Just back from a week away. I had hoped that some of my time would have been devoted to "Douglas-ing", but instead lots of Rudge-ing, tidying, and tool-ing.
Re the taps: I've just posted the the following message on another post, and it applies to these taps:
"Interesting discussion. I would have said that all petrol taps of this size have 1/8 BSP male threads on the inlet, but measuring a range of tapes - Douglas and non-Douglas - I've found that while all have 28 tpi, some are BSP (0.383" or there-abouts) and some are 0.375" (i.e. 3/8-28). Bizarre!
"I'm even more confused at the outlet end. Most early (pre-1930) taps are about 7/16" major diameter. All seem to be 19tpi, but some are about 0.430" major diameter, while others are the full 7/16" (0.4375"). I suppose that the 0.428" 19tpi is a pipe thread of some sort, and the other is some form of 7/16-19.
"Yes there's only about 8 thou difference between the threads, but there are certainly different sizes! In practice, the different parts could be screwed together providing there is not too much wear."
Three of the petrol taps/filters for sale seem to have 1/8 BSP threads on the inlet end, and two seem to have 0.375-28. I don't understand this, but from my measurements on non-Douglas taps these two sizes appear in other taps. I now understand why I've experienced taps so tight that they almost seize in the tank fitting (0.383 tap, 0.375 hole), and others sloppy enough that a bit of teflon plumber's tape is useful (0.375 tap, 0.383 hole).
Michel - Doug tells me he has seen the combined tap/filter in some EW catalogue photos, but decide for yourself if it's the tap you want and let me know. The price is $A 125 + postage.
I'll try to get to the post office in the next day or so to get prices on postage. I think all the taps are now spoken for.