Compression Fittings

There are compression fittings, and there are compression fittings, and they're not all created equal.

At the left in the above photo is a pre-fabricated, gasketed fitting on the end of a faucet supply hose. Those are superb -- absolutely fool-proof and reliable.

In the centre is a plastic faucet supply tube and compression sleeve. I get fine results from those as well.

At the right is a length of plated soft copper tubing with a loose sleeve and nut that one assembles in situ. My hat is off to anyone with the skill to consistently get those things assembled correctly -- I have a devil of a time with them.

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I installed a hose faucet by my workshop's sink recently, and learned a few things.

First off, compression fittings and soft copper tubing are useless for creating any sort of rigid, mechanical structure. I was attempting to add a tee fitting to a faucet supply valve with a very short length of tubing, like so.

That was fallacious to start with. Compression fittings have little tolerance for mechanical stress. They're strictly for making connections where the only load on them will be the weight of the undisturbed tubing itself. Just tightening the two hoses attached to the tee introduced stresses that were more than the lower two fittings could cope with. They leaked.

Even when correctly applied, though, compression fittings can be an iffy affair.

For a pressure-tight seal, the sleeve must compress straightly and truly at both ends. The least bit of crookedness in the sleeve's compression will result in a leaky fitting that no amount of tightening will correct. I tried a little experiment that I hoped might yield an improved fitting. It didn't work out, but here's what I did, for what it's worth.

At the left in the above photo is a standard 3/8" compression nut and sleeve. Note that the nut has the same bore as the sleeve. That strikes me as fallacious. It seemed to me that if the nut's bore were bigger, the assembly might be much more inclined to self-align correctly, so I bored out a nut to 27/64". It's shown at the right.

That turned out not to work any better, but a subsequent discovery told me I was on the right track.

While rummaging about in my little drawer of 3/8" compression fittings, I found some items that I'd paid no attention to in the past -- compression nuts with captive sleeves. Here's a view of one next to a standard nut and sleeve. (Down in front at the right is a captive sleeve that I knocked out of a nut.)

'Captive sleeve' nuts are also known as 'self-aligning' nuts. As you can see, they have a very different sleeve construction, one that makes a lot more sense to me than the conventional style.

I still wouldn't attempt a fallacious assembly like the one in the second photo above with these, but if I ever have need of a compression fitting on a legitimate soft copper tubing installation, I'll do it with captive sleeve nuts.

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One last point about compression fitting installation; lubricate the fitting before assembling it -- WD-40, salad oil, spit, something to help it go together smoothly and snugly.

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