MG T-ABC

Could someone please explain why the TC petrol cap has a coil spring inside?

Because the cap is hinged.

That spring ensures there is pressure on the sealing surface.

Not that gas can't pour out the vent hole on the first hard right turn with a full tank...

All I have is the dismantled components. The end of the central rivet/pillar has been cut off to enable a new cork seal to be fitted. I have drilled it out and tapped a 1/4" thread. How long should the screw be? Should it be long enough to allow the top cup washer to drop away from the head of the screw when the assembly is compressed against the spring? If it does this there is no seal...or is this how the vent works?

If the screw is short the assembly will not compress further and consequently there is nowhere for the tank to vent...

Or at least that is how I am seeing it at the moment. ???

I don't know if these pictures will help.

The Schematic drawing above is very helpful to see the final assembly of the Petrol Cap.

However, regarding the visual presentation of the inner mechanisms of individual parts of the disassembled MGTC Petrol Cap:
A photographic view can be seen under "Technical Tips" on the Web Site provided by FROM THE FRAME UP at https://fromtheframeup.com/pages/tech-tips-1.
The Petrol Cap can be seen under their technical tips for "CR182 – TB/TC Gas Cap Assembly"

So many Petrol Caps do not have the spring installed properly. These are essential for proper functioning of the Petrol Cap -- and if the cap does not have the proper spring open action then also a few Concourse points may rightfully be deducted from the overall score.