Ordinary 1/4 BSP dowty seals don’t work. I bought these on eBay. More like soft o rings bonded to the inside of a washer, designed to seal the thread when tightened flat. They are a tight fit and almost need to be screwed onto the tap. The plain washers are a must.
What is the thread size and type for the tap?
Les beat me to it. Stat-O-Seals are a common industrial item here in the USA. FYI, General Motors has been using them in it's automotive air conditioning systems for the last 10-15 years. Give me a day or two. When I have time, I'll go through my Stat-O-Seal assortment and see which size fits the fuel taps [aka petcocks]. Some links you might find useful.Yes, they are actually Stat-O-Seals, not Dowty seals.
Thanks Dave on the tap OD i will be applying thread sealant as well14mm
actually 567 is actually pretty good resistance against E-10