Building new fuel tank

I need to build a new fuel tank for my -4. Everything else on the wing is done, so all the nutplates for mounting the tank are there so I can?t match drill them. Figuring out how to get holes in the skin to line up with all those holes is giving me nightmares. A hole finder won?t work on a highly curved surface. Any ideas out there?

You could match drill it using the old tank skins, this would require disassembling and cutting up your old tanks.

If the old tank is still attached, I'd draw pairs of lines that intersect over the centers of each existing hole. Then drill at the intersection.

Kyle Boatright said:

If the old tank is still attached, I'd draw pairs of lines that intersect over the centers of each existing hole Then drill at the intersection.

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That's how I did it on my RV-4, more or less...

1. Get a compass (for drawing arcs) that has a screw adjustment. Set it for maybe 1.5", don't remember;
2. Put masking tape on the wing just aft of the tank;
3. Put all the screws in that hold the tank in place. Doesn't matter if the tank is there or not. Adjust the screws so that the indentation in the screw is at the same height as the skin of the tank would be;
4. At each screw, with the pointy end of the compass centered on the screw, draw a part of an arc on the tape, one inboard, one outboard;
5. Put tape on the new tank covering where the screw holes would be;
6. With the new tank held in place, draw intersecting arcs on the new tape. It doesn't matter where on the old arcs you put the pointy end of the compass. This will locate the new screw holes accurately. For best accuracy, the arcs on the new tank should be close to perpendicular.

For the screw holes not along the spar, do the same thing.

Easy enough to try this with a piece of scrap and maybe four adjacent holes.

It's really not that big a deal and works really well. I didn't say that it was a lot of fun...

Thanks Ed. Never heard of that method. I have tried the intersecting lines method but struggled to get it bang on. I will try it.

i use heavy clear tape. the thicker the better. use two pieced stuck together so that it is not sticky on either side. with the old tank on the plane with the screws removed, tape that on the wing (not the tank). have the edge of the tape just past the screw holes. you can do it with one piece of tape the whole length of the tank or pieces for every couple of screws. use a #40 drill bit to punch a hole in the tape in the center of the screw, the#40 just about perfect as it fits just inside of the nutplate with little slop. slide the new tank in place and use a sharpie to mark the holes on the new tank. I drill one hole at a time starting in the middle. take the tank off drill it, put the tank on, put a temp screw in and go on to then next hole.

bob burns

RV-4 N82RB

I remember using clear Lexan plastic type sheets of the same thickness as the skin, using straps to maintain those in place on the ribs and spars, drilling the holes thru le sheet and then mark the aluminum ones.

Intersecting lines works very good for most cases. For the spar attach (single plane) then a sacrificial skin can be made for the whole row of spar fasteners in one piece. Lay a strap of aluminum 4" x (length of tank) on the skin with the fwd edge just past the hole edge of the fastener. The rear edge is taped down with aluminum speed tape, forming a hinge. Locate the fastener holes in this new "flap", and fold it out of the way. Install the new tank and fold the flap down on the virgin tank skin. Drill the skins using the flap as a template.

Some really great ideas here. Thanks everyone!