1/8" BSP has an outside diameter of 0.383" by 28tpi.
The closest ME series thread would be 3/8" diameter (0.375"). If it's only a bit finer thread, then it'll probably be 3/8" x 32tpi, if it's quite a bit finer then it could be 3/8" x 40tpi. Both are part of the ME series
Of course with it being on a miniature, it's perfectly possible that someone decided to use something like a metric thread: M9x1.25 would equate to 0.354"x31.75tpi, whilst M10x1.50 would be 0.393"x38.1tpi. I ended up acquiring a 5" gauge "Sweet Pea", where above a certain size the builder seemed to use what ever taps he had to hand - there are BSP threads in there, and best of all, the safety valves are UNC of all things - no buying those off the shelf! Lol
The best way of finding out what thread it is would be to measure the diameter and count up the tpi: Count the number of threads (peak+groove) then divide by the length over which they occupy - of course not all that easy on a female thread, especially a small one! For the sake of £5 the easiest way would be to get a set of
thread gauges, armed with those, a micrometer/vernier and a
Zeus Chart, one never need scratch one's head over threads again!
