Canopy Slumping (But in Miniature)

Recently, my airpark neighbor Ed Banks was on the hunt for a leak-down tester. He was in the process of doing the annual condition inspection on his RV-6, and when he fetched his vintage (1990s?) E2 tester, one of the gauges was broken. Since I like to fix things, I was curious what the problem might be. It turned out that the plastic crystal had yellowed over time and became brittle.

Although the gauge worked perfectly without the crystal, Ed was right in not wanting to use the tester. Any slight bump or slip-up risks damaging the pointer needle or inner workings of the gauge.

Ed managed to borrow a leak-down tester and finish his annual. Afterward, he mentioned he was going to buy a new one. That was cool, but I thought it would be a shame to throw out a perfectly good tool because of a cheap plastic part. My first thought was I could probably find a replacement crystal on Amazon. I found one that might have worked, a 52.4 mm domed crystal for $20. The only hitch was two weeks for delivery.

I happened to have some scraps of 1/8-inch-thick acrylic sheet (Plexiglas) that I thought might work. I started by measuring the diameter needed to fit the groove in the bezel: 2.070 inches. I chucked a pre-cut square (see photos) in the lathe and made a disk to that diameter and turned a 0.060-inch-thick lip to match the original. The flat disk crystal did not work: The groove in the bezel was at (or just below) the pointer movement, so the crystal had to be domed to allow the pointer to swing.

So, we needed a domed crystal. The process of making the crystal was more or less the same as slumping an airplane canopy, just on a miniature scale.

All you need is a form and some heat. In this case, my part was too small and too thick to slump under its own weight. But with a little persuasion—in the form of the back side of a big spoon—I got it to conform to the mold.

That’s it for now. Time to get back in the shop and make some chips!