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Weird Pipe Threads

Has anyone seen threads like this on old steam systems?  I've been in the trade for 45 years and this is new to me.  Look at the pic - it is a shallow fine thread like on a lav pop-up tail piece, but is tapered.  I have laid a standard thread next to it for reference.  Regular 1" and 1-1/4" radiator valves are screwed onto these threads with little damage.  I needed to extended the riser due to a higher floor and planned on using an extension piece.  You all know what kind of job I got into replacing the riser.  I was able to screw the old valve onto a regular pipe thread.  Why the fine thread?  The other end of the riser has a standard thread.  This one pipe system is probably approaching a century of service.


  • ChrisJChrisJ Member Posts: 8,631

    I'm not a pro so maybe this is a stupid response, but any chance it is NPTF?

    I believe that is a finer thread designed for sealing without dope.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment

    Steam system pictures
    Central air project pictures
  • Charlie from wmassCharlie from wmass Member Posts: 3,925
    looks like they were double threaded

    maybe they tried to rethread the nipple and got it cross threaded.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • Big ShrubBig Shrub Member Posts: 9
    Not NPTF

    According to this site  one cannot visually distinguish between NPT and NPTF.  These threads are really fine.  The radiator valve squealed like a pig while removing it.  Not unusual, but more than normal.  Why not regular threads for a regular radiator valve?
  • Big ShrubBig Shrub Member Posts: 9
    Not Double threaded

    I tried to get better pics without success.  It is hard to tell from my photos, but it isn't a double thread.  It is definitely a single continuous thread.  It seems to have a bit more taper than standard.  It amazes me that the brass radiator valve still screws (with difficulty) onto standard threads.  Every radiator in the house has this same thread at the valve - rules out an accidental bad thread of some kind.  All other threads are standard.
  • JStarJStar Member Posts: 2,668

    My guess is that the original installer used a machine-screw die to thread the pipe. Maybe his NPT die broke on the job.
    - Joe Starosielec
    [email protected]
  • Big ShrubBig Shrub Member Posts: 9
    Not Machine Screw Threads

    They are tapered as much or more than regular threads.  Machine screw wouldn't have a taper plus it is a really fine thread.  Both the 1" and 1-1/4 risers to the radiator valves have this thread, but only on the end with the valve.  They weren't done by mistake because every radiator is the same.  It amazes me that the valve bites onto these threads and doesn't leak.  I tried to screw on a black pipe cap but couldn't get it started.  The soft brass will take hold, but doesn't seem right to me.  Where is Dan when you need him?  Anyone have a direct link to see if he has encountered this thread? 
  • Hap_HazzardHap_Hazzard Member Posts: 1,349
    Can you measure them?

    I have to be honest, that looks like nothing I've ever seen before, and I don't see anything even remotely like it in the standards tables, but it might be useful if I had some actual numbers to go by. Do you know how to measure thread pitch and taper?

    At this point I assume it's academic, right? You're just going to thread them to NPT because you wouldn't be able to find fittings for it even if you could identify it. Still it would be good to get some numbers before you destroy it.
    1983(?) Peerless G-561-W-S
    3" drop header, CG400-1090, VXT-24
  • Big ShrubBig Shrub Member Posts: 9
    I'm saving one

    I don't have any idea how to measure the thread pitch and taper.  I can look at most any thread and tell you what it is, but not this one.  As I said earlier, it looks a whole lot like the fine thread used on a tailpiece for a lav pop up - only a lot longer and tapered.  I'll be keeping the nipple in the photo.  I was able to get at the cast ell under the floor and break it.  I then replaced the ell and installed a new riser 3/4" longer to accommodate the new floor.  The pipe in the photo is 1-1/4, but there are 1" ones the same way with standard valves forced onto that thread.  I've saved it on the work bench next to my lead pipe and valve collection.  There has to be a reason why they went to all that trouble maybe 85 or 90 years ago on this one pipe steam system.  I've been doing this kind of work since Lassie was a pup and never encountered this thread.
  • Hap_HazzardHap_Hazzard Member Posts: 1,349
    edited October 2013
    Measuring Threads

    If you can tell a 1" BSP thread from a 1" NPT thread (11 tpi, 55° vs. 11.5 tpi, 60°) you're a better man than I am. I always measure if there's any doubt.

    If you don't have a thread pitch gauge (they usually come with tap and die sets), you can use a digital caliper or vernier caliper. Just measure the distance in inches between the farthest apart thread crests, matching the tips of the caliper jaw to the crests. Then you count the number of threads, and divide the number of threads by the distance to obtain threads per inch. If it doesn't come out reasonably close to a known standard, convert to millimeters and check the metric tables.

    To measure the taper, put a caliper over the end of the pipe so it contacts the crests of the largest diameter thread at the widest point, then measure the distance between the caliper jaws and the smallest diameter thread (the last whole one at the end of the pipe) on both sides. Average this measurement and divide it by the distance between the largest and smallest diameter threads. For a 1" NPT pipe you should get 1/16" per inch. If you need the taper in degrees, get out your trigonometry tables and ask your kids how to use them. :)
    1983(?) Peerless G-561-W-S
    3" drop header, CG400-1090, VXT-24
  • ToymotorheadToymotorhead Member Posts: 54
    I know what that looks like.

    It looks like someone tried to thread a pipe, but with the teeth installed in the wrong order in the die head.

    Now I am tempted to go try it.

    If you can't be good, at least be good at it.
  • ChrisJChrisJ Member Posts: 8,631
    And then

    They did what I recently learned.

    If it doesn't move, use a bigger wrench!
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment

    Steam system pictures
    Central air project pictures
  • Big ShrubBig Shrub Member Posts: 9
    What could be the purpose?

    Thanks for the refresher on the thread specs. Hap_ Hazzard is correct that it is difficult to distinguish between BSP and NPT.  These threads are far from either.  It might be a day or two, but I'll give it a try to come up with some details on the thread pitch, TPI etc.  It isn't so much identifying it as to learning what is the purpose of threading all the radiator risers with this fine thread ----- and then forcing regular NPT radiator valves onto them.  I have removed both 1" and 1-1/4 valves and they were NPT.  The 1-1/4 took a 2' wrench with a 2 foot cheater and both feet pushing off the wall to get it broke loose.  I figured it would egg the valve, but it screwed right onto the NPT I cut.  The home owner figures the system was installed in the late teens or early 20's.

    I'd be happy to lend this nipple to Hap-Hazzard if you promise to return it.  It is another piece in my collection.  Not long ago I ran into wooden plugs hammered into fittings with a metering hole drilled in them.  Our predecessors had their ways of making things work properly without all our fancy gizmos.
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