Ewarts fuel tap threads

Just dropped on some Ewarts taps -all seem to have same threads for inlet and outlet.
Tank side is approx 9.5 mm diam and, using a die to check, looks to be 1/8 BSP. A bit of web searching seems to confirm.
However the fuel line side is around 10.9 mm diam with pitch of 19 tpi?  Guessed at 1/4" BSP but that's to big. Can't find any reference to an intermediate size -  3/16" BSP ?
Can anyone shed any light?

Looks like a special if you measurements are correct: The following is stolen from one of my references.

BSP parallel fittings and their specifications can be identified by a completing few calculations.
To find the thread size:
1) Measure the O.D (outer diameter) of the BSP thread.
2) Take the O.D measurement (in inches) and subtract 1/4 inch (.25”).
For example, a BSP parallel male thread measures out to O.D 1”. Subtracting 1/4 gives a
thread size of 3/4, also known as “dash” 12.
Once you have the thread size, you then need to determine the number of threads per inch to
verify that it is a BSP fitting. You can do this by counting the number of thread crests over a
1/4” length, then multiply it by 4 to get the number of threads/inch. For example, if over a
1/4” distance you find 3.5 thread crests:
1) Multiply 3.5 by 4 to get 14. This is the number of threads per inch.
2) Combine the thread size with the number of threads per inch and refer to
the following chart to find your desired size and its related specifications.


Inner Tap
Outer Diameter Threads Pitch Thread Drill
Nominal Dash Diameter Male
Thread Pitch per Diameter Height Diameter
Size Size (mm) (mm) (mm) inch (mm) (mm) (mm)
1/8" 02 9.728 8.566 0.907 28 9.147 0.581 8.7
1/4" 04 13.157 11.445 1.337 19 12.301 0.856 11.6
3/8" 06 16.662 14.95 1.337 19 15.806 0.856 15
1/2" 08 20.955 18.631 1.814 14 19.793 1.162 19
5/8" 10 22.911 20.587 1.814 14 21.749 1.162 20.75

Thanks for the info Ray.  
Nearest I could get with that formula would be 11/64" - which I doubt exists. I wonder why they went for such an oddball thread when the taps are used all over the place. And, of course, you need matching nuts to fit over hose tails and the like. Very strange.

I FOUND IT
In order to determine the gas thread needed please see the guide below:

Gas Thread Measurements
1/8" Gas 3/8" dia x 28 TPI
1/4" Gas 1/2" dia x 19 TPI
3/8" Gas 21/32" dia x 19 TPI
7/16" Gas 7/16" dia x 19 TPI
TPI = Threads per Inch

Stolen from the following site who have the appropriate bits:
http://www.burtonbikebits.net/Taps.htm

Do I get a finders fee ?

Cheers Ray - have two merit points.