All: I am trying to figure out how my old (10-15 years +) 4" nicro solar vent is mounted. Nicro website did not help. Would it more likely be a threaded ring that the vent is on? It's old but works fine. Just want to rebed. Thanks!
My old Nicro was an outside pvc pipe with flange fastened and epoxied to the deck going through the deck hole. The vent fit insideand slid into the pipe with attachments at the top. Your old Nicro vent should unscrew and slide out of the deck pipe and you should be able to replace it with the newer 3" or 4" model depending on what you hole diameter is without redoing the outside thru deck pipe. I did this project with a new Nicro solar day/ night this year.
Some of the Nicro vents used a bronze deck ring and screwed into the ring. These came with a bronze deck plate that allowed you to seal the hole if the boat was going offshore. These vents were pretty expensive and not too common.
The rest of the Nicro vents used a deck fitting that looked like a piece of PVC pipe, and just snapped into place. There were no screws holding them in....just a ridge in their collar that fit into a groove on the deck ring. If you pull on these or push on them from the inside, they should just pop out.