Mystery Thread

Have been asked to look at an old Ewarts motorbike petrol tap. Thread is 19tpi and dimension over the threads is 0.430" Owner says it's 3/16" BSP, it's smaller than the common 1/4" BSP but can't find any reference to such a size. Can anyone throw any light on either confirming such a size existed and if yes all dimensions, or possibility it might be known as something else.

Cheers

Graham W

Must be a really ancient BSP. Even my 1948 Machinery's Handbook does not list 3/16 BSP.

OD was probably originally .437" nominal 7/16" . Threadform would be standard Whitworth 55 deg. Taper 3/4" per foot or 1:16 as per standard BSPT.

Depends how old the petrol tap is? Very early days, carburettor, and by extension perhaps fuel fitting, makers were not averse to using their own "special" threads.

Is the thread on the end that screws into the tank? Or is it the end that the olive-nut screws on to to hold the end of the copper fuel line? I remember the latter on various old Brit bangers I rode as a spotty yoof as always being some oddball thread that nothing else ever seemed to fit.

Edited By Hopper on 09/06/2020 13:22:10

Hi Graham at .430 it could be a 7/16 size. 7/16 is .437. cycle thread it 20 tpi and BSF is 18tpi easily miscounted hope this is some help or check Tracy Tools on the right side of the page . Graham

You might contact vintage bike specialists Draganfly. Their website mentions 3/16 BSP petrol tap threads.

The sizes given with the taps listed are British Standard Pipe or BSP thread sizes, tank end is given first then the outlet. A tap with a 1/8 BSP thread will need the 27-8436 adaptor with filter or A16-96, a tap with a 1/4 BSP thread will need the A16-90 adaptor. BSP thread sizes are confusing because the size relates to the size of the pipe not the thread. Nominal thread sizes are as follows (male then female):- 1/8 BSP .375, 0.35; 3/16 BSP 0.425, 0.375; 1/4 BSP 0.515, 0.468; 3/8 BSP 0.650, 0.609. All sizes of threads are 19 TPI except the 1/8, which is 28.

It appears that 3/16” BSP may have quietly been removed from the British Standard when ISO was involved.

Meanwhile ... here’s a useful link about sizes other than the one in question: **LINK**

https://www.thehosemaster.co.uk/bsp-pipe-threads

MichaelG.

Thanks guys. It's the connection to the carburettor feed, definitely 19TPI, he wants to use the original tap for authenticity's sake ????? also the thread that screws into the tank looks to be 3/8" BSP. If he'd go with the 1/4" outlet it would be easy enough to make an adapter to suit the tank but the readily available ones look quite different to this one. Tried Draganfly but they say they buy the taps in and wasn't forthcoming on the supplier, probably someone in the far east. Tracy tools has no knowledge of 3/16 BSP nor do Wisemans so Michael's suggestion might well be the reason, there's no mention of the size in any of the Machinery handbooks I have or on the web. A puzzle, it sounds like he's gonna have to compromise.

Cheers

Graham W

Just a thought - is it perhaps an 'Enots' thread? From what I can remember Enots used their own thread standard but it was close to BSP.

The other question I have is what make motorcycle is it and what year / model is it? I take it the 'Ewarts' name refers to the pet cock manufacturers name. If its the connection between tap and carb is it an olive nut that holds a tapered 'nose' on the pipe into a chamfered socket in the tap? Or is it a banjo type bolt? A picture would help if at all possible, I have a 1947 Ariel Square 4 and have some petrol cock fittings from that, I had a new tank for it about 3 years ago and still have the original sieve (that is leaky original tank!)

Edited By Oily Rag on 09/06/2020 16:03:18

You don't say if it is a parallel or tapered thread, if it is a parallel thread where the seal is made on a joint, then Tracey Tools stock 7/16 x 19tpi Whitworth form tap and dies in carbon steel. This might be what you are after. Your O/D could be the result of a slightly truncated thread form.

If your threads are tapered then this will not be any help I'm afraid.

I hope that you can solve your problem and let us know the outcome.

Neil

Or you might have to just get some hex bar and screwcut your own thread in it at 19tpi and diameter to match the tap.

Didn't see that when I searched Tracy Tools site, looks like it would do what I need, thanks for that Neil. Can't get 19tpi on my lathe so not an option Hopper. Cheers GW

There you go. So it must have been some standard thread at some stage.

Sorted now so thanks to everyone for advice and information. Great site, came through again.

Cheers

GW