Ron,

Before you guys had hit upon a agreed TPI, I had already decided to go directly to the source to answer your question. Here is my inquiry to Burlen Fuel:

I follow the Brickboard, a Volvo owner's discussion group. A
question came up concerning the thread pitch of the jet adjustment nut
on the SU HS6 (in particular) carburettors. There have been so many
different answers I thought I would come straight to the source. What is
the thread pitch of the jet nut? How far down the needle does the jet
move for each flat on the nut? Whitworth or BA? Thank you.

And their response:

Dear Sir/Madam,

Thank you for your recent enquiry and we now have pleasure in submitting
the following information.

Confusion rains on this subject due to the fact that there are two
different thread pitches, a fine and a coarse version. The original
(coarse) thread is 3/8'' x 26 TPI Whitworth. The later(fine) thread is
3/8" x 40 TPI Whitworth. The fine thread was introduced for more precise
jet adjustment with the advent of emission controls. The original
recommended 12 flats down initial jet setting applies to the coarse
threads only. The amount of jet movement created by a turn of one flat
of the nut is not really relevant as this measurement would not be
needed for tuning purposes.

Hope this helps.

Kind regards,

Customer Services

So, there you go, directly from the folks who still build these things.

Hi Ron,

I'm new here, my first post, thought I'd start off by answering a question first, before asking one!

Checked the threads on a spare carb using a thread gauge, and the 27 TPI blade seemed to fit very close, but my middle age eyes seemed to see a slight bit of gain over the length of the gauge. Checked a BSF thread chart and it shows a 1/4" x 26 TPI thread, which is a smaller diameter than the jet holder, but perhaps SU used this thread count on the larger diameter to get finer adjustment capability. I don't have a 26 TPI gauge, so can't verify if one would fit better than the 27 TPI.

By my math, which I admit can sometimes be questionable, a 27 TPI thread would give .0062" movement per flat, while a 26 TPI thread would give .0064" movement, both figures rounded to the nearest tenth of a thou.

Hopefully this will be close enough for what you are after.

Regards,
Mark
'67 122 and '77 244