Seal for top of original AC compressor?

[frostrobo]

Hi,
My AC was tested for leaks recently before getting it refilled and it was all good aside from the plug on the top of the compressor. The original seal for that has deteriorated and needs to be replaced.  This doesn't seem to be a seperatly available part, but is there a cross reference for the type of seal/o-ring that I can get hold of? My local garage didn't have anything suitable to fit on the day, 
The photo shows the nut with the missing seal.
   
Thanks
Simon

[Rissy]

Take the bolt out, measure the thread diameter, and buy an HNBR O ring. Wet it with PAG100 oil before fitting it into place. I’m sure that will be all you’ll need. I’m sure it’s nothing special.

[frostrobo]

Thank you, I'll give that a try!