Fuel Tank Prep Begins - Lots of Research, Reading, and Marking and Drilling the Z Brackets

I finished drilling the access hole for the right wing tie down ring through the right LE skin following the same procedure I outlined in detail for the left side.

Then, since I needed to wait for more parts to arrive from Vans before I could continue the work on the LE mod, I decided to start working on the Fuel Tanks. Other than reviewing the instructions in the manual from Vans, I pretty much ignored just about everything I have read about the fuel tank prep because, as many, many builders before me have already attested to, their instructions for how to do this well really suck. I have Dan Checkoway's CD and have reviewed the fuel tank section of his posts several times. I have also found several build sites with relatively good write ups and pictures, but alas, there are still little details about certain things that go missing. So I will once again try to fulfill my roll to assist other builders by providing those additional pics and details where warranted. We will start with the marking and drilling of the center bolt holes in the tank attach brackets otherwise known as Z-brackets.

They are called Z brackets because that is exactly what they are. When I started working with them I was a bit shocked at just how small and seemingly thin and flimsy they appeared to be. They are made from 1/16th inch thick extruded aluminum, so they are very thin. There are a total of 14 of them for the RV-8, so that's 7 per tank. They are shaped like the letter Z so that the flanges on either side run in opposite directions from each other.

The tank is removable for servicing, and it attaches to the wing in a number of different ways:
1. Skins attach to the main wing spar using a large number of #8 screws, all of which fit into all those nut plates that were riveted to the wing spar a long time ago.
2. The inboard leading edge of the tank attaches to a bracket that comes out of the fuselage.
3. One end of the Z bracket is bolted to the main wing spar web, while the other end is pop riveted to the baffle plate, or the rear plate of the fuel tank.

The bolt holes for the Z-brackets in the main wing spar web are pre-drilled (thank god!), but not so for each Z bracket. The challenge is to get the Z bracket bolt holes drilled in exactly the right spot so that when the tank is mounted to the LE of the wing it forms a gapless joint with the LE skins and the main wing skins. Because of their odd shape, and the fact that they are mounted between the fuel tank rear baffle and the main wing spar web, it is difficult to figure out exactly how to drill the bolt holes in each Z bracket and still end up with the skins meeting up close to each other as desired. The method for this is what Checkoway and many others have perfected and documented, and it seems to work well, so I will follow that lead.

Step 1
Measure and mark drill lines for each bolt hole.
Study the plans and make sure that you orient each bracket properly. It looks like the inboard and outboard-most brackets are oriented differently from the others. Of the 7 brackets for each tank, all but the most inboard bracket will be bolted from the back(rear) side of the main wing spar web, into nut plates mounted on one flange of the Z bracket. The inboard bracket is accessible from the front of the wing from the gap between the fuselage and the wing that is covered by the wing fairing.

So drawing the correct reference lines and marking the precise bolt hole locations on the Z bracket flanges becomes the first challenge. When you draw the drill lines for the Z brackets, do NOT mark the true center line of the flange as Vans instructs you to do. Instead you need to offset these lines so that the ones on the 6 outer brackets are about 1/16th of an inch CLOSER to the bracket web, and the holes for the inboard bracket are about 1/16th of an inch farther AWAY from the web of the bracket. The reasons you do this are:

1. Inboard bracket - allows more clearance for the thickness of a 3/8 inch socket head to be placed over the bolt head. I ran a couple of tests using two different tools to verify this. I took an AN3-4A bolt, turned it upside down, and slipped both a socket head AND a double-sided wrench over the bolt to verify how far out from the Z bracket web the bolt would need to be to allow for the wrench/socket head clearance.

One of the above pics shows a line that I had already measured and drawn, and as you can see the bolt appears to go through that line as expected. So, lets cover some measurements, since I cannot find them listed on any builder sites or in Vans instructions. Remember that your mileage may vary, so be sure to measure your own brackets and arrive at your own measurements, and use these as a guide:

a. The length of each bracket is about 4.5 inches, making the perpendicular center point of each flange at 2.25 inches. That was the easy part.
b. Now for the longitudinal axis of the the flange. This can be tricky, so here how I did it:
- The underside (bottom) of each bracket flange from end to end includes the thickness of the center web of the bracket, which is about 1/16 of an inch thick. The width on this side was about 20/32nds of an inch, making the true center on this side to be 10/32nds of an inch.
- The side of the flange where the bracket web is located is of course about 1/16th of an inch less than the former measurement, or about 18/32nds of an inch, so the true center on this side is about 9/32nds of an inch. Both of these measurements are shown in the following pics. Disregard the poor drawing of the bolt head. I was trying to see how far away the bolt head would be from the web of the bracket of the flange of the bolt was at the farthest outer edge of the flange of the bracket. This ended up not meaning a thing, so disregard.

The first pic is the top side, or the measurement of the flange area up to the edge of the web of the bracket (18/32). This is the true amount of flange that you have to work with where either the bolt head or a nut plate must reside, and a bolt head must clear the center web of the bracket so that it can turn freely without hitting the web. The second pic is measuring from the bottom side of the flange tht includes the thickness of the center web (20/32).

What makes this even more difficult is that there is a radius of each flange, both on the inside edge and the outside edge, so you have to do your best to try to get the "real" measurement of the flange width. So, the shorter side flange (correct) centerline would appear to be at 9/32nds of an inch, and moving it out an additional 1/16 of an inch would make it 11/32nds. After double checking my measurements, I think I went with 10/32nds of an inch, or about 1/32 inch LESS, as my drill line for the short side of the flange, because the center of the bolt head with the tool around it seemed to be closer to this measurement. This would make the measurement on the other side of the flange 12/32nds instead of 11/32nds. So this was the offset line measurement for the top and bottom of the INBOARD bracket only, NOT for any of the other remaining brackets, which require a hole CLOSER to the web, not farther away from it. Hopefully that makes sense.

Looks can be deceiving, so double check all measurements before drilling to make sure they are correct. The hole for the majority of the brackets seems way to close to the web of the bracket when you stare at it long enough. but when you realize that the only a nut plate is being attached to this flange you realize that you only need enough clearance for the bolt hole and the width of the nut plate on the short side, and you don't need the additional clearance for the bolt head because it resides on the rear side of the main wing spar web and never comes into direct contact with the Z bracket.

With the measurements for the inboard Z bracket bolt holes determined, I marked and drew the lines. 

Now to continue this little math lesson, we move to the remaining 6 brackets, each of which require a bolt hole that is closer to the web. This allows a little more room for the rivet puller to be able to pull the pop rivets on the other flange of the bracket that attaches to the rear tank baffle/rear flange of each tank rib etc. Here is a pic of the line drawn for these holes.

The measurement for these holes will be 8/32nds on the bottom of the flange, and 6/32nds on the other side as shown above, again to account for the thickness of the web. When you mark that line it looks way too close to the bracket web, but in reality, it works out fine because this is just about the perfect width for the required nut plate installed on these flanges, as shown in the slightly blurry pic below:

Once the drill lines were marked and I found the adjusted center point on each flange for the center bolt hole, I marked the point with a center punch, and then set up my drill press. I started with the left wing and right wing inboard Z brackets, since they both have the same layout and measurements. Then I reset the drill press for the remaining 12 center holes in the other 12 Z brackets for both fuel tanks and drilled them with a #13 drill bit followed by a number 12 reamer. I'll post more pics of all that tomorrow.

There is a lot more detail about this process to share....... For now, I just wanted to edit this post to include the correct measurements that I used. Again, YMMV so do your own analysis and take your own measurements. Hopefully mine will work out as planned.