This past summer I decided it was past time to replace the vent hose on a water tank under the starboard settee. Not a big or difficult job, it did require removing the deck of the settee to access the hose, some time with a drill and screwdriver bit and the deck was off and I had access to the fitting. While disassembling the settee I notice a little water beneath the outlet. No problem I thought, I'll just tighten the screw on the clamp and it will be good.
To double check, I filled the tank with water and went for a beer. I wanted to make sure I had fixed the problem before putting everything back together. I came back, the water had returned and I pumped the tank empty. With the tank empty, I removed the fitting, put new teflon tape on the threads, attached the hose and tightened the clamps. After filling the tank with water I went home.
The next day, upon returning I found, no surprise, more water. Fearing the worst, a split seam on the tank, the tank was drained and I finished removing all the restraining parts to remove the tank. After wrestling with the tank, it yield to my persuasion. The good news, the seams in the tank were intact. Yahoo! And the leak was found. In the bottom corner of the tank that was tight against a bulkhead was an unused threaded hole for an outlet. The outlet had been sealed with a brass reducing bushing and a black iron plug. After 20 or so years the black iron finally failed and created a very slow leak.
Discovering the leak solved a couple of other concerns, a musty smell emanating from the region and a swollen drawer on the other side of the bulkhead that had some mildew and would not slide easily. Fortunately the leak had not damaged the bulk head yet, probably because the boat spends half the time out of the water on the hard so it can dry out.
Mystery of the leak solved, I swung by a big box hardware store on the way home and bought a 50 cent ½" PVC plug to seal the hole. On the next trip to the boat with plug in hand, I discovered the plug was too large. It must be a ⅜" plug I thought. So off to the nearest big box hardware store, a 50 mile roundtrip, to purchase a $5 ⅜" brass plug. Back at the boat and ready to be done with this project, I inserted the plug only to find, you guessed it, it was too small. WTF, it was another chance to practice my sailor speak and look for new words in the Profanisaurus.
Back home, with micrometer in hand I struggled to solve the problem. Perhaps it was a metric thread? A trip to the local industrial fastener supply center bore no fruit, a matching thread could not be found. Then it was off to the plumbing supply house, again, to no avail. Back to the internet and a deep dive into the world of pipe thread standards. Therein lies the answer. In the world there are 2 major pipe thread standards, National Pipe Standards, the NPT and NPS standard we all know and the British Pipe Standards (BST and BSS) and a couple of lesser known and rarely used other standards. My best guess was the thread was a ½" British Standard Taper (BST). But where to find a BST threaded plug in the US?
Where to find arcane bits and pieces? McMaster-Carr, there buried deep in their inventory was a ½" SS BST plug for just $3.10 plus $7 shipping. After waiting anxiously for a day or two for the part to arrive, the threads on the new and old plug matched. Off to the boat, the plug was wrapped in teflon tape and it fit!
While the tank was out, the areas was cleaned and treated with a mildew and mold killer. The bulkhead was inspected and found to be solid. The tank was installed and filled. After a few hours there was no evidence of a leak. Problem solved.
Now all that remained was trying to remember how it all went back together two weeks after starting this quick and simple project.