Locking Fuel Cap Installation

My thinking going into mounting the fuel caps was that if I was particularly careful when cutting the outer hole and bonding the fuel cap assembly to begin with, I might not have to do much, if any, sanding or grinding to get the hole around the fuel cap to look good, and to make the cap flush with the outer wing skin. I was reasonably successful in achieving this.

I found that the very best results I was able to get was with a plain old wood hole saw. You DO NOT need a special hole saw with a grinding edge designed specifically for harder materials such as fiberglass. Those hole saws typically cost from about $45 to $125. The ones I used were about $12 from Home Depot, and I was very pleased with the results. An important point is that the hole saw should have a center pilot drill bit. Most of the wood hole saws have this.

The construction manual calls for a 2 1/2 inch hole saw for the hole in the outer skin, and a 4 1/2 inch hole in the inner wing skin. The fuel cap depicted in the manual looked different than the locking fuel cap option that I had ordered with the kit. I measured the dimensions of the fuel cap flange, and the measurements were also different. According to my measurements, the outer wing skin hole needed a 2 3/8 inch hole saw, and the inner skin needed a 3 3/4 inch to 4 inch hole saw (the size of the inner skin hole is not as critical). A 2 3/8 inch outside hole fit very nicely around the hole in the fuel cap assembly for the removable fuel cap. The photo here shows the smaller outer hole, and the larger inner hole. The Nomex core has also been removed where the fuel cap flange will bond to the outer wing skin.

The advantage of using a tighter hole is that once I bonded the fuel cap assembly in place with Hysol, and then cleaned the extra squeezed out Hysol before it cured, there wouldn't be any sanding or grinding needed to finish the fuel cap assembly on the outer skin.

There were several things that I did that made the job easier. The most significant items include:

To make sure that the fuel cap assembly did not shift position on me once I Hysol'ed it into place, but before it cured, I drilled eight 1/8" holes around the flange. Then I drilled corresponding holes through the outer wing skin. This allowed me to use clecos to hold the assembly in place. To make sure that the clecos did not get permanently bonded to my wing, I removed them every couple of hours, one by one replacing each with a fresh cleco. Each removed cleco was then soaked in a acetone bath for a while. Jim Nordin suggested that I use the clecos before I apply Hysol, but then use regular 1/8" diameter pop rivets. Once the Hysol has cured, I would drill out the rivet heads, and then remove the rivets. Either way, this made sure that there were no gaps between the wing skin and the fuel cap assembly flange. It worked very nicely. I also used the lead weight for the rudder that came with the kit to place on the fuel cap assembly to make sure that the wing skin would not have an indention like it tends to have when you remove the Nomex core and the inner prepreg skin. Once the Hysol had cured, I followed the instructions in manual. After I had put the BID layers in place on the backside of the fuel cap flanges, I filled the 1/8" holes with epoxy/micro.
When I abraded the fuel cap assembly for bonding it in place, I sanded the outer surface of the flange back enough to allow the fuel cap to come closer to flush with the outer wing skin. I also ground the very slight curve to the flange to allow it to conform a little better to the curvature of the outer wing skin. Both of these steps really helped raise the fuel cape ever so slightly.

There were a couple of lessons I learned about cutting nice round holes in areas that will show in the completed plane with the type of hole saws I was using.

When possible, cut through each side of the prepreg material by cutting through each side from the outside. Don't cut all the way through, cutting the second skin from the inside. Resume cutting the second side of the hole from the opposite side. Enter cuts are always nicer than exit cuts. My proposed method gives both sides an enter cut.
As soon as the hole saw cuts through the first prepreg skin, stop cutting. The Nomex core helps hold the material inside the hole in place while it is being cut from both sides. Continuing through the Nomex core when the second side is being cut will typically result in an out of round hole on one or both sides when the Nomex suddenly gives way.

The picture below shows the completed fuel cap installation. The fuel cap flange is bonded into place, and the removable cap is the black and silver device on the right. The fuel cap pictured is the locking fuel cap option that I ordered with my kit.

Fuel Cap Flange Mounted in Place (Left)
Locking Fuel Cap (Right)

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