Tonight I was able to countersink the rivet holes in the Z brackets for the nut plates. That's 36 holes in 6 brackets, with 6 holes per bracket. The first problem to overcome is a very familiar one. The Z bracket flanges are only about 1/2 inch wide, so they don't provide much of a base for the microstop countersink tool flange to rest on. This is not too much of a problem with the inner holes, but the two most outer rivet holes on both ends of each bracket flange are so close to the edge that it is difficult to ensure that the tool is resting flatly on the flange. So in order to provide a better base for the tool, I came up with this solution:
By butting the ends of two flanges together and clamping them on the drill board, with a gap in the middle to allow the countersink pilot to clear, I was able to rest the tool on both flanges to countersink all of the end holes in each bracket.
Here is one flange completely countersunk:
Next is a test of the rivet depth. I should also mention that before I started countersinking the bracket flanges, I tested the depth on piece of scrap aluminum. Here is a pic of a rivet sitting in one of the holes. It sits just a little deeper than flush, but a layer of primer on each bracket should allow all of these rivets to set flush when they are driven.
It is also pretty easy to see the slightly offset line of the flange on which the nut plates are to be installed. This was done intentionally to try to allow a bit more room for a pop rivet puller tool to be able to apply pop rivets into the rear baffle of the fuel tank when the time comes for that. All of he nut plate holes are about 1/16th of an inch closer to the center (vertical) Z bracket flange where the bolts will secure the tank to the wing spar. This should mean that the holes in the other flange for the pop rivets should be about 1/16th of an inch farther away from the same vertical flange, giving a little more room for the tool.
The last thing I did tonight was to match drill the remaining rear baffle plate - to - rear tank rib flanges, since the rear rib flanges had not been drilled to final size yet.All those holes were match drilled to #30 as shown by the copper colored clecoes.
There are tons more holes to match drill in the fuel tank, but I would really like to get the LE mod completed soon. I'll be giving my tech counselor a call for another visit, but the holiday is fast approaching, so I imagine I may be doing more fuel tank prep work for the time being. No worries - plenty of work to do all around in the airplane factory.
I'll need to prime the Z brackets, but I need to mask off the flange that gets riveted to the baffle plate, because I will need to apply proseal to that flange since the rivet holes are a potential area for fuel to leak out. You cannot apply any primer to any areas that may require proseal or that may come into contact with fuel, as this could contaminate the fuel or cause blockages if pieces of proseal or primer start breaking off and end up blocking the fuel pick up line. And that's not good. I think I will try to plan a primer session for the Z brackets and the top main wing skins, and will use the epoxy primer for this job as it is a bit more durable, and I think also a little more fuel resistant, than the NAPA 7220 self etching primer.
After the Z brackets are primed I will install the nut plates. Sure hope the weather stays warm enough for me to do the the priming in the next several days.
KPR.
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