Words And Photos: Jeff Smith
Does it seem like the whole world of AN lines and fittings has become a bit more complicated in the last few years? Not all that long ago, all you had to try to remember was that AN fittings were different than production car fittings. But now in addition to tapered National pipe thread (NPT), there’s straight NPS thread and then something called ORB fittings. We decided to unscramble all these acronyms and define what’s going on and also point out some important tech tips about threaded connections and sealing techniques that can help you the next time you have to ensure that fluid connection stays tight and leak free. It’s easier than you think.
First it’s important to know some definitions of components you will need to remember when it comes time to make those fuel line or transmission cooler line connections. First of all, let’s be clear that this discussion is all about fittings specifically for oil or fuel fittings. It’s a whole different topic to get into bolt threads or brake fittings. We’ll save those for a different discussion.
The most popular automotive sealing fittings employ what are called National Pipe Threads (NPT). Many believe that NPT stands for National Pipe Taper because the threads are tapered at a 2-degree angle, but the important point is that pipe threads use this tapered arrangement to help seal the connection. Thread dimensions can be confusing because the size designation – such as ¼-inch pipe thread – defines the inside diameter (I.D.) of the pipe, not the outside diameter (O.D.) of the fitting. If you study the size chart for a moment or two, it will begin to make sense.
Less common are National Pipe Straight (NPS) fittings that are sometimes called National Pipe Straight Mechanical. These threads, as you can probably guess, offer the same thread pitch as NPT threads but are straight – not tapered. This means a 3/8-inch NPS female fitting can accept a 3/8-inch male NPT fitting. This does not mean they are particularly compatible, but in a pinch, the NPT thread will seal when accompanied by thread sealer or Teflon tape. While this is an obscure thread, you may have already run across it without realizing it. As an example, the GM 4L80E, four-speed automatic overdrive transmission uses female NPS threads in the transmission case to mount the adapter fittings for the cooler lines. While a ¼-inch NPT male fitting will work in this application, the proper fitting would actually be a ¼-inch NPS fitting with a sealing O-ring.
This brings us to the next level of fluid transfer fittings that are becoming increasingly popular especially with fuel delivery systems. You may have already seen references to ORB fittings – which stands for O-ring boss fittings that, much like a NPSM fittings, use a straight AN thread pitch combined with a nitrile rubber or Viton rubber O-ring placed over the base of a male AN thread fitting. The O-ring seals the connection and is very easy to service should a leak occur.
The most popular place for ORB fittings is with high performance electric fuel pumps, regulators, and filters. This design offers the advantage of large fitting sizes to increase flow using large O-ring sealing surfaces that can be easily adapted. ORB fittings use standard AN thread size and pitch configurations to make choosing the proper size easier. If you’re trying to determine the size of a female ORB connecting, for example, just thread in a male AN fitting. For example, your fuel pressure regulator has a large female hole and you need to determine its ORB size. The most popular of these are either -8 or -10. Find a -8 male AN fitting and see if it threads into the hole. If it fits, then you know that it’s a -8 ORB. If the -8 is too small, likely the size is a -10 ORB fitting. You can also use the sizes in our AN fittings chart as reference.
Working on your car can be a hassle sometimes, especially when it comes to high performance fittings. Making sure all those fuel and oil fitting connections are tight and leak-free is a great way to make the high performance life a little more enjoyable.
(Tube or hose I.D. will vary)
|AN Size||Tube or Hose OD||Tube or Hose ID||Fitting Thread Size(SAE)||O-ring size|
|-6||3/8”||.297 – .343||9/16-18||2-113|
These are AN standard specifications but actual component can vary slightly.
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