PS 1/4 BSP fittings are usually readily available from "Vintage Supplies" and "The Green Spark Plug Co"

Ron

I've checked 5 standard M20 fittings and the breather fitting thread is 7/16" x 20 Cycle thread as far as I can deduce..20tpi is the 'coarse' version of Cycle Thread and is used for various applications on BSAs. The 19t thread gauge is definitely not a perfect fit to the thread on the fittings I have...

Regarding the petrol taps/pipes, my M20 came from a chap who'd owned it since demob and one other owner, both of whom I knew... It had a standard type petrol pipe fitted..Neither owner had altered anything on the bike from its 'demob' specification. However, both the internal thread in the tank adapters and the tap threads and petrol pipe nuts were all 1/4" BSP...Postwar replacements perhaps?...

Of course it's entirely possible an alteration has been made in this case....As a point of interest the OHV M/B Series postwar, timing cover mounted breather assemblies use a 1/4BSP thread and use a larger diameter breather pipe...Ian

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

Hi Ian, I’ve just got a pair of eyes, attached to a 24 year old, to double check the pitch. You’re exactly right, it’s 20tpi! In my defence it was 05.55 this morning when I dropped the pitch gauge on! The early hour and my newly acquired ‘special Austin fuel thread’ info. Made it seem like 19tpi was spot on.
In conclusion, the 7/16 diameter and 20tpi must make it BSC coarse, as you told me it would be, thanks. My two fuel taps have the same thread on their outlet also.
Thanks for clearing it up,

Tim.

email (option): timmykins@icloud.com

Just to confuse matters here, Draganfly list an Enots type tap for the 1940 - 1947 'M' series as 1/8 upper x 3/16 lower and state that all sizes are BSP. 3/16" BSP seems to be an obsolete vehicle size that was never an industry standard.

https://draganfly.co.uk/product-category/bsa/sparesparts-for-bsa-a710-b3133-c101112-m202133/oil-petrol-tank-abcm/petrol-taps-pipes-abcm/

On the basis that BSP is often given as dimension + 1/4", both 3/16" BSP and 7/16" would have a .4375" OD and it would seem likely that the BSP thread would be 19tpi as per 1/4"....A brass thread of 19 or 20 tpi that has been forced could easily give a misleading measurement.

So......I’ve checked my fuel taps, the thread on the outlet side is 7/16 x 19 tpi. The thread on the crank case breather outlet is 7/16 x 20 tpi.
In conclusion then, the breather thread is 7/16 bsc coarse and the fuel tap thread looking to be the elusive 3/16 bsp. Possibly adopted as a brass saving measure and used on, amongst other things Austin 7 fuel systems.

email (option): timmykins@icloud.com

Enots used a special thread for the pipe fitting 7/16 x 19 TPI.
This from RudgeWhitworth.com website.

It would seem though that the lower fitting has a 60° female taper as per BSP....so if it was unique to Enots then it followed BSP conventions.

Some careful examination is called for to check if it has a 55° thread angle. If so then it is likely that Benton & Stone created their own intermediate 3/16" BSP size....

For my breather pipe, which thanks to Ian we’ve established is BSC Coarse 7/16 diameter 20tpi. I only managed to source a 7/16 diameter 19tpi fuel pipe union (the ones that fit the Enots fuel taps, also discussed in this thread).
Using a 1.25 pitch metric thread file (60 degree thread form like BSC, much cheaper than a BSC thread file and has got me out of trouble with lots of BSC thread issues) I’ve ‘eased’ the thread in the union. So it now screws onto the breather with slightly more effort than my sausage fingers can produce. I’m happy with this compromise. Once my workshop is built and I have bought a lathe, such things will be a thing of the past!

email (option): timmykins@icloud.com