Frank - here's a not very good pic of the taps I've been using for a while. The big one is a 10mm internal diameter - 15mm gland nut/olive. Good for oil feed lines - but MUST have ignition cut-out rigged up unless memory is perfect! No good for fuel. Haven't got one on a BSA - they're reserved for oilier problems in my shed.
The smaller gas tap comes with gland nut and olive for 8mm pipe, and the internal diameter is 5.5mm or thereabouts. Perfect for fuel. These taps have 1/4 gas thread / BSP threads both ends. Not many fuel tanks have 1/4BSP threads I think - 3/8ths BSP and 1/8th BSP seem more common depending on marque and model. So to use these gas taps, you probably need an adapter to get into the tank, and a new bit for the fuel pipe side. A bit of hex drilled 6mm right through, threaded 1/4 BSP internally at one end and with a short round section at the other for the fuel line to go on is what I have made. Brass or stainless for choice - the one in the pic is only mild steel.
Also need filters. In the pic there is the sawn off top-half of a tap from a BSA tank. I tried to bore and thread it to take the new tap direct as an experiment and to save having to think about adapters and filters, but no go - not enough meat. Scruffy thing anyway. So had to make an adapter which accepted the gas tap and screwed into the tank - and could accommodate a filter. The 2 brass gauze jobs in the pic are threaded 3/8 by 20 BSF at the bottom to go onto tap or adapter, made from brass bar drilled 6mm, with gauze sheet soldered on. Long one is main tap, short one is reserve. They're ready to go.
The other thing in the pic is a dead push-pull original style tap for scale comparison.
Easy enough to male use of the things, but you do need access to some threading tackle and either to be good at soldering or have a lathe. Good luck.