Assembling NPT threads

[Bat21]

Not sure where to place this one but as it’s a project on my 80 I thought I’d post it here.

I am in the process of fitting a larger gearbox oilcooler and the fittings are 1/2 NPT.

When screwing the fitting into the cooler it only goes in by about 2 threads by hand, I have then tightened it a further 1/2 a turn to king up the 90 degree elbow to where I want it to be positioned, this final half a turn seemed to make it quite tight.

Having now read this http://www.entegrisfluidhandling.com/Product.aspx?G=1054 it seems I should be tightening the fitting by a further 4 turns past hand tight. Obviously I don’t want to strip the thread in the cooler or fitting by over tightening it.

I realise NPT is a taper thread but how much force can it take before before it ends in disaster?

 

[Chris]

Sounds like a lot to me; 4 turns? I have split NPT fittings before now. The VIAIR stuff is all NPT but seems to mate up well. Have you used any tape or some oil on the threads at all? 1/2" is a decent size and maybe it needs a good bit of force to seal it. Hard to know unless you try it under pressure and then nip to seal.

 

[chapel gate]

the temp probe I tapped into my exhaust manifold for my EGT gauge was NPT. I didn't know about the four turn thing until now, I just nipped it up until it felt right and its never leaked.

 

[StarCruiser]

I would say do it up till it feels tight. If it were taking 2000 pounds of hydraulic pressure I might say you need more threads but presumably you’re only talking 100 psi or probably far less (I don’t know maybe someone knows better). I would say to use a good thread sealant and to have at least 2 threads in total ideally more.

Are you absolutely certain they are NPT? I’m not saying they aren’t but it seems wrong that only 2 threads would be possible with correctly matched fittings. There’s not much difference between NPT and BSPT but what you are describing sounds like one against t’other.

 

[Bat21]

Yes I’m 100% sure both are NPT.

I’m using PTEF tape and yes the pressure in the cooler will be relatively low so hopefully 3 threads will be fine.

I asked because after coming across that link I thought maybe I was missing something and there was Black magic involved while mating these threads together.

 

[Towpack]

the temp probe I tapped into my exhaust manifold for my EGT gauge was NPT. I didn't know about the four turn thing until now, I just nipped it up until it felt right and its never leaked.

Mine also. Half a turn from finger tight felt ample but then it isn’t sealing in fluid under pressure.
So what pressure is auto box fluid under, a few psi maybe? I agree though, a further 4 turns from “feeling quite tight” sound too much, especially if the cooler and/or fittings are aluminium. I’d go with half turn increments until there’s no sign of leaks.

 

[StarCruiser]

I have come across female taper threads that are cut tight, in other words thevtapered tap hasn’t gone in far. I’ve also come across real sloppy fit ones too. Loctite do a good thread sealant. PTFE is good and LSX is also good especially when used with PTFE. How many turns of PTFE are you using and is it standard off the white reels or thick gas tape off of the yellow reels?

Lots of farm stuff used to use standard BSP taper threads on low pressure hydraulics. Not the best setup but I’ve seen it done a few times. It’s more the mechanical ability of the joint to stand up to its life in the truck than the pressure holding aspect in this case.

 

[Bat21]

As they are 90 degree elbow connectors I only have the option to go one full turn at a time due to where I need them facing.

I am using the standard White tape and there’s probably about 3 winds on there.

I’m sure the with the PTFE tape the 3 threads will be fine as the pressure in there shouldn’t be that great.

 

[Chris]

You know you can get swivel elbows?

 

[Bat21]

You know you can get swivel elbows?

It was hard enough finding these fixed 1/2 NPT to 3/8 barbs, has to get them from the US.... this is where you tell me my local corner shop does the swivel ones

 

[chapel gate]

Sainsburys, with the sweets at the checkout. ..

 

[StarCruiser]

It was hard enough finding these fixed 1/2 NPT to 3/8 barbs, has to get them from the US.... this is where you tell me my local corner shop does the swivel ones

Not seen swivel ones yet but there’s loads of NPT fittings on EBay such as this https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/232608341245
A local hydraulics supplier would be able to supply NPT fittings. A banjo would probably be good. https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/172844533084

How come it’s 1/2” is that just what the cooler has or is it 1/2” hose from the gearbox?

 

[Firewout]

When screwing the fitting into the cooler it only goes in by about 2 threads by hand, I have then tightened it a further 1/2 a turn to king up the 90 degree elbow to where I want it to be positioned, this final half a turn seemed to make it quite tight.

When it's tight and it doesn't leak, leave it that way. Don't overthink it too much, it's just a fitting.