kev
your gauge looks great
heres how to take one apart
use an old kitchen knife and put the dull side against the cover and tap with the handle of a screwdriver gently tap all the way around and it will pop right off,no threads
then gently pry the pointer off with a screwdriver
using a jewelers screwdriver take the two screws out and the cover will fall off
inside is a bourdon tube ,,as the pressure builds up it expands, and through links and gears it moves the pointer
you can see the bourdon tube ,its the flat looking arc just inside the edge,it starts at the left where the air comes in and curves to the right to engage the pointer
go to google and look up bourdon tube and it shows many examples and explains the function,better than I can
tom