Thread Fitment Mystery

Equipment doesn't work? Troubleshooting? If you're handy, members can help.

#1: Post by Tillamook »

I am trying to replace the Manometer on my ContI Prestina, as the original appears to be dead. Pictured below (top) are the threads to the original dead manometer. The threads below it are from a manometer from an older Zacconi Riviera. They look identical to the naked eye, but they are not. The Zacconi manometer will screw in about 4 turns before binding up, while the original goes all the way in 13 rotations, smoothly. I dont have a thread ID tool, but Im hoping someone here can help me identify the threads I need to purchase a new manometer. Thank you.

Team HB

#2: Post by ira »

Looks like the top one is tapered pipe and the bottom is straight pipe. same thread pitch but not interchangeable.

Tillamook (original poster)

#3: Post by Tillamook (original poster) »

So any manometer that is BPST will work in this size?

Team HB

#4: Post by ira »

It looks like the one you took out is tapered so it's likely NPT or BPST, not familiar with the difference or if they interchange.

Tillamook (original poster)

#5: Post by Tillamook (original poster) » ... 07WM27TRV/

So this should allow me to use most manometers on the market?


#6: Post by DeuxInfuso »

BPSS, BPST, & NPT have same threads per inch for a given nominal dia (I think), but the BS0 threads are different from NPT in other ways than taper.

BPSS is standard in espresso machines from Italy & Europe and UK, but there are exceptions. Key here is clean off all thread down to shiny brass with wire brush & pick. I always remove all soft-set pipe dope as it sometimes has dirt or whatever.

Best method of reassembly depends on joint design, of course: sealing element compression, thread-interference or metal-metal compression. Silicone grease on all threads is my motto. Teflon tape or rectorseal5 for the 2nd, silicone grease on all threads and shanks or flanges for the 3rd.

For metal/metal do clean & polish lightly, but remove no metal and inflict no scratches. Sometimes factory scratches are found, explaining an old leak and scale-sealed joint. These take more time, can be carefully buffed out, and sometimes need a custom annealed copper washer to adapt imperfect surface.

Lube with silicone grease (Dow111] and hand tighten. When hand tight, snug just ~1/6 turn more. VERY light touch here, it's easy to damage brass & copper; and most of us have iron or steel muscle memory! Short wrenches help, but a soft touch is critical. If it leaks just tighten a wee bit more. If it still leaks... naw it wont leak!

I left out BSPP male to female w/o any compression element. I used to dislike these, but now love 'em. Just form a hump of tight teflon tape on the male part (6 or 8 turns of tape) where you want the joint to stop. With a trial or two you can time an elbow easily so it binds up just where needed. I prefer these now as they are so easy to make and never leak.

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#7: Post by homeburrero »

DeuxInfuso wrote:BPSS, BPST, & NPT have same threads per inch for a given nominal dia (I think),
Slightly different pitch. 1/8" BSP is 28 tpi, NPT is 27 tpi, and the BSP major diameter is slightly smaller than NPT. NPT is always tapered. BSPP (which is typical on Italian manometers) is not tapered, but BSPT is. The thread profile is also a little different. Because of the NPT taper, a male 1/8" NPT will thread into a female BSPP, but not very far before it binds. The most popular US made adapters for converting La Pavoni Europiccola for pressure gauge fittings were female 1/8" NPT, and people were able to fit stock 1/8" BSPP male Pavoni gauges on them with the help of some teflon tape.

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#8: Post by WWWired »

Hi Tillamook :) Fantastic photo demonstrating the complexity of this type of issue.

Awesome insights from DeuxInfuso, homeburrero and ira above :) Will be very hard to identify a thread exactly that has undergone a compression/crushing to form a seal on the threads. This does, as mentioned above by other posters, appear to be a taper thread.

If indeed this is a "tapered" thread fitting (versus a "parallel" thread), it might then fall into one of two more common "tapered" thread types of: (1) NPT ("National Pipe Thread" tapered); or (2) BSPT ("British Standard Pipe Tapered"). To possibly determine which of NPT or BSPT it might be, have look inside the end port of the fitting to see if it has a 30° chamfer (angled mating surface for a pipe-termination/nipple to form a seal against). While not definitive, NPT may commonly have a 30° inner-chamfer, while BSPT quite often does not (since the seal is formed on the thread with tapered threads, instead of as in BSPP or other parallel threads on an o-ring/thread-locking-sealant/Teflon tape or a mated pipe-termination/copper-nipple and inner-port-fitting chamfer assembly). That said, espresso machines are rare beauties and so some BSPT threads can show an inner-chamfer as well, but this could be more rare and the 30° chamfer on BSP ("British Standard Pipe") threads are usually associated with BSPP (parallel) not BSPT (tapered).

The photo appears to show several measured crushed/compressed thread and flank angles of approximately 65° (greater than both the 55° of BSPT and 60° of NPT thread angles). This larger thread and flank angle than the BSPT and NPT angles may not mean too much as the metal is expected to be crushed after installation, broadening the flank and thread sweep angles. Secondly, it appears there are about six threads in the top fitting in the photo compared to seven threads for the lower fitting (indicating a lower threads per inch pitch count on the top thread possibly). Grab a ruler and count the number of threads per inch to get the Thread Per Inch (TPI) which can then be converted to "Pitch" (also possible for centimeters etc.). Below is a screenshot (original photo credit to Home-Barista and Tillamook) using an online protractor tool to guesstimate thread and flank angles although it should be noted again most strongly that a thread that has undergone compression/crushing (as designed to form seals on the threads in tapered threads) may show an altered, usually broadened flank (the valleys) and thread (the peaks) angle due to the compression/crushing forces . . .
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Tillamook (original poster)

#9: Post by Tillamook (original poster) »

Wow this is more complex than I realized. Thank you all for the feedback. I will clean up my threads and report back.


#10: Post by DeuxInfuso »

Thank you, homeburrero, I was too lazy (and sleepy) to look up the thread details. Nice responses Tillamook & WWWired. Most fittings are BSPP, in my limited experience with La Pavoni commercial machines, one Fracino Londinium, and a KdvW Mirage, and reading in this forum over the years.

I read somewhere that the British standard originated in availability of British car fuel-line parts in post-war Italy, coinciding with the espresso machine maker creativity rennaissance that blossomed in Naples in the 1950s. Folks used the fittings they could find, which was apparently British sports car parts!
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