Hi. Hoping someone might be able to help identify the BSP sizes of each end of my petcocks, as the BSP sizing is new to me and I'm confused :stuck_out_tongue_winking_eye:. Draganfly has a nice petcock and fittings write-up on its website...and mine "appears" to match what is listed as correct for a '44 M20 (part 27-8552, 3/16 x 1/8), but there are other sizes of that petcock type listed, too (1/4 x 3/8 and 1/4 x 1/4).

The unions for my fuel pipe at the petcock side are smaller then the union at the carb (what BSP is union at carb?), if that helps at all.

I'd like to switch over to BAP, which appears to be 1/4 x 1/4, but not sure if I can with my current tank adapters and fuel pipe...



Thanks all for your help!!

email (option): aturner312@gmail.com

The fittings on Amal 276s are usually 1/4" BSP.

It's difficult to see from the photo what sizes you have. It looks to be a tap threaded 1/8"BSP x 1/4" BSP with a 3/8" to 1/8" adapter...Can you measure the diameters in mm or decimals of an inch to confirm ?

BAP taps are normally 1/4" x 1/4" and were supplied on late Triumph twins. They're quite chunky - make sure you have enough room to fit and operate them (they don't fit Norton 16Hs). If you want to use them then you'll proably need an adapter in the fuel tank - these are available in brass from plumbing suppliers.

email (option): aturner312@gmail.com

The only acceptable repro's are sold in the Czech republic, see all sizes here:

https://www.vintage-replica.com/323--petrol-tap-with-whitworth--thread/323--petrol-tap-with-whitworth--thread-2/


BSA M20 had adapters screwed in the tank, to take the same size taps as Norton.


Cheers,


Lex

email (option): welbike@welbi**.net (think about this!)

Aaron, many of us grew up using BSP and it's still an industry standard for pipe fittings so I see it every day and don't have to think about it (with metric hexagons but that's another matter entirely !) but if you don't know them then it may be worth 'Googling' BSP dimensions.

You'll see that 1/8" BSP has a diameter of .383" - You're measuring at 3/8" which is actually .375"

1/4" BSP is .518" and you're measuring at 1/2" (which is of course .5")

I don't know what you're measuring as 7/16" (.4375")...There are several float bowls and they are not all intended to take pipe fittings directly - The male thread on the banjo fitting is the critical thing.

Does the male thread on your tank adapter measure .656" ? If so then it is 3/8" BSP (It will not be 5/8" or 11/16" !) and you will need a 3/8" to 1/4" BSP adapter to use the BAP taps. If you're in the US then you will only find National Pipe fittings which are a similar system but not suitable (and rare with a parallel rather than tapered thread).

Rik - the 7/16 I measured is the bottom threads on the petcock (that fuel pipe attaches to). That being said, I was using 1/16" graduated calipers, so not a precise number - sorry.

That being said, Dragonfly lists the following flat-slide petcock like my current ones for "M" models: 27-8552 - Petrol tap, flat slide, 1/8 x 3/16

https://www.draganfly.co.uk/bsa/bsa/a710--b3133--c101112--m202133/category/955-955-petrol-taps-pipes

It shows 3/16 BSP as .425" diameter, so guessing that is what mine are(?).

I buttoned everything up to go on a ride before I saw your question on tank adapter size. I'll check next time I pull apart.

email (option): aturner312@gmail.com

Lex - thanks for the link! I'll likely grab a set from there.

email (option): aturner312@gmail.com

Just seen this thread and to add that the taps for a WM20 where originally 1/8 BSP (3/8") at both ends. But a lot have had the tank fitting changed over the years to accept the easier to obtain 1/4 BSP taps (1/2"). Ron

email (option): ronpier@talk21.com

My M20 came from the Army with 1/4" BSP fittings at both ends of the taps...Possibly a later adaption by the Army?

1/4" BSP tank adapters are available but if they can't be found the originals can be drilled out and tapped to that size without too much difficulty...BAP taps will fit OK on the M20. I've used them on mine for years as I don't have the determination to spend weeks trying to get original taps to stop leaking...

As BSP thread diameters are not the same as the stated size it's worth looking up the thread form on the internet to familiarise yourself with the overall dimensions and specifications...

The stated sizes are derived from the inner diameter of the tube for which the thread fittings are intended...So 1/4" BSP is for for use with a 1/4" I.D. pipe, 1/8" with an 1/8" I.D. pipe etc. etc....Ian

email (option): ian@wright52.plus.com

To make life easy I just phoned Draganfly and they sent me BAP petrol taps with adapters for the BSA M20 tank.