Q&A – Threading compression fittings

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Q: I have a difficult time threading the compression nuts on the ends of hydraulic lines, such as on a tilt/trim mechanism, and high-pressure diesel injector fuel lines. Are there any tricks that make this easier?

A: There are high-pressure steel or stainless steel pipes between the injector pump and the injectors on diesel engines, on various hydraulic systems, such as outboard hydraulic tilt assemblies, and in other applications. Usually the pipe is fastened with a nut that threads over or within a socket. The pipe passes through the nut and has, as part of its end, a small, rounded face with the orifice in the middle. This is sometimes referred to as a ferrule or “olive.” The high-pressure seal is made by compressing the ferrule against the rounded, smooth matching face within the socket by carefully tightening down on the nut.

To the surprise of the uninitiated, it’s often very difficult to align the threads and screw the nut on once you remove the compression fitting for servicing or perhaps to replace the line. The result can be stripped threads and possibly the need to replace very expensive parts, or spending hours trying to get the ends lined up.

This usually happens because you have to pull out the pipe from the socket far enough for the ferrule on the end of the line to clear the socket. When you pull out the end of the line after backing off the nut, frequently the pipe bends just slightly so that the fittings aren’t aligned perfectly when you reassemble. Even if you’re installing new pipe, it may have been slightly bent during packing or shipping. Be very careful when you remove the pipe to not bend it even the slightest. Loosen the nuts at both ends — each a little at a time — so that as you reach the end of the threads at each end of the pipe, you can lift the entire pipe off without bending any part.

I’ve used the following method successfully many times. (Others prefer other methods or techniques.) When you’re reassembling the pipe, put both ends in place and get the first end started, being sure the threads line up correctly. Do it by hand, turning it in far enough to be sure it’s threaded correctly and then backing it out until it’s just hanging by a few threads. I usually start with the least accessible end because the first threading typically is the easiest. You may find it otherwise. The pipe will be loose, even though you’ve started threading it at one end, because you haven’t compressed the ferrule on that end.

Center the pipe squarely over the other compression port if it’s not already in place. Insert the pipe into the port most of the way down but not tight against the compression face such that you will misalign the compression fitting. As noted above, it’s best to have this end in place when you began with the opposite end. Move the compression nut into or over (as the case may be) its threaded connection.

Unless you’re lucky and it threads correctly right away, gently press the compression nut against the threads with your fingers. Then turn it counterclockwise as though you’re unscrewing it. Listen closely and feel carefully with your fingers. As you turn it counterclockwise, you’ll eventually feel and maybe hear a click as the ends of the threads move past each other and snap back in place at the beginning point. Immediately as you feel this, stop backing the nut off but maintain the gentle pressure. Carefully start turning it clockwise. Because the threads have been positioned at the right point to grab, they’ll usually catch and will be correctly aligned so that you can safely tighten. If not, keep trying until they are. (You can also use this method to start the first end if you have problems at that point.) Never use a wrench until you’re absolutely sure the threads aren’t crossed. Hand tighten until there is no question, and eventually use a wrench to snug the nut tight.

The nuts on high-pressure lines must be tightened enough to keep the ferrule pressed tightly against its mating surface to prevent leaking or seepage of the fluid. But you must not overtighten; this can damage the fitting and/or crack the ferrule. Tighten carefully until it feels snug, start the machine, and, if necessary, tighten just enough to stop any seepage. Usually “just enough” is very little.

Check regularly to be sure no seeping develops from vibration or slippage. Be careful: in some applications, such as diesel high-pressure fuel pipes, the pressure of the fluid should there be a leak or breach can be high enough to puncture your skin or worse.

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