Drilling and Deburring the Remaining Fuel Tank Rivet and Screw holes

Spent almost a whole day match drilling and deburring the remaining rivet holes for the left fuel tank. It never seems like much work until you start doing it, and you suddenly realize just how many holes there are.

I also decided to go ahead drill the #19 holes for the all the remaining screw holes along the bottom of the fuel tank skin that attach it to both sides of the main wing spar flanges. This has come up in several build logs over the years as something that many are not sure how to tackle. Some will step drill these holes until they get to the final #19 bit, and others will just drill using the #19 bit without any upsizing the hole whatsoever. I chose to do the latter. The reason why some decide to step up the hole a little at a time is because if you follow Vans instructions, they would have you believe that you just run a drill bit through the skin without any kind of backing behind it.

Problem is - anyone who has been building for a while, which would be most of us if you are at this stage of the build, would know from experience that drilling aluminum sheet without anything backing it up is prone to leave some very nasty burs on the back side of the hole. Some of them can be a pain to remove. So, as a rule, it is always best to have something behind the metal when you drill to help prevent this from happening, or at least keep the burs as small as possible. SO was my solution to this problem. I found a piece of wood lying around the garage that was just ling enough and wide enough to fit behind the fuel tank skin just above the rear baffle.

I was able to match drill all the screw holes, and almost all of the rivet holes. The only rivet holes I have not drilled yet are for the stiffeners on the bottom of the fuel tank skin. I have not fabricated the stiffeners yet (on my list of to-do's for this weekend). So I don't want to drill those holes in the skin until I have the stiffeners clecoed in place per the instructions. However, I did manage to get all the rib flange to skin rivet holes drilled.

So next steps at this point are to fab the tank skin stiffeners and cleco and match drill them to the skin. Then I can start deburring the rib holes and the stiffener holes after I disassemble the ribs. I was able to debur and disassemble the rear baffle plate, and this allowed me to also resolve another pesky problem I encountered after going through all that "special method" to position and drill the rear flanges of the inboard and outboard ribs and baffle plate holes to the Z brackets on the wing spar.

When I reassembled the tank assembly and tried to cleco all the rib flange holes back to the skin, the forward most hole on the top and bottom rib flanges of the outboard fuel tank rib would not align with the holes in the skin. With the tank completely closed up it was impossible to get those small tab holes in the rib to line up with the skin holes for some reason. Usually this is a sign that more rib straightening and fluting may be necessary, but I was pretty sure 3+ years ago that all the tank ribs had been straightened adequately.Then again, with these leading edge, stressed skins, anything is possible. I did not want to start match drilling any holes in this rib until that forward-most set of holes was lined up and clecoed in place again.

Anyway, to solve this little problem, before I removed the rear baffle plate, I wanted to see if I could realign the holes in those tabs in the rib flanges without removing the rib, since I still needed it to remain in place so I could match drill all the rib flange holes.  I decided to leave the baffle plate in place, but to remove the clecoes that were holding it in place in that outboard bay along the flanges of the baffle plate, and then I removed all the clecoes from the rib flanges for that outboard rib. This meant that the tank was still closed up, but only loosely "closed" in that bay. So I could not just easily reach down from the rear and move the front of the rib around, but things were now lose enough that could manipulate the rib a bit to try to line up those forward holes with the skin.

How did I do this? Well, I thought my solution was quite ingenious. I took the largest and longest allen wrench that I own - about 3/8s of an inch as I recall, and 6 inches long. Then I realized that the rib in question was right next to the hole in the skin for the fuel tank fill port. So I was able to take that allen wrench and slip it inside the fuel port hole in the skin, and turn it so that the small "L" portion of the wrench was parallel to the rib web, and positioned it as low toward the front the rib as possible, and lightly tapped on the rib web. I'll be darned if that did not align the holes right up. SO I immediately clecoed all the rib flange holes, starting with those and working my way toward the rear.

Then I was able to match drill all the rib flange holes in that rib - FINALLY! After that the rear baffle plate was able to come off for deburring. So ends another fun-filled day of airplane building.

KPR.