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A Lesson in Thread Standards

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Thought I'd share a lesson I've just learnt the hard way, to assist others in avoiding a nice shamozzle...


We've been chasing an annoying and intermittent air leak around our fuel system, nothing to do with our new engine, which is great. But does causing it to intermittently not start without priming or to miss infrequently at high revs.


We found we couldn't get the old fuel / water separator bowl to seal properly (had a minor leak), so we replaced it. The problem basically persisted. I set up a system to pressure test the fuel system, by installing a priming buld (aka outboard primer) and a shut off valve in the discharge line. Immediately found the new fuel / water separator was weeping at the fitting. On closer inspection it had a stuffed thread in the alloy filter housing - non of the fittings I had on the boat would thread in, despite all being 1/2 in.


Went and purchased a new fuel / water separator, and specifically asked for the correct fittings to join it to the pipes. Out of curiosity I read the spec sheet before installing it - threads are 18 NPTF (Parker Racor filter)


Googling up the one we just stuffed, Ryco, takes 20 UNF.


Upon checking with the retailer who sold me the Parker Racor filter yesterday, they supplied me with 1/2in BSP threaded fittings not 18 NPTF. These fittings line up and appear to fit, but about half way in start binding up. They didn't have any 18 NPTF, don't stock them.


So we went and got a Ryco filter (fraction of the price by the way), but went around 4 retailers, including specialist hydraulic suppliers, and none of them stock 20 UNF fittings. One actually asked what the f*ck we'd want UNF for?


So we phone Ryco and asked them how we're supposed to connect hoses to their filter we just brought - and they are the only people they know of that stock the fittings that fit their filter. Accept none of the retailers who sell that line carry the fittings... Ryco head office are couriering us fittings overnight...


As far as I can tell, every other fitting on the boat is BSP. All of these fittings are 1/2 in, and look the same. Some even 'fit' each other, but cause issues when tightened. Like little weepy leaks, and stuffed threads.


So for anyone installing or swapping fuel / water separators, check the specs and ensure you get the right thread specification on your fittings. Otherwise you risk having a tiny wheeny air leak in your fuel system that will drive you bat sh*t crazy trying to find...

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Yip, threads are a freakin nightmare.
  The type used depends on where the unit was made and what it is going to be used for. The US has their own range of threads, being NPT(national Pipe Thread). Brittan has theirs, being BSP(British Standard Pipe) and Europe tend to use Metric. So it is helpful to at first identify where something is made and that at least may give you a clue.
However, within each Country, there are different variations of each type as well. For instance, BSP has BSPP = Parallel and BSPT = tapered.
The reason is that each thread type is for a specific purpose. Fuel, Gas and Water. So knowing that helps as well. Then there is Pipe, Tube, Hose. Pipe is measured internally and has a heavy wall thickness and can usually be threaded. Tube is measured externally and tends to have too thin a wall section to be threaded.
Then there are the SAE(society of automotive engineers) fittings. They have the tapperd ends on them and work by screwing into a socket with the opposite taper and the surfaces mate together. These fittings do not need sealant and as the name implies, they are found in the Vehicle industry and common for Fuel and oil etc.
      On top of all that, you then have flared fittings and Olive compression fittings for various Tube types. And then when it comes to copper, there are individual size ranges dependent on use as well. For instance, Water has a pipe size range that is different to Refrigeration and refrigeration uses their own version of the SAE fitting.
     I am shocked you could not find the right fitting for the filter though. ENZED should have carried the correct fittings as they are Parker. You start running into problems getting these fittings when you go to the other guys like Perko or Ryco etc. The reason for Hydraulics being so varied is that as time has gone on and new equipment developed, pressures have dramatically increased and so completely new Hoses and ways of dealing with such pressures have had to be developed. It's a nightmare and a mistake can be costly.

So yes, it is a nightmare, but understanding a few of those important points can be helpful to identify what a thread may be.
Where it is made.
What it is for... Fuel, Oil, water.
Look in the hole and does it have a concave socket at the bottom, or a seat around the neck for an O ring, or is it a tapered thread.

By the way, the reason the Parker/Racor unit has the Oring fitting is due to the unit being made for late model Diesel engines in Trucks and heavy machinery, which tend to use high flow rates and pressures and the hoses used are often a Hydraulic type hose. The fittings need to be rugged and take a lot of strain. Other fittings are too light weight and could be snapped off.

Slightly aside, but NZ is about to have a major change in the plumbing industry. We are about to change over from BSP to Metric, which means the plumbers will need to carry two ranges of fittings to meet the old stuff and the new.

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Beta marine diesels Japanese Kubota engine with British conversion bits.

Two different threads and Hex sizes in certain places , duh.

I've not come across that problem on our Beta yet. We're really happy with it. Have rationalised the spanners we need down to 3, being a 7 mm, 10 mm and 14 mm. (But I do keep a full set on the boat).

Previous engine we needed every size in both metric and imperial... Was a Japanese engine with British gearbox and NZ marinisation... Isuzu motor, PRM box and Lees HE and water pump. 


Prior to this whole thread shamoozle, I was familiar with UNF and UNC, unified fine and unified course, and BSP obviously. We needed UNC on our prop shaft when we replaced that, to match the propeller nut.

But this NPTF threw me - National Pipe Taper - Fuel. Its an obscured American standard for fuel pipes only... The only place it is noted on the Racor filter is on the spec sheet that came with the filter, that I read after buying the wrong fittings.


As for the Ryco filter, the only place we found information that the thread was UNF was on the Ryco website, where it was advertising matching fittings. If we didn't have a problem with the Racor fittings, we wouldn't have checked the Ryco thread standard, and bunged another BSP thread in the hole... I could have easily stuffed the Racor filter by doing that ($280), but luckily, we've only stuffed one Ryco filter ($80) during this whole exercise, not to mention a fair amount of time.

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Yip, threads are a freakin nightmare.

  The type used depends on where the unit was made and what it is going to be used for. The US has their own range of threads, being NPT(national Pipe Thread). Brittan has theirs, being BSP(British Standard Pipe) and Europe tend to use Metric. So it is helpful to at first identify where something is made and that at least may give you a clue.

I agree threads are a nightmare.

As my stepfather used to say: "Everyone likes standards. Thats why we have so many of them!"

Actually the BSP standard was adopted into the metric system as  ISO 7 and  ISO 228.

ISO 7 generally relates to what used to be BSPT (British Standard Pipe Taper  where a seal is formed on the thread as the male thread is tapered and tightens as you screw it in. ISO 228 relates to BSPP British Standard Pipe Parallel where the seal is not formed on the threads but on an end face gasket.

Still has the classic Whitworth thread form too!

(I regularly cringe as i watch todays "I did a 5 minute training course" tradesmen attempt to seal parallel threads with lots of thread tape.)


For stuff originating from Europe you will see these designated as: 

  • G, external and internal parallel (ISO 228)
  • R, external taper (ISO 7)
  • Rp, internal parallel (ISO 7/1)
  • Rc, internal taper (ISO 7)
  • Rs, external parallel

with G and R being most common.


As for NPT, its like Trump. Should be kept within the USA and never exported. We are a metric country and NPT is not a metric standard so suppliers and importers have no business bringing this sh*t in.

Good luck

Edited by Abel Seaman
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