The 3rd director of my 5 elements Yagi was missing the insulator/separator as designed by 9a4qv (http://www.qsl.net/9a4qv/Antenna_insulators.htm). Got hold of a rubber pipe section, dug the 10mm boom housing and drilled a 3mm through hole. Here's the result:
Once the element was mounted on the boom I noticed I could tighten the wing-nut indefinitely. The cause was the bolt pressing and flexing the insulator. So I cut in half the rubber pipe, keeping the lower part (upper in the picture).
Here's the XYL checking the result