Carburettor mounting nut identification

Can anyone tell me what the markings on these nuts indicate … thread type or grade perhaps ?
They are 5/16" UNF plain nuts.

Usually indicates a left hand thread.

Most commonly used for fuel gas fittings, or in situations where a right hand thread would tend to undo.

Have you tried them on a bolt?

+1

They are definitely 5/16" UNF right hand thread.
I have searched and found similar markings which indicate the grade of a nut but they appear on all 6 points of the hexagon, not on just the 2 opposite as these are. I wondered if perhaps it’s crimping to form a stiff nut or lock nut but I thought that was usually done on the flats, not the points as here.
Anyone seen these before ?
Thanks.

Hey Roger. Think they may be “Identification-Marks”. Similar marks on older British Built Aircraft, BA/BSF Nuts & Bolts. (Americans use a completely different Identification and numbering system. AN NAS MS etc)

CAIP’s (Civil Aircraft Inspection Procedures, give you a fuller picture)

archive.org/details/CAIPPART1/p … 3/mode/2up

Hi John (Freddie),
Thank you for posting that link.
The nuts I have are definitely UNF and the identifier marking shown for that in those standards is different so I think my markings mean something else.
They are used on my S4 for the Stromberg carb and adaptor plate mounting and are original fitment as far as I know.
I’ve done a bit more research and came across this description…

Access to the carb mounting nuts, particularly on Stromberg carbs, is very tricky so using reversible locknuts does make sense … to me at least !

The markings are different on what’s sold now but these are nearly 50 years old so things may have changed!

Anyone else have them on their Strombergs ?

I’ve just used regular nylocs. In fact the same six regular nylocs for as far back as I can remember. I’m not sure heat is a major issue for those nuts - I’ve measured 50 - 55C around the carb area on a really hot day (testing temp compensators) and I’d have thought nylocs would be ok with that.

Hi Stuart,
I’m sure nyloc nuts are fine … not suggesting otherwise, I was just trying to identify what the markings on my originals mean and I think a reversible locknut is a likely explanation. The 6 securing the carbs have been on and off a good few times over the years and there’s not much “locking” left … I should probably replace them … but they have never come loose yet. Do you also have nylocs holding the adaptor blocks to the head … is the recess deep enough?
Cheers.

Roger & Stuart.

If space is a bit tight on your SU Carbs, you could consider Nylock Half-heights.

Half a dozen should be cheap-ish on Flea-bay.

ebay.co.uk/itm/UNF-ZINC-PLA … Sw5VFWH8VP

John.

My carbs with crossover functioning get quite hot; so maybe the reason.

Just regular (somewhat rusty) nuts holding the adaptor blocks on. There’s enough thread sticking out past them that nylocs would have something to ‘grab’ onto if I changed to them but I’ve never had issues with them coming loose.

Out of interest and following up on an earlier thread, I measured the carb to block gap with some feeler gauges earlier - 0.043’" on top and within a couple of thou of that the rest of the way round. Now just got to take it all to bits as one of the carbs is flooding. :frowning: