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I have two oil pressure sensors on my engine (Mitsubishi K4).  The further one in this picture is the low pressure alarm, and this works fine (gets tested every time I stop and start the engine).  And when I change the oil it beeps for a couple of seconds as the new oil is picked up by the pump and gets to the sensor  -- all as normal. 

 

The nearer one is connected to a standard sized dial on the control panel, and this has been behaving rather sluggishly for a few months. I don't think I have a substantive problem with the oil pressure, so I am planning to replace the sensor and the dial with a new pair. (I also grounded the sensor wire, and the gauge flipped over to maximum pressure, so I think the circuit is OK - I had been about to check the connector plugs...)

 

So my question is: is the threaded hole in the engine block a standard size? The parts list for engine also includes a couple of plugs for these holes (I guess in case they are not in use) which are labelled "PLUG, taper (PT-1/8)".  I can find quite a few sensors on the internet, labelled 1/8" NPT, and as far as I can see PT and NPT are not quite the same (pipe thread and national pipe thread).  It may be that there is an adaptor between the engine block and the sensor.  So do I need a PT to NPT adaptor and then an NPT sensor ??

 

Many thanks for reading this far ....
DSC_5014.JPG.8d83f916386bd7a5e1ad53b6ed78d242.JPG   

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13 hours ago, Tracy D'arth said:

PT (pipe thread) and BSPT (British Standard Pipe Thread) can be considered interchangeable  at 1/8" size.

 

NPT is not the same Thread pitch or size.

 

See.

 

https://trimantec.com/blogs/t/thread-identification-guide#PT-Chart

       

Thank you,  that is helpful.  Who was it who said "the good thing about standards is that there are so many of them" ...?

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9 hours ago, Scholar Gypsy said:

Thank you,  that is helpful.  Who was it who said "the good thing about standards is that there are so many of them" ...?

 

Some sizes of NPT are very close to BSP, they will fit together and screw up for a few turns before it all goes wrong (screws up 😀).

BSP comes in two flavours, parallel and taper, sometimes written as BSP-P and BSP-T.

The taper version will seal on the threads, maybe with a bit of ptfe tape or whatever, whilst the parallel will use an olive or soft washer etc to make the actual seal.

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When I worked for British Gas's Midlands Research Station between school and university all the pipe fittings we used in test rigs (for gas, water and air) had BSP parallel female threads and taper male threads. With a couple of wraps of ptfe tape around the male thread the fitting would seal on the entry to the female thread, and not all the way along. For adjustable length connections we used a parallel male thread, with a locknut against the end of the female fitting, and a load of a gloopy red sealant liberally applied before tightening up.

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On 08/10/2021 at 16:07, David Mack said:

When I worked for British Gas's Midlands Research Station between school and university all the pipe fittings we used in test rigs (for gas, water and air) had BSP parallel female threads and taper male threads. With a couple of wraps of ptfe tape around the male thread the fitting would seal on the entry to the female thread, and not all the way along. For adjustable length connections we used a parallel male thread, with a locknut against the end of the female fitting, and a load of a gloopy red sealant liberally applied before tightening up.

 

Whilst out "on the district", few plumbers were even aware there are two types of BSP thread. If the joint leaks, apply more jollup or PTFE obviously. Problem solved....

 

 

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1/4" BSP is 28 tpi, 1/4" NPT  is 27 tpi, both are about 0.4" diameter mid-taper. The only pipe sizes where the pitches coincide are 1/2" and 3/4", 14tpi for both standards,  but 55° flank angle for BSP and 60 ° for NPT.  NPT is used in the USA and countries where  US practice is followed, the rest of the world uses BSP. 

 

The discrepancy between the nominal pipe size and the actual diameter goes back to the time when lead pipes were made by wrapping strips of lead sheet around a mandrel and soldering the seam. The nominal size was that of the mandrel, and thus the bore of the pipe, not the external diameter. 

 

Converely, the iron pipes originally used by the fledgling gas industry in the early 19th century were actually the barrels of governent surplus muskets from the Napoleonic wars, which had a 1/2" bore. These were joined using loose iron sockets.  Into the gap between pipe and socket was packed  a mixture of iron filings and a chemical that would produce rapid rusting and hence expansion of the mixture to produce a permanent gas-tight mechanical joint. A bit like joining  modern solvent-weld plastic pipe.  Developments in pipe manufacture allowed the production of pipes with thinner walls than musket barrels, and for compatibility with the original pipes and fittings, the new pipes were made with the same external diameter as the musket barrels..This is why the bore of a nominal  1/2" iron pipe is significantly greater than 1/2", and plumbers still refer to iron pipe as "barrel". 

 

Likewise, the nominal sizes of Imperial copper pipes were the internal diameter of the original types that had significantly thicker walls than later types. Hence the original 1/2" copper pipe had an external diameter of slightly greater than 15mm and a bore of 12.5mm, and with the development of thinner wall pipes, the external diameter of the original thick wall type was retained.   

Edited by Ronaldo47
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8 hours ago, Ronaldo47 said:

Converely, the iron pipes originally used by the fledgling gas industry in the early 19th century were actually the barrels of governent surplus muskets from the Napoleonic wars, which had a 1/2" bore. These were joined using loose iron sockets. 

 

A very romantic idea I've heard mentioned before, but is there much evidence of this? 

 

It reminds me of the folklaw that Springers are made out of scrapped gasometer steel. I have a sneaky feeling Mr Springer was more likely to have just ordered the steel he needed, in the sizes and thicknesses he needed, from the local steel stockholder just like every other steel fabricator. 

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I first read it in the 1970's  in a book I got out of the public library  written by a plumber who had been given a scolarship to research the history  of pumbing. I have also found references to this in other books on pumbing I have come across.  While I trained as an electrical engineer, dad was a pipe fitter, and from an early age I have always taken a keen interest in anything to do with pipes snd plumbing! 

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5 hours ago, Ronaldo47 said:

I first read it in the 1970's  in a book I got out of the public library  written by a plumber who had been given a scolarship to research the history  of pumbing. I have also found references to this in other books on pumbing I have come across.  While I trained as an electrical engineer, dad was a pipe fitter, and from an early age I have always taken a keen interest in anything to do with pipes snd plumbing! 

I heard that many years ago, not that they were surplus muskets but that was how they made them in the same place and manner of musket barrels.

All I can say is thankfully they were not French muskets else we would have metric pipes.   Oh damn, we already have.

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We don't really have metric pipes.   We, and the rest of Yurrup mainly  have imperial pipes, and connecting threads,  measured in metric units. Hence a 1/2 in (ID) pipe will fit into a 15 mm (OD) solder or compression fitting and a 1 in pipe into a 28mm solder or compression fitting.  The exception is 3/4.  The thread series G, Rp and Rc are all defined in SI units but are interchangeable, inter-connectable and have the same  thread angle as the BSP series from which they are copied/derived.

I also believe some German manufacturers still use the 26 tpi 55 deg thread angle British Brass series. Note it is not BSB, as there is no published Standard.   The Whitworth series of threads also sometimes still makes an appearance from Europe!

 

N

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8 hours ago, Ronaldo47 said:

I first read it in the 1970's  in a book I got out of the public library  written by a plumber who had been given a scolarship to research the history  of pumbing. I have also found references to this in other books on pumbing I have come across.  While I trained as an electrical engineer, dad was a pipe fitter, and from an early age I have always taken a keen interest in anything to do with pipes snd plumbing! 

If you had gone on the tools instead of being an engineer you could have bashed some solid drawn conduit

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More than 25 years ago I was tempted into buying a die stock with a 20mm conduit thread die from a second-hand stall in Romford Market for £2. I finally got to  use it for the first time a couple of years ago to instal some additional sockets in the flat my son had bought. I don't have a pipe vice, but clamping the conduit in a Workmate and preventing rotation with a Stillson wrench worked OK. I would love to get a conduit bending machine, but they are a bit too expensive to buy new and I haven't seen any at car boot sales yet. 

 

When visiting my late brother-in-law in France, I found that the French have gone their own way with pipe sizes. While we commonly have 10, 15, 22, 28mm, their series is  10, 12, 14, 16, 18, 22, 28 mm. French plumbers do not usually use soft solder with copper pipes, preferring to make brazed joints.  

The old German electrical steel screwed conduit thread (Stahl panzer rohre gewinde, abbreviated to "PG") was defined in terms of "Gewinde pro englisher zoll" (threads per english inch), and was supposed to have been superseded by metric sizes a decade or so ago, but there still seem to be plenty of fittings for the old sizes available, even in the UK, for compatibility with imported continental stuff. . 

Edited by Ronaldo47
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13 minutes ago, Ronaldo47 said:

 

 

 . I would love to get a conduit bending machine, but they are a bit too expensive to buy new and I haven't seen any at car boot sales yet. 

 

 

A meter length of wood with a hole in the end about 100mm square or over your knee, but not with big stuff

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Thanks again to all for their advice, and history on the making of pipe joints.

 

I bought a BSPT to NPT adaptor, and a new sensor and gauge (VDO). In the end I was able to remove the old sensor while leaving the existing adaptor in the engine block, and I was able to satisfy myself (using the redundant adaptor) that the old and new sensors were both NPT.  A bit of PTFE and some crimping, and I am now getting  over 4 bars.  It will be interesting to see again how this drops as the engine warms up. I also have  a bit of kitchen towel in place, to check that the joint is not leaking.

 

Also very pleasing that the buzzer (the other sensor) comes on exactly as the needle gets down to zero - I've never had that before.

 

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On 12/10/2021 at 17:05, BEngo said:

We don't really have metric pipes.   We, and the rest of Yurrup mainly  have imperial pipes, and connecting threads,  measured in metric units. Hence a 1/2 in (ID) pipe will fit into a 15 mm (OD) solder or compression fitting and a 1 in pipe into a 28mm solder or compression fitting.  The exception is 3/4.  The thread series G, Rp and Rc are all defined in SI units but are interchangeable, inter-connectable and have the same  thread angle as the BSP series from which they are copied/derived.

I also believe some German manufacturers still use the 26 tpi 55 deg thread angle British Brass series. Note it is not BSB, as there is no published Standard.   The Whitworth series of threads also sometimes still makes an appearance from Europe!

 

N

 

Ahhh - but which 'inch ?

There were many many inches.

 

 

 

Screenshot (653).png

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To be pedantic, in copper pipe 1/2" is a bit bigger than 15mm pipe. Same with 1" and 28mm.  It is possible to get a fit with enthusiastic cleaning!

 

22mm is substantially larger than 3/4"

 

The imperial sizes were based on the internal size, metric on the outside diameter. 

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