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Thread Protector Couplings

EBEBRATT-Ed Member Posts: 13,454
Just a note about "Thread Protector Couplings" that some may not know. Of course their made of steel and are not malleable iron or cast iron made of the same material as the pipe

1/8" -2" these couplings are tapped with straight threads and are not to be used in a piping system although we have all seen them used (with a lot of Teflon and dope) Their leakers especially if you have any pressure. They make in too far.

2 1/2" and up the couplings come tapped with tapered threads the same as the pipe and are perfectly fine to use.

Why the couplings are straight thread 2" and under and tapered 2 1/2" and up I have no clue.

But you can look it up on pipe specification sheets.

Yes, I have had several bouts with gas inspectors who have questioned their use on larger gas line 2 1/2 and up.

haven't lost one yet


  • j a_2
    j a_2 Member Posts: 1,801
    I tried threading one of them there new plastic protectors in my pipe once....So of a gun it did not hold the test....lol....Ed I new enough not to use them but never knew they were running threads...Never used tape on gas pipe, but that's just me I guess
    JUGHNE Member Posts: 10,580
    Out here the thread protectors or merchant couplings have been used forever.
    They do not appear to be running threads....having run a fair amount of rigid galvanized threaded conduit and using those couplings that bury completely with no threads showing.

    The ones that come with my black pipe (T & C) will not go on any farther than the MI ones. I just tried each on a 1 1/4" nipple stuck in a chain vise. I end up with the same amount of nipple threads showing with the same wrench effort.

    I don't use the steel ones anymore as pipe wrenches slip and they will stretch from being over tightened.

    Also why "protect" only one end of the pipe.....they could have just used the plastic caps......$$$ if you buy merchant couplings.
  • j a_2
    j a_2 Member Posts: 1,801
    I am not sure there a listed fitting...meaning approved....but could be wrong
    EBEBRATT-Ed Member Posts: 13,454
    Find a 2" thread protector and then a 2 1/2"-4" one. Put your fingers inside it. You can easily tell the ones that are tapered and the ones that are not.
    EBEBRATT-Ed Member Posts: 13,454
    • Merchant coupling dimensions and materials conform to ASTM A865
    • Merchant coupling threads conform to ASME B1.20.1
    • Merchant couplings from 1/8” to 2” are NPSC (straight threads),
    2-1/2” and larger are NPT (tapered)
    • Recessed couplings are API 5L style for extra heavy pipe, all sizes have NPT threads.

    Copied and pasted the above from a pipe specification brochure
  • rick in Alaska
    rick in Alaska Member Posts: 1,420
    I was told merchant couplings were used for the military, where they would thread the pipe on with them, and then weld the coupler. The coupler was mostly used to keep the pipes together to make it easier to weld. That is also why they are steel.
    So I was told, anyway.
    I also try and take them off right away when I am hauling pipe as they have a tendency to rattle off the pipe and bounce off my hood.
    EBEBRATT-Ed Member Posts: 13,454
    @rick in Alaska or worse on the car following you!!
    rick in Alaska
    JUGHNE Member Posts: 10,580
    Ed, so I think I understand now. The only reason the straight coupling gets tight is that it runs into the increasing taper of the pipe. Without the matching taper of the coupling then only the first few threads of the coupling get tight onto the pipe?? It then lacks the integrity of perfectly matched tapers for male and female.

    So what is the story on RGC couplings that run all the way onto even a field made thread or a factory galv water pipe nipple?
    EBEBRATT-Ed Member Posts: 13,454
    don't know unless they are RIDGID CONDUIT couplings?? Electrical threads are straight on the pipe and the coupling, the pipes are supposed to meet in the middle of the coupling....I know U already know that.

    I just wonder why 1/8-2" pipe couplings are straight and 2 1/2-4" are tapered??

    I'd like to find someone who can answer that