Assuming we're talking about the same thing, ime you'll have more difficultly getting "Dowty" washers:-
... to seal on the petrol tap threads than Stat-O-Seals:-
>1/4" bsp applies.
Exactly the thread of Triumph and later BSA petrol taps. Triumph (and later BSA) use a Stat-O-Seal on each tap against the tank and a plain steel washer between the Stat-O-Seal and the petrol tap nut. Reason is the central seal of the Stat-O-Seal is thicker than the surrounding steel or aluminium, without the plain steel washer, turning the nut against the seal simply pulls it off the surrounding steel/ally.
>I need two washers to get petrol tap at the right angle
Sounds as if you want do exactly what Triumph did - fit plain steel washer and Stat-O-Seal to a tap thread, screw tap into tank, set desired tap position, tighten nut, steel washer and Stat-O-Seal against tank to lock tap in position and seal tap thread? Only other thing you might wish to do to avoid petrol leaks down the taps threads is wrap a couple of turns of plumbers' PTFE tape around the tap thread before fitting the washers?
Another handy thing about Stat-O-Seals in preference to Dowtys is, because the central seal is thicker than the surrounding steel or aluminium means the latter won't scrape on the tank, unless you do up the petrol tap nut very tight ...
If the Triumph parts sound like your desire, 70-7351 is the part number of the Stat-O-Seal and 83-0002 is the part number of the matching o.d. plain steel washer. Be aware, if you enter those numbers into an internet search engine, the wasters on eBay and even some bricks-'n'-mortar Triumph spares dealers will offer you Dowtys as shown above.