Tillamook wrote: From what I have learned here, am I correct to think this adapter is the answer to my problem?
https://shop.prmfiltration.com/collecti ... -bspp-male

It seems fairly likely that your machine has a 1/8" BSPP fitting and your manometer from the older Zacconi is somehow NPT. That would explain the binding. But would be good to verify that and you could use that adapter as a way to verify that the fitting on the machine is indeed 1/8" BSPP. But I'm not sure you have room for that adapter and still get the gauge to align correctly, based on what I see in an old image from drgary here on HB: Conti Prestina Espresso Machine Restoration 101 (Completed and Indexed)

If you learn that the fitting on the machine takes a 1/8" BSPP, then I think you should try to find a replacement manometer that fits that and also has the dimensions you need to line up right with the case.

Also note that they say "BRASS NOT FOR USE WITH POTABLE WATER" in the spec. This adapter seems to be of a brass that doesn't meet the modern 'lead free' standards. Probably not a real issue, especially out of the flow on that manometer fitting.

P.S.
I think we are assuming that it's 1/8". You can quickly verify that with a 10mm metric wrench - - 1/8" would be close to 10mm OD whereas 1/4" would be closer to 13mm, and 1/16" just under 8mm.

Tillamook,
you do not need to use a tapered gauge even if the original was. If the fitting threads on until it bottoms out, with a metal on metal seal, then all you need is a little silicone grease or Teflon tape and gently snug tight.. Put the tiniest amount of silicone grease on the metal sealing surfaces.

If the seal was previously formed on the tapered threads (less likely), you can re-create the taper effectively using controlled tape thickess, forming a thickness taper, of Teflon tape, that acts like tapered threads to form an interference seal.

Most gauges I've replaced (3 out of 4 anyhow) were metal to metal sealing and the threads simply developed the compression force on the tiny metal faces. Some Teflon tape could be used but I prefer silicone grease. Be sure to check for dents and dings to the threads or metal sealing surfaces. Brass is soft and easily damaged. It is also easily fixed. Use a hand-lens or loupe to check, and rifle files (or a die) to clean up dinged threads. Dinged sealing surfaces are more work, using a lathe to reface, or make a tiny copper washer: I've also fixed leaky gauges with small silicone o-rings, but finger tight only.

-Deuxinfuso

Great additional photos and homeburrero and Deuxinfuso also further lighting the path brilliantly! Based on the photos and the link homeburrero posted to DrGary's superb Conti Prestina posts, this manometer appears to be Panel Mount Center Back Connection model (while not strictly mounted/fastened to the panel, but rather just behind it and mounted to a pipe fitting). Sometimes knowing the exact right magic words "abracadabra" or the name of Rumplestiltskin can open the doors . . . a search on that electronic bay place using "Panel Mount Center Back Connection Pressure Gauge 20 Bar" gave some sellers, one of which looked a bit like this allowing some drop down menus to choose fitting types (BSPT for example), sizes(1/8th etc), and diameter of the manometer panel hole . . .


. . . and here's a pretty great link I think I noted posted by cafeIKE one day . . . https://www.mcmaster.com/pressure-gauges/pipe-size~1-8/thread-type~bspt/ (although I'm not sure about whether their gauges offer many Bar reading manometers).

And then there's also that Ali place Expressing stuff to sell as well that will show some options if the magic words of "Panel Mount Center Back Pressure Gauge 20 bar (again not strictly a "panel mount" as the mounting of the gauge in the Conti-Prestina appears to be just to the pipe fitting and the gauge sits just behind the panel, and the gauge does not appear to be bolted or screwed in any manner to the front panel)," or something like that, is put in to search for lol . . .


And finally, not to be out done, no Amazonian Princess would be left out of selling such great pressure gauges online:


As for the threads, give the local big box Depot for Home store a buzz and ask if they've got a thread gauge tool (most do and will identify the threads for you). These big box stores also usually sell thread gauges so it might be possible to take a look at some of the thread gauges on the shelf next to the actual manometer threads

For anyone wondering about the Conti Prestina . . . here's a wonderful short 4 minute video (very low resolution from the earliest days of YouTube haha) by Coffee Community legends Doug and Barb showing off a 1970's model of this impressive lever espresso machine . . . All credit to: Doug and Barb of OrphanEspresso and Youtube

. . . and not sure what type of manometer this is, but if it is a speculated bourdon tube type, here's a fantastic video by a brilliant youtube creator mrpete222 (Lyle Peterson). . . these are repairable . . . All credit to: pete at mrpete222 and Youtube
Notes:

• What Mr. Peterson is calling a "quadrant gear" is sometimes referred to also as a "Sector Gear;"
• The video shows an Air (Gas) pressure application, but fluid (Water) bourdon tube manometers work the same way;
• The "Brass Block" referenced is also sometimes called the "Receiver Block" and joins the inlet pipe (running inside the Receiver Block) to the Bourdon Tube;
• The mechanism involves a thin spur gear reducer system where a larger sector/quadrant gear (what looks like a pizza slice with gear cogs as the crust) rotates a small pin with gear cogs on it that has the gauge's needle attached to the opposite end and as the pin is rotated shows the calibrated pressure change in the bourdon tube.