Who is selling custom plugs for the 328 sampling tube ports?
If you mean for the sampling tube ports on the exhaust manifolds, Ferrari already makes a part 110860 (used with a 12mm x 16 or 17mm x 1.5mm thick Copper sealing washer): https://www.ferrariparts.co.uk/part/ferrari/110860?id=82064 Image Unavailable, Please Login But it's just an M12 x 1.25P threaded plug about 10mm long thread with a 17mm AF hex head -- many much less expensive places to get them. Something like this would work OK IMO (and look reasonable): https://www.amazon.com/Stainless-Slotted-Copper-Washer-Oxygen/dp/B08F4YJPGZ/ref=sr_1_3?crid=MGSSX8I6FU6U&keywords=m12+o2+sensor+plug&qid=1646239058&sprefix=M12+O2,aps,184&sr=8-3 The trouble with making them out of a M12 x 1.25P hex head bolt is the hex head is kind of bulky. If you mean threaded plugs for the air injector nozzle ports = see Mike's post.
Indeed the rear header sampling ports (early cars) are already plugged with those caps. Thats why they have a part number.
Metric bolts can be purchased at many places but are too long. I cut the longer bolts off after running a die all the way up on the threads, then unscrewed it reshaping the thread end. A little smoothing with a file finished the job. Viola, I had plugs. Of course, it was the front sampling tubes on my 328 that corroded so it was labor intensive. The old screw ins had to be drilled out and removed with a broken screw extractor. Replaced all 4 of them while in there.
+1 -- it's probably too late now for every F that came equipped with these type of exhaust sampling tubes to ever be removed without totally destroying them, but, if anyone buys replacement sampling tubes, they should just keep them in their tool kit and use the threaded plugs on the exhaust manifolds. The chances of a Mechanic needing to use them is so low IMO that it's better to not cook them to death and just pay the Mechanic a little more to install them if ever needed to investigate a problem. Certainly on US TR, reinstalling the exhaust sampling tubes is trivial so my replacement set are safely in the tool kit .
Well, even with cabs a Sync Tool determined flow at the intake throat and as long as I was ever emissions tested, they sampled at the exhaust tips. JRV if he wanted to tune individual cylinders he swore by the glass bodied ColorTune plugs!! You can see the actual color of the flash off.
