Rule 405FC pumps - what threads do they use?

[IDBoating]

I'm getting conflicting info from the manufacturer's website. This page:

http://www.ittflowcontrol.com/marine...-cartridge.htm

...says they use a "straight 3/4 inch pickup tube". Yet their PDF at on the same site:

http://www.ittflowcontrol.com/files/itemdoc134801.pdf

...says the 405FC has an inlet (item E on the illustration) that is "3/4-14 NPT" which would be tapered threads.

It makes more sense that the pump would use NPS (straight) threads since the PDF also instructs to "saw off the excess length of the pickup tube". Cutting NPT (tapered) threads doesn't make sense. But the PDF is consistent... all "FC" models of the pump are shown with NPT on the inlets.

Anyone with one of these pumps know what is REALLY going on?!?

[chpthril]

They are NPT.

I think this state on the web page is misleading and not actually referring to the threads.

For Thru-Hull Applications Straight 3/4" (121mm) Pickup Tube

[IDBoating]

They are NPT. I think this state on the web page is misleading and not actually referring to the threads.

OK, thanks for the confirmation.

NPT threads on the pump make it easy to screw it directly into a valve body. But that leaves the other side... aren't most thru hulls NPS threads so they can be cut to length? I've never seen a valve body with NPT on one side and NPS on the other. And the thru hull (NPS) to valve body (NPT) is the crucial joint. No matter what, you end up with a thread mismatch on one side. How do you properly handle that? (Jason at WakeMakers says he has custom NPS-NPS ball valves but that doesn't resolve this problem.)

Proper flanged seacocks, such as those from Groco, handle the NPS to NPT transition but we're talking SERIOUS money and lots more real estate on the hull. How are people doing this for ballast systems?

[boatwakes]

You CAN run NPS threads and NPT threads as the pitch is the same but obviously the NPT's are tapered. HOWEVER, it is not advizable to do so because the NPS need a collar to compress and tighten at the hull. I've changed a handfull of thru hull valves on ocean boats kept in the water and all correctly built boats and their respective raw water systems above 2" inlets were flanged seacocks with NPS in and NPT out. Most of the 1", 1.25" and 1.5" were standard ball valves but you never knew. It's fine to run an NPS threaded to NPT without worry in our boats IF you use a silicone sealant such as 3M 4200 fast cure. Tighten them up and they'll seal just fine. Be sure to apply pipe tape in a clockwise manner as you're looking down the threads from the top otherwise the tape gets screwed down in the wrong direction and it'll leak. Good luck.

[IDBoating]

It's fine to run an NPS threaded to NPT without worry in our boats IF you use a silicone sealant such as 3M 4200 fast cure.

Yes, it's possible but not advisable. I believe ABYC (sp?) regs specifically prohibit it, and I've heard that some insurance companies have disclaimers for watercraft for mismatched threads on thru hulls. I'd like to avoid it.

You're correct, formal seacocks solve this problem. So do thru hulls with what are called "combination threads", where the portion through the hull is NPS and the end is NPT. The problem with those is they cannot be trimmed to fit the hull thickness, so you can end up with a whole lot of extra threaded length - a potential problem for ballast systems using centrifugal pumps because they need to be below the waterline to be self-priming.

[Timmy!]

On the ones with the combination threads, you won't need to trim it to fit. You will have just enough thread there to put the nut on and then the ball valve and maybe have 1/2" to 1" of thread left depending on your hull thickness. The hull on my boat where I put the thru hull in was not very thick, only like 3/8".