A long shot, but does anyone know the thread size of the little tap in the picture? It's on the 777a control cock/preheater.
Sometimes you can get a good idea by counting how many threads are in a centimetre, I.e if there is exactly 10 it’s a 1mm thread pitch, or if there is exactly 15 it is a 0.75 mm thread pitch, if it is a 0.9 mm there will be 11 (well a microscopic amount over) , not very technical I know but might help
Is British Brass thread 1/8th then?? I've done the best I can trying to get the TPI correct, it does seem to be 48. But it's difficult doing it purely by eye.
Mmm… British (Standard) Brass thread is 1/8” to 2” at 26 tpi (as far as I know). @ColinG might have an idea. Tony
@Ian Bingham Funnily enough I was researching all that last night, until quite late. It appears I had a couple of, what I think are, 1/8 BSW 40TPI in the bowels of my toolbox, but they only screw in so far. I've just ordered some 1/8 BSF 48's, but I'll try the 5BA next if no cigar is forthcoming...
BA was a very popular imperial thread type back in the day. I'd offer to do some research into the exact thread but last time I tried something similar I got roasted something rotten over on the sister site because I used a range of taps and dies plus a thread guage to find out what the exact thread spec was. I won't be repeating THAT mistake any time soon... well not on an open forum anyway. Anyway, good luck in your investigation!
@ColinG: Hope you're well and I know what you mean about being roasted on these forums, though why on earth anyone would castigate you for such a valiant effort is beyond logic imo.
BA provides a nicely spaced offer of sizes. The proportion, both diameter and pitch, are a base 10 logarithmic progression. This avoids the awkward leaps prevalent in other thread systems, including metric where the usual pitch becomes too coarse on smaller diameter - and a clumsy change is necessary.
I received a 5BA this morning and it threads to conclusion in one of my BR49's. Unfortunately though, the control cock I had in mind for this turned out to have it's previous tap, or some such similar, snapped off inside about halfway. I have decided not to attempt drilling and tapping it though for fear of rendering it useless. I'd wager getting such precision probably too risky and it not sealing properly. It does work as a standard control cock would, so I'll leave it be, but it's good to know for future reference of course if I, or anyone, happens upon one in future without one. Also, when i tried it in the initial lamp it did airseal with 10 pumps in the fount, but not with fuel.
So here's that control cock with the bypass tap chamber plugged and blocked. I thought I'd jazz it up a bit and give it the operable look, by putting an M3 screw in the bypass tap end, cut down and filed to something similar. I then took another M3, heated it and bent it to look like the pipe coming out of the preheat cup on the original article. This should mate up nicely to a Vapalux spirit cup I'll use on the impending lamp build. I have a BR49A cage and 169 vapouriser as a start point to build something worthwhile, if only half-hearted so to speak. All good stuff.
Thanks for the confirmation of the thread from Ian B looks great what you are fettling there had a look at my one and theres not much room for error .. trying to put a drill up there… hmm
At one point I did seriously consider the drill option tbh. I was about to buy a 2.65mm tapping bit and 5BA tap off eBay, but at the last second reneged. Whilst it would be easy to drill tentatively, until you see an orifice appearing, the exact tolerances are just too precise to guarantee a seal. The amount of thread to put in with the tap though, would be directly proportionate to an existing tap, even though the three original ones I have, have slightly different ends and thread lengths. Go figure!
Many thanks @podbros . I'll do a proper full line up pic sometime. My intention with some of the lamps was to get hold of at least two complete Tilley preheaters, certainly for the Speculum and gold one on the right. But, as the prices have gone crazy, I reckon they'll stay as is. It must be said also, if current prices of complete, good condition and working BR49's (of any variant) is up towards £300+, then at least £250 of that is in the preheater, 169 vap and leg extensions.
