Hi Tom;
Preface - this is a new boat to me, and it’s been 20+ years since I owned one. Apologies for the paragraph numbering, but I find this helps me, at least. Also, I’m no expert in this stuff, so don’t take what follows as gospel.
Tom, I’d not use an iron (cast, or steel alloy) cap. It’d probably last a lifetime, but if there’s a problem, the Insurance company might(?) spoil your day. Yes, check them when the boat’s out of the water and make a note in your log book. If you think they are ok, then, fine. The higher the “schedule” the thicker the pipe, so these things come in different strengths. IIRC, Schedule 40 is used in hot water heater systems, and these last a couple of decades, at least, but have an oxygen-starved closed loop system. I have no idea how they’d fare in Lake Ontario water.
(spoiler alert, go to paragraph 9 below for the condensed version)
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Pipe taper threads (“pipe”, NPT) where they are kind-of ice-cream-cone shaped. They taper, and, kind-of self seal. You can only screw them in, until tight, and they won’t go much further. LOTS of plumbing stuff is this way; black iron water pipe, etc, etc, so you’ll see it everywhere. (note that “1/2 inch” is NOT the outside diameter, but the inside diameter; the gate valve has a 1/2" diameter hole through it when open)
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Think about through hulls. They go in from the outside, and, on the inside, a large nut and washer screws down on the through-hull, until it’s tight. Think of a large bolt and nut. These are parallel threads.
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ON MY BOAT, I removed the Bronze-cast raw water shutoff, and brought it home with me. I measured it with a dial caliper (“eye balling” it), and came up with a thread OD of 21mm, 14tpi. (apologies for the metric - these are from my notes) It’s actually a really well made valve, but see item 9.1) below.
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NPSM threads, 1/2 NPSM, major diameter 0.823 inches, = 20.9mm. (ref https://www.engineersedge.com/fluid_flow/straight_pipe_threads_13376.htm) That’s really close to my eyeballing of 21mm. Note, as with NPT, the ID is the size, so it’ll have a 1/2" minimum hole in it.
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1/2" NPT is 14 threads per inch (TPI) as is 1/2" NPSM. So, they fit, but think about putting the end of an ice-cream cone in to a deep hole, it’ll fit, but the engagement is only a short bit where you can see it. Maybe people attack the parallel thread end with a file, but that’ll only affect the outside thread crests, not the minor diam.
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Marelon shutoff valves. Ordered some today. From the manufacturers’ site:
(ref https://www.forespar.com/products/boat-marine-plumbing-ball-valve-thru-hull.shtml)
note the sentence “All valves sold throughout the U.S.A. have standard NPSM type threads. If specified, B.S.P. threads are available on valves sold outside of North America.” Also, their “tech tip” on thread forms says “… thru-hulls all have a parallel threads. They are “NPSM” style threads but are not tapered…”
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I ordered Marelon 905100 ball valve(s), and some tail-pieces. I think the water intake valve (stuck) in the bilge for the foot pump is also this size. (I’ll find out in the spring when I’m down at the boat) Note the next paragraph about the tail piece, as I don’t think finding a commercial replacement will be easy.
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7/8" tail piece. I measured the diameter of mine at 21.25mm. Note that 7/8" is 22.22mm, so smaller by about the thickness of a dime. The hose comes off easily. I think it has an NPSM thread on the end, but have not unscrewed it and looked at it. I have the machinery to replicate this stuff, so will likely pick up some certified bronze of a known composition, and will machine one up. Deciding on what alloy I’d prefer. (i.e. not something of unknown composition from my scrap box)
8.1) Sure, if I’m making 1, making a couple more is not a problem.
- Does mixing the straight and tapered threads matter? Probably not. However,
9.1) A marine survey suggested replacing the gate valve with a ball valve (do not ignore advice from professionals) and:
9.2) I did find that, when working on my W13 diesel, that the through-hull was always in the way, and easily bumped. With my luck, I’d bump it, and it’d break due to small amount of thread engagement. So, I’m changing it, and will make the replacement fit well.
There you go! Yes, too much information above, but… If I did not answer your questions properly, please ask again, and I’ll try to do better.
John A. Stewart
NS26C #046
On the hard at Kingston, ON.
