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Left Hand Pipe Taps

Mike_in_Maine

Aluminum
Joined
Mar 16, 2004
Location
Biddeford, Maine
We were recently cleaning out some old areas of our tool crib and came across 6 brand new 10 year old 1/4 NPT LEFT HAND pipe taps. Does anyone know where one might actually use these? I work with machinists with as much as 38 yrs experience and none can think of a reason for these taps.
 
Poor man's pipe union? A pipe nipple threaded LH on one end and RH on the other could work kind of like a turnbuckle. It would be tricky to get it started just right so both ends tightened up properly?????

Stu Miller
 
No kidding, it just hit me... Acetylene regulators. I know the hose fittings are left hand, wonder if they use LH pipe threads for the hard fittings?
 
Part of an answer is that as late as 1957, the trade joined pipes with right and left hand threads, instead of unions. I have a 1957 Walworth Co. Catalog which shows a figure 7125 right and left hand coupling,in sizes of 1/2" or larger. Nothing about 1/4".

So, it is only part of an answer.

The only reason I can think of for a machine shop to have small left hand taps would be if they were building machinery that required cooling or lubrication from a manifold. If there was not room for unions, left and right threads might work. This is a bit contrived, I know.

Thermo1
 
I also ran to some old catalogs to try to figure it out, I have a BBB (Binghams Best Brand) Catalog No.37 which appears to be from the early '30s (no date anywhere). Most of the listings for Pipe tooling say LH is available. Briggs Standard LH Pipe taps were available up to 2" dia. at list prices, over 2" at a different discount rate. List price on a 1/4-18 was $1.20.

Maybe they used them for plumbing fixtures exported to the southern hemisphere (so the counter-clockwise rotation wouldn't unscrew the pipes) :D
 
Not sure about the southern hemisphere thing. Every thing is Left handed down here, how else are you going to export it and have it mate up when it gets to the other side of the world? (I’m kidding).

But Left hand threads were common, still are here for fuel gases. Acetylene, L.P.G etc. Normally have a vee parted into the nut A.F flats to specify them as fuel gas / Left hand. Bar-B-Que bottles here are left hand thread, but B.S.P. Acetylene regs and hand pieces are left hand. I think if we follow it, it was probably a British thing.

Regards Phil.
 
I have some older CO2 regulators that are LH 1/4-28 NPT on the inlet side. I think they did that so some goof wouldn't confuse the high pressure and low pressure sides of the plumbing. Kaboom.

Allan
 
The "specials" section in the toolroom had LH pipe taps up to 2 IPS as I recall. They were all dusty and the hot dip strippable plastic that protected the threads was so old it was brittle.

Not to say that LH NPT pipe threads weren't used, available, whatever but it my 40 year of diversified experience in heavy ndustry mostly associated with shipbuilding and repair I've never seen LH NPT used in actual application.

Many straight flare and O-ring gas threads are based loosely on pipe threads (9/16 - 18 UNF is very close to 3/8 - 18 NPT for example) but flammible gas threads and some others (too lazy to look them up) were all made left hand to ensure exclusive assembly.
 
I routinely specify left hand pipe taps on some machines with particular applications. Here’s an example. You have a rotating shaft say 6” in diameter. You want to run cooling water through the shaft. So you bore the shaft for the cooling water and tap the end with a pipe tap, screw in a rotary union and attach your hose. This setup works fine as long as the rotating shaft tends to want to tighten the rotary union to the shaft. However if the shaft is rotating the other way, over time it will unscrew the union, unless of course you use a left hand NPT tap.
 
at least until 20 years ago, unions were forbidden for gas in Massachusetts (at least), not sure why, so you had to use LR nipple/couplings. Potentially leaky? Wow, I had much more trouble with LR nipples than unions, making them up at least; a really terrible design in my view.
 
Have seen left hand pipe threads used in heat exchangers that were used on electric street cars and steam locomotives for the compressed air . They had forged 180 degree return bends and the pipes threaded into the bends in a bank. The setup formed a serpintine back and fourth lattice of pipes. No unions were used and the left hand threads were needed to get the bends to make up at the end of each pipe....needles to say we are not talking about 1/4" here more like 1 3/4 -2 1/2 iron pipe.
Cheers Ross
 
Scottie that must be a Canadian thing. I work in HVAC field and while I have come across this on some stuff dated B.C. in the Detroit area. LH threads have'nt been in use in the last 50 or more years here. We pipe the stuff all day long and would'nt think of not using unions and its been that way for some time. But some do have the right idea we do use unions with the proper ratings on them so as not to aid the leaking process an all.
 
I once had to do a 'rush' job for American Can Corp... (about 5 years ago) called for two parts with 3" NPT, and two with 6" NPT LH. No gages, no mating parts, just cut 'em to the numbers; they couldn't take the mating parts out of the production line. Evidentially, they are still using LH pipe threads.

RAS
 
This brings back nightmares of when I was a young lad working in the shipbuilding industry used to spend a lot of my time working on a Cincimatic and never had a problem with any job I was given,except one tapping a large NPT thread it just more of less reamed the hole couldnt work it out as the tap was in a holder and never had the need for swapping holders as that m/c had loads of tooling done same job at least 3times with same results it really depressed me why I couldnt get it right(saved by helicoil),eventually it dawned on me it was a left hand tap.Such a fine thread that I rarely used it was difficult to see at first glance-Thats my excuse,so someone had been using it on Sulzer engines but unfortunately I was the new guy and everyone who used to work on those machines were long gone,taking advantage of the generous redundancy packages that was offered to guys in their 60s getting more than they could earn if they had seen their time out
 








 
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