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I'm installing a 1" Watts LF7R dual check valve backflow preventer and this model has a female NPT thread on both ends, according to the documentation, and it does have 7 threads and looks like it's tapered. It's brass.

I have two male copper 1" NPT fittings I'll be connecting it to. From everything I've read, I should be able to hand tighten it 3 to 5 turns and then I should be tightening it with a wrench 2 to 3 more. The thing is, when I hand thread the fitting into the valve (either end) I can only get about 1 and 1/8th turns in before it starts to get hand-tight. (That's without putting teflon tape on it yet.)

Should I be worried? I'm not used to NPT threaded copper/brass connections. Is this normal?

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  • I would get some different male 1" NPT fittings and try them. Go to a plumbing supply not a big box.
    – Jim Stewart
    Apr 2, 2017 at 2:23
  • @JimStewart - using a 1" CPVC fitting with NPT male threads, I can get it into the backflow valve about 1 and 3/4 turns by hand.
    – Scott Whitlock
    Apr 2, 2017 at 14:25
  • Well it would then depend on how many additional turns you could get once you start wrenching on it. I don't know what the right number or length of threads that need to be engaged, but I would think it would be at least 3/4". Someone here probably can give an informed opinion. Do you trust CPVC ? Would these be as safe as the best quality copper?
    – Jim Stewart
    Apr 2, 2017 at 19:48
  • Do you plan to have a union(s) or other joints close for servicing or changing the anti-backflow valve at a later time? This is also called a "check valve", right? Where is this valve located? in the basement? in a box in the ground?
    – Jim Stewart
    Apr 2, 2017 at 19:59
  • @JimStewart - It will be in the utility room in the basement. Yes, it's a check valve, and I was planning to put a union right after it, but the check valve is assembled as a union itself, so that may not be necessary.
    – Scott Whitlock
    Apr 3, 2017 at 9:34

2 Answers 2

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Sounds like the threading machine was set a bit two large, many universal dies are adjustable. My Ridgid 535 I have to set the adjustment for just under the size marked on the gauge to get proper sized threads. You may be able to take the pipe to a plumbing supply store that can recut the threads. If a fixed jaw die is used the threads need to be cut the full length of the cutting die to get the proper taper. Usually the female threads are cut with a die if not deep enough the same problem can happen.

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  • Pretty sure you're right. Ultimately I couldn't make them seal. I had one fitting in the bench vice, wrapped in at least 3 layers of teflon tape, screwing on the check valve, and pulling on the valve with a pipe wrench to the point where I couldn't move it anymore. I took it downstairs and assembled it, and that joint still leaked. I took it out, and gave up, and just sweated in a length of copper pipe instead. Instead of a check valve I'm going to use anti-siphon valves later in the system, which is the approved alternative.
    – Scott Whitlock
    May 7, 2017 at 14:39
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You report reading that you should be able to hand tighten the 1" NPT connections 3 to 5 turns before starting to wrench. That agrees with my understanding and (limited) experience. I think you are right to be concerned that your fittings cannot be hand tightened less than 2 turns before meeting resistance.

It would be good to get an answer from one of the experienced and competent plumbers on this site. I give my recent experience below so you can judge my level of competence.

Recently I have had trouble with NPT fittings purchased from a volume homestore. The threads did not engage well. I think you should try to get the best quality fittings to go into the valve and see if those work better than the ones you have that are going in only 2 turns by hand.

On the other hand obviously if the male NPT goes in too far then it may seep. I had that problem recently too. The last two NPT jobs I did both seeped after repeated reassembly.

One case (clothes washer shut-off valves) seeped for a few days, but finally stopped. I had to use pipe dope; when assembled with teflon tape both hot and cold flat out leaked. That was 3 years ago and it has been OK since.

More recently I installed two new outside faucets in our tract house on a slab in a warm climate. There are two outside faucets on 1/2" copper tube emerging vertically from the ground, original faucet just sweated on. Thirty years ago we had a freeze which damaged both faucets and I sweated on a female 1/2" NPT. Recently I replaced the valves with new ball valves which happened to be 3/4" female NPT. I just saw and liked these valves "Garden valve made in Italy". IIRC I made the connection with brass fittings a short 1/2" MIP x 1/2" MIP followed by a 1/2" FIP x 3/4" MIP to go into the valve. So three threaded connections instead of one! I should have found faucets with 1/2" male NPT.

The fittings threaded in nicely by hand and with teflon tape leaked seriously. I took them apart and reassembled with pipe dope and they seeped. Took it apart again and used more pipe dope this time on the female threads too. Seep was reduced and maybe it has stopped by now. It has foam insulation around it and I can't see. It is outside and I don't care if it seeps.

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