carbs, pump). The timing chain adjuster nut on the engine is also BA (7/16")

 

You should not find any Whitworth/BSF threads on the car. Almost all will be UNF with UNC where the thread is in an aluminium casting. As said, BA is common on the small sizes, particularly the electrical stuff. BA is a pseudo-metric standard almost always used for electrical components in all places.
You can read all about it here: -
https://en.wikipedia.org/wiki/Britis...nd%20modelling.
The only real way to know what a bolt is, is to measure it with a caliper gauge to determine diameter, (a ruler is no good), plus a thread gauge.

 

MK1 & Mk2 Jaguars are full of UNF nuts and bolts.
I keep a hundred UNF10-32 nuts on hand as I replace old rusty nuts when I am doing any work. In the last couple of days I have removed and replaced front door handles held on by 10-32 nuts and also a MK2 front grille ,also held on by 10-32.
I also keep a hundred 8-32 nuts on hand for the occasional smaller ones that I encounter.

If you have any difficulty locating these nuts try aviation sources as American aircraft are full of UNF/UNC threads.

Bill Mac
MK1
MK2
S3XJ6
X300
15 previous Jags MK5 to X308