Filler Neck Hole

[PaulSA]

On the Bravo Quickbuilds the bottom skin is unfinished so you have to find a way of centering the filler to make the cut in the top skin. Here's what I did:

Place the tank in the bay and mark where it is likely to sit. Measure everything up and work out where you think the center of the filler hole is going to be. Drill a 1/8" hole in the top skin. Now, you HAVE to be within 1" of the right place....... right?

Cover the tank filler hole with some tape and mark a cross in the center. Put the tank back in position and drill back through the skin and the tape on the tank. Take the tank out and you should now be able to see exactly how much and in which direction you need to adjust the center of the hole. Then you can make your 2" cut in whichever way suits you. I made a series of 1/8" holes around the perimeter and finished off with a vixen file.

My first drill was about 3/32" out and the final cut was spot on. Now only 3 more tanks to go......

[svyolo]

I think it is a bit more complicated, and less precise than that. Bad combination. There are a lot of variables. In a perfect world, with perfectly formed tanks straps, when you tension the straps, the for and aft force will be equal. So you have to calculate it which isn't that hard. But with small variations in strap length, it will change, and will change with time as the straps elongate or "relax" with tension.

Fore and aft distance, the tank should be close to center between the main and aft spar. I believe a bit skewed towards the aft spar. I swagged it which was good enough. The second tank I used 10-15% closer to the aft spar the the front. It worked out good.

[PaulSA]

I get that. My final plan is to enlarge the hole slightly and then fit a ring with a neoprene sleeve - so I'll have some wriggle room once I see where things settle down with the straps tightened.

[Mark Goldberg]

Builders do this differently. I like to either put a pin centered in a wood plug in the filler neck to mark the center. Or lipstick on the filler neck.

In a general way - place the tanks outboard and to the front. The aft side of the tank is angled to make access to the tensioners a little easier. And the inboard side has all the fittings. Mark

[svyolo]

Mark;
Marking the spot isn't that difficult. Putting the mark in the right place is the problem. You can choose the side to side location, but the for and aft location is chosen for you by strap tension. If you simply put the tank in, mark a spot and make the whole, you could easily be 1/2# off. I will take some measurements today.
My first tank was just slightly off, maybe 1/8" The second one was spot on.

[PaulSA]

The pin/lipstick thing doesn't really work with the top skin riveted. You are likely to scratch the skin or just spread lipstick all over the place as you are positioning the tank.

[JBC]

The wood plug with a center pin worked perfectly for me (and I can screw anything up). The center pin was sharpened and a light tap on the skin after the tank was in place left a dimple to mark the center of the 2" hole. I still have the plug; if anyone wants it send me a mailing address. Pass it on when you are finished.

John Ciolino
Patrol

[rv8bldr]

I wrote an article for the BearTracks last year (Q2 2017)about how I did it. If you have access, Jared has put it here:

https://bearhawk.tips/3030

[Bcone1381]

After the fuel tank installed in its final position, the filler neck penetrates the skin and so it sits higher when compared to the position of the tank when the filler neck hole is marked.

Please explain what are the issues that make this job difficult. What are the common errors people make? Will the tank migrate as the straps are tightened and the tank rises up? Is it difficult to get the tank positioned repeatably in exactly the same place?

It seems like its more than just getting an accurate location of the filler neck hole prior to drilling.

[svyolo]

Brooks;
You get to choose where the tank sits right to left. I put it so the outboard edge is just about flush with the tank bay skin. You want to maximize the space on the inboard edge for plumbing. Fore and aft, I don't believe you get to choose the location. The tank strap tension is going to choose the location for you. It is a tension only system. When all tensions are equal, the tank stops moving. So the challenge is to guess where the tank is going to want to sit.

I have B model. My tank bay is 8.5" deep at the front, and 7.5 at the rear. The A model might be different as it has a different airfoil. You can calculate where it will sit if it were a perfect world. But with variations in the tanks, welds, and how you make the straps, it is not possible to get it exact.

So on the B model, I had a total of 3 inches of space fore and aft of the tank to the spars. I figured the tank would sit just a little (10-15%) aft of centered in the bay. Maybe 1.6 inches of space from the front spar, and 1.4 aft. I believe it came out OK. I would not recommend marking the holes for the plumbing until the straps are tightened. I did on the first wing and they were off by quite a bit.

I would try to make the adjusters be as close to the bottom (you are working from the bottom) of the tank as possible. This makes getting to the screws easier.

I also use a strap around the tank as "handle" to pick the tank up. It is very loose, just a loop. I use my other hand to control one end. That makes it much easier to move the tanks in and out of the tank bay, as you will do it many, many times.

If anybody else wants to comment on this, please do. It was the best I could come up with. Variations in the tanks and straps will definitely affect this.

[Rollie]

The lipstick thing worked fine for me. Even though it seems like once the straps are made the tank position will be fixed, there is actually a bit of adjustment possible. I'm not sure how many times I had my tanks in and out after the holes were cut and the straps were in, but they lined up differently each time if I wasn't careful. left to right was easy, it was the front to back (the way you wouldn't think they could move) that I had to be careful. It could change based on the order in which I tightened the straps and I could also put a bit of pressure on the tank to hold it where I wanted it while I tightened the straps. I'm not talking a lot of movement, just enough for the filler neck to not be centered in the hole.

My point is that while you want to be as precise as possible, there is a bit of wiggle room, even fore and aft when mounting the tanks, so don't get too bogged down on trying to get it just perfect.

[svyolo]

I used the lipstick as well, but either technique would work. I put pieces of soft foam around the filler neck, just thicker than the filler neck is tall, and on the other corners of the tank. I put the tank where I wanted it, and thought it would come to rest finally. Then just pushed down to make the mark.

I drew around with a magic marker around a jar of the right size. I drill a bunch of holes, used the dremel between the holes. Then I used the biggest round grinding stone I could fit in the hole, I think it was 1 3/4" around. This made it extremely easy to "grind to a line". Nice and round, although you may have to adjust it after making the tank straps. The first wing I did have to adjust, second one I think is OK where it is.

I did find some interesting abrasive hole saws that would have worked, but 50 bucks a pop seemed high for 2 holes.

[Rollie]

Once I had it marked I just used a harbor freight hole saw. It worked fine, no more to de-burr than any other drilled hole.

[svyolo]

I was afraid to try a normal hole saw. Thin aluminum, not laying flat, kind of scared me off.

[PaulSA]

Well, that worked out OK ..................

[Chewie]

Has anyone tried installing the tank first and then locating the top skin to it (before drilling it to the ribs), rather than the other way around? Then you could cut that hole ahead of time in the approx location. But then I imagine locating the leading edge bend and stiffeners would be the next challenge.