Picture of Vacuum Formed Fuel Cap Cover
Aviation and automotive vented fuel caps are designed to keep water out of fuel tanks but they are sometimes not entirely effective.  Rainy and windy conditions can lead to water leakage through the vent of an otherwise functioning fuel cap.  It is especially important to keep aviation fuel free of water for flight safety.  This guide describes a method to make a protective cover for eliminating leaks in vented fuel caps.  This method should be universally applicable to essentially any fuel cap shape.  I made it at TechShop in San Jose, California (www.techshop.ws).
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Step 1: Materials

1/16” HIPS sheet (available from Professional Plastics in San Jose) cut to a 13 ¼” x 13 ¼” square

Vented fuel cap

Step 2: Equipment

Picture of Equipment
Formech vacuum former (available for use at TechShop in San Jose, CA)

Dremel/rotary tool with diamond cutting disc

Step 3:

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Power on the vacuum former by rotating the red knob clockwise.  On the control touch panel, select NEW/EDIT.  Open the HEATING ZONES menu and set the heater power to 70% on zones 1 and 2 and leave the remaining zones at 0%.  Hit the back arrow to return to the edit menu.  Enter the HEATING TIME menu and set the time for 80 seconds (in the time panel hit 8, 0, then enter).  Hit the back arrow again.  Save the protocol under any number (default is 12) by selecting SAVE and then selecting any number.  Wait ~20 minutes for the system to warm up.

Step 4:

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While waiting for the system to warm up, load the form you want to use to define the shape of your part in the vacuum former.  In this case, we formed the sheet over a vented fuel cap.  To load the cap, push the lever on the right side of the instrument to raise the grid to the top position.  Place the fuel cap in the center of the grid.  Now pull the lever back up to lower the grid into the instrument.