I have an intake manifold that has two 3/8" NPT threaded holes that I need to plug. I understand how NPT fittings work with their taper. This leaves the plugs proud of the surface. I don't dig that look. I'm looking for a plug that will thread in further. I'm looking for a non-tapered plug if there is such a thing. If there is such a thing, what would it be called and where could I find one, preferably in stainless steel.
Sounds like you're looking for an ORB fitting:
https://brennaninc.com/fittings-101-orb/
Edit: These wouldn't be totally flush with the surface because they seal with an O-ring, but they wouldn't have extra thread sticking out like an NPT fitting.
In reply to GameboyRMH :
ORB is a different thread diameter and pitch.
Not sure if they are available in SS but Levl-Seal plugs are designed to install flush to the surface. May have to google around with the part number to find a desirable package qty or we sell them at work so I could source some if needed.
In reply to EvanB :
Ah I thought the question was looking for a different system...yeah ORB is incompatible and a plug with an inset hex head like fasted58 linked (Edit: Or yours) would work best.
We sell these
So I have to add that I have two of the recessed hex plugs-one in aluminum and one in SS-that protrude above the surface. I also have an NPT hex head plug (uses a wrench or socket to drive) and it sits way above the surface. Ford used the square headed NPT plug as do plumbers. I couldn't find an ORB style that uses whatever 3/8 NPT equates to in SAE fastener thread pitch, if it does.
The manifold did come with two steel recessed plugs that do fit flush but they're plain steel and will corrode. That tells me something is out there but I don't know what to search for.
In reply to EvanB :
That looks like what I'm after! Is NPTF a fine threaded variant of NPT? Does F stand for flush?
wawazat said:In reply to EvanB :
That looks like what I'm after! Is NPTF a fine threaded variant of NPT? Does F stand for flush?
Usually F stands for female. What about using a 3/8 NPT tap and just run down a little further, which would move the taper down in the hole and thus the plug.
In reply to carbidetooth :
My thought as well.
I too would tap the hole a turn at a time until it was flush
But also.
NPT means National Pipe Taper.
NPS means national pipe straight.
3/8" NPS is the same 18tpi thread, just not tapered. But you would need to run the NPS tap through and then rely on a Loctite product to seal and retain the plugs
Why not just grind the plugs down until they are short enough to be flush?
In reply to carbidetooth :
Your suggestion seems easiest if the hex plugs can be sourced locally.
If it's already installed on the engine, running the plug through a die to let it sit a bit deeper might be cleaner than tapping and trying to remove all The chips from the passage.
wawazat said:In reply to EvanB :
That looks like what I'm after! Is NPTF a fine threaded variant of NPT? Does F stand for flush?
F stands for Fuel, it is a tighter thread tolerance than a standard NPT to provide better sealing. Hydraulic adapters and higher pressure fittings will typically use it.
If you'll type "3/8 npt recessed plug" into Amazon you will find pages of them. Stainless, brass, whatever material you choose.
Thank you Cousin Eddie! I'll look in to those.
Since everyone likes pictures, here's what I'm dealing with.
3/8 NPT hex head plug fully tightened
3/8 recessed hex plug fully tightened
Unknown recessed hex head plug that came with the manifold fully tightened
This plug is not coated and is a bit crusty. It is the fit I'm looking for however.
I like the external hex head plug as it doesn't have a recess to catch and hold schmutz like the recessed hex plugs could.
The intake is not yet mounted though it has been powder coated, so I can tweak the threads by hand but wouldn't want to hog them out by machine.
In theory pipe thread seals by the threads "crushing" hence using Teflon tape to help seal. You're not supposed to thread/unthread NPT a 1,000 times as the threads would theoretically wear.
To get the plug lower the idea of using a tap makes sense but can be tricky.
Now we can discuss NPT "thread Engagement".....
These holes will be plugged and not accessed again during my ownership that I can foresee.
If the two non supplied plugs sit THAT high up, it looks to me that they didn't tap deeply enough. Or their tooling is worn.
Either way, they can be tapped deeper.
Maybe a little silver paint on the ones that seem to be made for it would suffice?
MiniDave said:Maybe a little silver paint on the ones that seem to be made for it would suffice?
Or a spritz of Boesheild T9?
If corrosion is the primary concern I would do that before removing the manifold to tap the threads deeper.
Alternatively, purchase a 3/8" NPT thread die and cut the taper deeper on the new plugs (this will be hard work on stainless) but you won't have to worry about metal chips in the intake
Thanks Trent and Pete. I can tap the manifold now then blow it out or run the plugs in to the die. The black plugs are aluminum so that shouldn't be too hard to do. The stainless ones will be a challenge if I choose to go that direction.
EvanB said:wawazat said:In reply to EvanB :
That looks like what I'm after! Is NPTF a fine threaded variant of NPT? Does F stand for flush?
F stands for Fuel, it is a tighter thread tolerance than a standard NPT to provide better sealing. Hydraulic adapters and higher pressure fittings will typically use it.
Learn something new every day!
MNPT and FNPT are the male and female designation. Never ran into an F on the end!
We used to weld metal hoses at one hose shop I worked at and they ignored the F (fuel) and just threw on whatever had in stock - most had no idea.
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