Rigid Threads

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jmellc

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Durham, NC
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Facility Maintenance Tech. Licensed Electrician
I don’t do much rigid these days but when I do, I have a lot of trouble threading locknuts on. Both factory & hand threaded. Couplings thread on fine & most consumers. Last week, I had to add about 1/2 inch of thread to allow 2 locknuts & a plastic bushing.

Anyone else having this issue?
 
It's probably the locknuts and not threads. Back in the day you could take a steel locknut off of an EMT connector and thread it right on the the RMC threads. Those days are over. Even though the EMT connector has a trade size straight thread the locknuts don't fit all the way down the thread on RMC.
 
infinity said:
It's probably the locknuts and not threads. Back in the day you could take a steel locknut off of an EMT connector and thread it right on the the RMC threads. Those days are over. Even though the EMT connector has a trade size straight thread the locknuts don't fit all the way down the thread on RMC.
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I didn’t know this. Guess we have to buy a few rigid locknuts. I do recall some years back that we had a batch of EMT connectors where the locknuts wouldn’t fit other connectors or rigid threads. The connectors themselves would thread into anything we used.
Just like a manufacturer to complicate something as simple as a locknut.
 
Take your diagonal cutters (dykes), carefully make a indent cut (not all the way through) at 12, 3 6 and 9o'clock then see how it fits. If need split those index's until it fits. It expands the ring for ya... a old industrial trick just for you.
 
If your running ridged you probably going to have a die so you can just run over it quick and clean up the threads as you already did. its just another case of bad tolerances these days. Nothing is made as good as the old stuff. You have to fight everything.

Even metal boxes the screws you always have to crank them or use a drill.
 
Eddie702 said:
If your running ridged you probably going to have a die so you can just run over it quick and clean up the threads as you already did. its just another case of bad tolerances these days. Nothing is made as good as the old stuff. You have to fight everything.

Even metal boxes the screws you always have to crank them or use a drill.
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Even newly cut threads are an issue. My only fix was adding extra thread.
 
Dsg319 said:
Have you tried deepening the treads any?
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I have done that in the past but that would depend on the type of field threading equipment that is being used. A typical stock and die cannot be adjusted to cut deeper. Won't solve his problem on manufactured threads already on the RMC.
 
This may be too obvious, but PIPE dies cut tapered threads, conduit dies cut straight threads. At least that is what I was taught - to be sure I was always using conduit dies on conduit, not pipe dies.
 
OldSparks said:
This may be too obvious, but PIPE dies cut tapered threads, conduit dies cut straight threads. At least that is what I was taught - to be sure I was always using conduit dies on conduit, not pipe dies.
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Boxes with integral hubs, myers hubs, conduit body hubs are all designed to be used with tapered thread.

There is non tapered thread items out there, not just electrical fittings. I have a mini excavator, foreign made machine every hydraulic line on it is same diameter and threads per inch as NPT threads but none are tapered. Have to inform them when making up a new hose that it is not a NPT fitting or they may make it up with wrong one. The male ends have a internal flare and the the female ends have matching flare that bottoms out when you tighten it to make the seal. Can thread a NPT cap on it temporarily to keep dirt out but it may still leak some oil while on there.
 
OldSparks said:
This may be too obvious, but PIPE dies cut tapered threads, conduit dies cut straight threads. At least that is what I was taught - to be sure I was always using conduit dies on conduit, not pipe dies.
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Like me you have been taught incorrectly. The NEC requires tapered threads just like plumbers, sprinkler pipes, and steam fitter pipe.
 
The factory threads on RMC/IMC is tapered as well. The couplings however are straight thread. They do jam as you thread it on further but do not necessarily make good enough seal for pressure like water, steam, etc. applications.
 
kwired said:
The factory threads on RMC/IMC is tapered as well. The couplings however are straight thread. They do jam as you thread it on further but do not necessarily make good enough seal for pressure like water, steam, etc. applications.
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The diameter of NPS (straight) threads appears to be about halfway between the min. and max diameter of an NPT (tapered) thread of the same nominal size. That allows you to jam straight threads into or onto tapered threads as you said.
But because locknuts for EMT fittings are designed for straight threads, they will be too small to go much beyond halfway up a tapered thread.
 
Our threader is a Ridgid 535, dies are NPT. We do have a couple of BPT (British) dies but are careful not to use them. Maintenance uses them for a few items.
 
Like I said before, it takes a genius to complicate a locknut. Locknuts are rarely used anywhere but electrical conduits & connectors. No valid reason not to have uniform threading across the board.
 
OldSparks said:
This may be too obvious, but PIPE dies cut tapered threads, conduit dies cut straight threads. At least that is what I was taught - to be sure I was always using conduit dies on conduit, not pipe dies.
Click to expand...
We had conduit dies back in the 1970's and thought they no longer made them. My thinking is conduit dies do not cut taper thread so not be legal in explosion proof conduit runs. I always ordered quality T& B steel locknuts for bench stock. Made the mistake of ordering a hundred 1/2 to 11/4" Aluminum locknuts for use on aluminum heavy wall conduit. If you hit the locknuts to tighten them had a tendency to split.
 
garbo said:
We had conduit dies back in the 1970's and thought they no longer made them. My thinking is conduit dies do not cut taper thread so not be legal in explosion proof conduit runs. I always ordered quality T& B steel locknuts for bench stock. Made the mistake of ordering a hundred 1/2 to 11/4" Aluminum locknuts for use on aluminum heavy wall conduit. If you hit the locknuts to tighten them had a tendency to split.
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344.28 calls for 3/4" per foot of taper on RMC
No straight threads.
 
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