FUEL GAS tank petcock adapter 3/8-1/8 Triumph Norton small fuel valve adapter
You get the 2 USA Made adapters just as pictured. This will make your 1/8" gas valve fit into a 1/4" B.S.P. hole. The filter threads in nicely. Check out our ebay store for buy it now pricing and more options and parts. Basically this will make the small British petcock fit in the large tank hole. I have only ever seen two sizes of British petcocks in my 12 years in this industry. We carry anything you need for your British or European ride so check out our store for more options.
If you are confused, maybe the below conversation will help you.