I recently posted looking for some guidance on replacing the plastic cover for my Alon Fuel Tank gage (left side, by the pilot.) Got good input – including a link to Hartmut’s site with a high-end aluminum and glass replacement.
I took another route and wanted to share it with the group.
Lexan is fuel resistant, so I got some, cut it down to approximately the size required. Exact fit isn’t any issue in that application.
I removed the metal ring that sits on top of the sight glass, and used that as a template to drill hole into the lexan piece. Note: on my plane, the holes are not evenly distributed around the circle, so I marked with a sharpie where things lined up so I’d get them the same way every time. The metal ring has standoffs to keep the interior panel off the glass.
I then put the lexan on the metal ring and turned it upside down. I put 6 number 6 screws in it to hold it in alignment.
The trick is how to get a dome into the lexan. Turns out, lexan absorbs water (and holds it) so you need to bake the water out. Looking around for the chemistry, it turns out that if you heat lexan at 250 degrees F that will do the trick. But, as with all recipes, you need to know how long to bake it. Turns out that for every .01” of thickness, you need to bake it for 1 hour. For the .093” material I had, that’s 9 hours.
I put the upside down metal ring (which provided a standoff for the lexan) on a cookie sheet and put it in the oven. I then selected 250 degrees F and set the timer for 9 hours.
Then, I raised the temperature to 375-400F. The material will soften enough to allow it to be shaped. I took a ¾” ratchet drive and used the side of the head to form the indentation I wanted (while in the oven). Then I took it immediately out of the oven and let it cool. That took only 5-10 minutes. Note: if the indentation you make isn’t deep enough, you can reheat the piece to 375-400 again if it’s within a short time (i.e. 10-ish minutes)
I found the baking had tightened the screw holes a little so used a screwdriver to remove the screws and sized the holes with a drill bit so getting the screws in and out was easy.
Using a cork sheet and the existing cork gaskets, I made new gaskets, reassembled, and filled the tanks. Voila! No leakage and a crystal clear sight glass.
Josef
