NPT fitting advice - Team Camaro Tech
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post #1 of 12 (permalink) Old Jul 19th, 02, 07:01 PM Thread Starter
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I need some info on NPT fittings.

I have a holly mechanical fuel pump that has 3/8 NPT inlet and outlet. I purchased the Earls 3/8 NPT to 6 AN fitting. When I screw them in, they only go in about 3-4 full turns and then come to an abrupt stop. Consequently its only engaging about 1/3 of the available threads on the fitting. I used a wrench and tightened them down some more to see if it would go in any further.

Well of course being and aluminum fitting, the threads pretty much stripped all to hell and in the process damaged the threads on the pump. I ended up having to tap the pump threads to get them fixed.

So whats the deal ??? Is this how a NPT fitting is supposed to work ?? Why wont the fitting thread into the pump farther before stopping ??

[This message has been edited by DRJDVM (edited 07-19-2002).]
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post #2 of 12 (permalink) Old Jul 19th, 02, 07:19 PM
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3 or 4 turns should be all you need with the tapered NPT threads. It won't screw all the way in.

Try some teflon paste on the threads, it reduces some friction and also helps seal.

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post #3 of 12 (permalink) Old Jul 19th, 02, 07:53 PM
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Holley inlets are SAE inverted flare fittings, not NPT fittings. Regardless of what Earl's may or may not call them. the difference is National Pipe Thread, NPT, seals with the threads becasue the plug is tapered. SAE Inverted flare fittings seal by holding a double flared steel line in compression against the inverted flare inlet fitting. They are a straight thread, not tapered. If you installed a NPT to AN fitting into a Holley, you did damage it. but it isn't that bad, you can buy replacement inlet fittings from Summit or any good speed shop.

Droptop is right regarding NPT fittings that you thought you had, sealers will help and it shouldn't take many threads before a NPT plug seals. However Holley carbs don't use NPT threads in the inlets. Inverted flare fittings should never be installed with any kind of sealer, if they don't seal, something is wrong with the metal to metal seats. this includes all fuel lines and brake lines using SAE inverted flare fittings. Same can be said for any flare fittings, sealers should never be needed or used. If it leaks you've got another problem, the threads aren't the sealing surface for any flare fittings. They do make a conical flare metal crush gasket to help seal flare fittings that may be slightly misaligned or scratched. Seco Seal is one brand of these.

Hope this helps,

[This message has been edited by SY1 (edited 07-19-2002).]
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post #4 of 12 (permalink) Old Jul 19th, 02, 08:28 PM
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All the above is correct. I think he's dealing with a Holley fuel pump, not a carb, so it is NPT and they only go in a few threads. Use liquid teflon paste and you're good to go.

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post #5 of 12 (permalink) Old Jul 19th, 02, 08:29 PM Thread Starter
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I'm talking about the fuel pump not the carb. The holes for the inlet and outlet are just holes..... there is not flare in them like brake fittings etc.

I kind of thought that NPT fittings slowly taper. If you look into the hole on the pump, it kind of looks like it gets alittle smaller as you got farther in. I thought I was imaging it, but it looks like my first impression was right.

I am using Teflon paste in the NPT side, but not the AN side.

I did figure I had enough threads for a seal, I just thought it looked kind of crappy to have the fitting sticking out that far.
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post #6 of 12 (permalink) Old Jul 19th, 02, 09:04 PM
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NPT - national pipe taper.

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post #7 of 12 (permalink) Old Jul 19th, 02, 09:20 PM
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I read your post again and see it is a mechanical fuel pump you're working on. You're right, it seems it would go in further than that. It's possible that they either didn't run the tap in far enough so the hole is on the small side causing your fitting to stick out, or it's intentionally left on the small side because there is a reason they don't want the fitting going too far in. I've got one on an engine in the shop I'll take a look at and let you know how mine looks.
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post #8 of 12 (permalink) Old Jul 20th, 02, 02:18 AM
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Is it possible that Holley has gone to a metric thread? I wouldn't be at all suprised. Seems like the english standard threads are being phased out of everything, anymore.
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post #9 of 12 (permalink) Old Jul 20th, 02, 03:57 AM
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I have the Holley 110 gph mechanical pump on my 383. I had the same concern when I installed mine. I work with pipe threads all the time and these just didn't seem quite right - they didn't want to go in very far like you said. But I did get them to seal and they haven't leaked after two years of use. I think maybe its because Holley puts plating on the pump after the threads are cut.

I use two wraps of the cheap/thin teflon tape (I don't like the thick stuff) with a light coating of teflon paste on top of that. Just make sure you apply the teflon tape and/or paste so it stays only on the threads or it will get clogged up in your fuel system somewhere.
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post #10 of 12 (permalink) Old Jul 20th, 02, 04:54 AM
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i recently had the same problem as you with an intake manifold. Once getting the fittings out, clean up the thread with an NPT tapping tool. This worked well for me. Then, use a good amount of teflon paste and put in new plugs.

To get mine out, I first soaked them with a liquid wrench penetrant with teflon. They came out fairly easily with that.

Good luck, and again thanks to all the guys who gave me advice on this a few weeks ago. = )
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post #11 of 12 (permalink) Old Jul 20th, 02, 08:37 AM
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The deeper the tap screws in the larger the hole, but there might not be enough room behind the holes to tap it that deep?
I'd just tap it deeper if the tap doesn't hit something.
I'd be careful using aluminum fittings where there is a lot of vibration. I had a dash four aluminum elbow break off my water outlet. Lucky it wasn't a fuel fitting.

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post #12 of 12 (permalink) Old Jul 20th, 02, 06:20 PM
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David good advice on the aluminum fitting. You are right there is a place to use aluminum AN fitting and for better strength or fatigue resistance steel AN fittings.

I checked my pump, I used a NPT plug that has an inverted flare on the other end so I could bend my own hard lines to the carb. My plug threads almost all the way in. Like David says, with NPT the deeper the tap is run in the hole the larger the hole will be due to the taper.
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