Finish Up Work on Fuel Tanks

Time: 6.5 hrs

Starting to finish up the fuel tanks since they have been tested for leaks and seem to be OK. I’d like to do a good leak rate test on them, but the gauge I purchased (Dwyer DWGW-04a0 isn’t working as I had hoped. I may just build a manometer as Pascal L. has suggested and do the leak test that way. In any case I’m not able to find any leaks using soapy water and saw no noticeable movement of the needle on my gauge while testing for 48 hours, though my gauge isn’t super accurate so it may not reflect any small leak.

In any case I felt it OK to proceed with priming the tanks so I got that done today. I’m waiting on some hard plastic grommets to use on the fuel vents so I can’t completely finish them until those arrive. I also need to rivet in the rib and short channel pieces and install the rivnuts.

Primer came out OK. I didn’t prime the rib because it’s already alodined so should be OK to leave unprimed. Once this dries up I’ll install the rib and channel pieces, plus the rivnuts.

I’m hoping to get the tanks installed by mid next week.

UPDATE 5/5/2020: Happy Cinco de Mayo!

I spent around 6 hours today getting the fuel tanks ready to mount on the wings. I installed the fuel vents, the remaining rib and channel pieces, rivnuts and also filled the rivets. The remaining rib didn’t fit so well, I kind of figured it would be an issue when I did the test fit. I ended up using my fluting pliers to get the holes to better line up, but still took some time to install.

I made a manometer for testing for leaks in the fuel tanks. It’s pretty accurate, but it does fluctuate with temperature quite a bit. Since the tanks are in the house and the temperature doesn’t fluctuate that much I just recorded various readings along with the temperature eons the gauge itself. After about 2 days of checking the markings against the current temperatures and finding that they were no different than the markings I had previously made then I could assume that the tank wasn’t leaking. Not very scientific but I think it’s better than the balloon test. I did also check all the rivets and fittings with soapy water.

Had to do some fluting on the rib to get the holes to line up.

For the fuel vents I ended up using a rigid plastic bushing rather that a rubber grommet. The grommets seem to dry out and fall apart so I thought the plastic might hold up better. I ended up upping the hole to 3/8″ and used some black RTV to seal it up and make sure the bushing didn’t move.

The fuel tanks are ready too go. Hopefully tomorrow I can bolt them in and then rivet them to the wings.

Fuel Vent/Overflow Bending

Time: 1hr

Just did a little work on the plane today by bending the fuel vent tubes. The right tank can’t be tested until Monday so getting these out of the way will be good so that I can get the tanks permanently mounted soon.

My bender kind of sucks, but at least the tubing didn’t kink and the bends cane out OK with a bit of effort. I tried to keep the bends such that there was always a bit of a downward flow, but it probably doesn’t matter since there really won’t be any fuel draining out of these. The first bend I kept to a little less than 90, then a bit more than a 90… maybe 100-110 and then a 90 again. All the bends are 9/16” radius. I may make a few minor adjustments to it, but seems to work ok. The other side is bent as well, but I’ll have to wait until the tank is off to fine tune it. Also I’m going to look for solid plastic grommets rather that the rubber ones for the hole where the end passes through and will need to cut a few millimeters off the end of the tube before installing.

Looks ok from this side. I kept the tubing as close as possible to the tank since the rib on the wing is pretty close to the one on the tank.

Also I received my digital pressure gauge yesterday, but I needed to get a fitting so I can attach it to the -4AN hose I’m using. Hopefully that will be here tomorrow or Monday and I can run a test on this tank.

Fuel Tanks Leak Test

Time: 2.0 hrs

Today I started the leak test on the left fuel tank. I let the tank dry for a week on the wing. The manual says 3 days, but since I used the Class B-4 sealant which takes longer to dry I gave it more time. I couldn’t find anywhere in the Sling 4 manual where the test pressure was. I remember someone had said the Sling 2 manual said 1 bar which is wrong and would be way too much pressure. The TSI manual says .253 bar which is about 3.5 PSI which seemed more reasonable, but checking with other builders the value seems to be 1 PSI.

For the test connections I just purchased some short -4AN ready made hoses off Amazon (for around $10) I connect those to the tank fittings and then my various other fittings to the other end of the hose. On the input hook up (connected to the fuel vent/overflow fitting) I had a the fitting from a -4AN hose tester kit which provides a screw type interface (sorry don’t know what it’s called) to the bicycle pump. On the fuel return fitting I used the pressure gauge from my hand pressure pump which has a smaller PSI scale so a little more accurate (at least in the sense that it’s easier to read small fluctuations in change) and fitted that with a -4AN to 1/4 NPT fitting. Using the bicycle pump was great because it’s very easy to put too much pressure in the tank and deform the skin, the bicycle pump allows you to add purchase in small amounts and watch the tank and gauge to make sure the tank isn’t getting over pressurized.

On the initial pressure test I pumped the tank up to 1 PSI and noted the needle dropping slightly on the gauge. The leak was in the -4AN to hose connection at the gauge. I tightened that up and then reestablished the 1 PSI pressure and noted no real drop while I went on to check the rivets for leaks, but when I move the gauge to read it I could swear the pressure dropped a bit and then stabilized. I continued to check all the fittings at the tank and all the seams and rivets and found no leaks. I was still a bit perplexed as to why the pressure would have dropped and then stopped, but noted it again when I moved the gauge around. I check the hose at the other end (the tank side) and sure enough there was a small leak that got slightly work when moving the hose. So after tightening that down more the needle seems to be stable. I added a bit more pressure and I’ll leave it like this for the day and hopefully won’t see any change.

Inboard side of the tank. I connected the gauge into the fuel return using a short pre-made -4AN hose. The fuel supply fitting is capped with a =6AN cap. The viton gasket seems to be holding up with no leaks and the fuel drain that I used the thread seal (with no fiber washer) seems good as well.

On the outboard side of the tank I have a short pre-made -4AN hose with a -4AN fitting which was part of a hose test kit for testing -4AN hoses. I don’t know what the threaded fitting that the bicycle pump fits onto is called, but it’s the same as what you’d find on a bike inner tube and that screws into a -4AN fitting that mates with the hose.

I put some tape of the fuel caps. I don’t think they will leak, but I saw other builders do this so I figured it can’t hurt.

This was the illusive leak. I though for a minute that it was the threads on nut on the AN fitting, but then remembered I put the nut on the inside of the tank so that couldn’t be leaking. This was fixed by just tightening the hose onto the tank fitting.

Well so far so good. It’s been about an hour and no change in the needle, but will see later today. Hopefully this will pass and I will get the sealant for the other tank in the next few days.

I guess I look at bending the vent pipe and get those ready to go.

UPDATE 4/20/2020: After 24 hours I checked the pressure gauge and the needle didn’t seem to move from the mark I had made. I also tested for any leaks again with soapy water. I think the tank is good, but since I need to wait on the right tank to dry and I already ordered a digital pressure gauge, I’ll wait until that arrives and check this tank again just to be sure.

UPDATE 5/3/2020: The right tank has also been tested and seems OK. I had purchased a more accurate digital pressure gauge that was capable of 0.1mbar and .002 PSI however the gauge has proved to be nit very usable. The gauge does certainly have the accuracy, but the it seems to read erratically so that makes it completely useless in measuring any kind of leak rate of the fuel tank. I’m checking with the vendor (Dweyer Instruments) to see if they have any recommendations.


Fuel Tank Closure

Time:11.5 hrs

Yesterday I got both the fuel tanks ready to be closed up. I did a quick test fit of the left fuel tank to verify that the floating nut plates worked, which they did. In fact it makes it so easy to put the bolts in I was almost thinking of doing it for all the mounts. I also came up with a way to put in the 4 or 5 rivets on the #10 (outer most) mounting bracket. There’s very little space to get the riveter in because the mount bracket is U-shaped so you either need to buy the expensive compact riveter which fits under the upper lip of the U or use some kind of angled shim to hold the head of the rivet flat while you pull the rivet. What I did was to just filed an angle into one of my 4mm rivet gun head adapters (since I have 2 or 3 sets for the manual pullers). Then I used my Stanley pivot riveter and tried it. It seemed like it would work fine.

I also when through each tank and made sure all the sealant inside the tank was good and added some where needed. I did a final adjustment on the fuel level senders and tightened the screw that holds the float arm. I also put some sealant on the screw to keep it from moving. After all the internal stuff looked good I fit the back channel and brackets and made sure all the rivet holes were good and that rivets fit well in each hole, match drilling if needed, then debur. When everything looked good I cleaned all the pieces with MEK and wiped it dry with a clean rag.

I was planning on continuing to seal up both tanks, but then realized I probably only had enough sealant for one tank and it was getting late anyways so I held off until today to do the left tank.

Everything went fairly well with sealing up the left tank. I just ran a bead of sealant around all the mating areas on the rear channel including some extra in the corners and not forgetting to put some on each of the rib ends. Then I lowered it into the open end of the tank. What I didn’t plan on was the mess that happened when the channel hits the side of the tank skin. I tried to be careful, but there were areas that had a bit of sealant to clean up before I could put it on the wing. After clecoing that in place with only a few clecos. I added sealant to the mating areas of each mount and clecoed those onto the rear channel. The 3 center mounts use closed end rivets which I dipped in sealant before installing. I forgot that there is one extra hole that doesn’t need sealant because the bracket doesn’t”t use it, it’s just a rivet into the end of the rib through the rear channel so I ended up making even more of a mess that needed to be cleaned up. The end mount brackets don’t get any sealant at all and the rivets are just regular open end rivets so these go faster. In the interest of time and not to mess up the nice bead that formed on the brackets I chose the clean up the rivet heads on the brackets later.

With that all done I put the tank onto the wing and put in some clecos into the 4mm holes to attach it to the wing. I then installed and lightly tightened the screws that go through the spar into the mounts just so I know they lined up OK. Lastly was to rivet all the 3.2mm holes to attach the rear channel to the fuel tanks skins and seal up the fuel tank….hopefully. Since I used B-4 sealant I’ll leave this on the wing for a week (not 3 days if I used B-2). I felt a little less rushed with the B-4 sealant and frankly I don’t know how anyone gets this done in 2 hours… well I guess the sealant doesn’t completely dry in 2 hours, but still it gets a lot less workable when it approaches is tack time. In any case I’m not in a huge rush so waiting a few extra days for the sealant to dry will be fine. I used B-4 on the ribs and it was nice to not feel so rushed. I’ll need to order another 3.5 oz tube for the right tank.

UPDATE 4/20/2020: I sealed up the right tank today and got it temporarily mounted to the wing. This tank went a liiltle better than the left since I knew what to expect this time. I reduced the amount of mess when putting the back channel on by using some clecos in the bracket holes as handles so I could better position the back channel on the tank and also I put in one of the long edges first and then just tilted it down into the tank rather than put the short end on the rib and dropping it down. I also remembered not to put sealant on the bottom holes of where the brackets since those just go into the ribs and not the brackets. Now I just need to wait a week to leak test it.

Here’e what I did for the #10 mount bracket rivets. I filed the riveter 4mm adapter at an angle. The fact that the handle swivels around was a bonus because it gets a bit hard to work inside the rear channel of the tank because the fuel tank skins get in the way.

Fuel tank ready to close up.

The left tank is on… temporarily. I put some saran wrap on the spar in case any sealant seems out. All the rivets are cleaned up and I even used the little bit of left over sealant to finish installing the stainless steel rivets for the #0014 service bulletin. I need to remember to also use the stainless steel rivets in the first 14 holes on the bottom and bottom when I attach the tanks to the wings.

So now this one needs to stay on here for a week or so. Once I get the sealant for the other tank I can do that one and let it dry. Next week I’ll test this one and hopefully there won’t be any leaks. If there is I also got some Class A sealant to use to fix the leaks. The idea is that you pull a vacuum on the tank and then the more watery Class A sealant gets drawn into the hole and seals it up. I need to also bend the vent pipes and attach those and I want to spray the back and fuel level side with primer before mounting it to the wing.

Fuel Tank Mount Fixes

Time: 5.5hrs

I received all the various things I needed to implement fixes for the few issues I had when I test fit the fuel tanks a few days ago.

Issue #1 was that the inner most mounting bracket (#10) had it’s mounting holes (really the bottom hole) slightly off from the holes in the spar. TAF said that the floating AN nut plates would probably do the trick and it seems like it will. I have to do a final test fit tomorrow, but it seems like the bolts will now line up fine. The floating nut plates are MS21059-3 which I purchased from Pegasus Racing. The mounting holes for the floating plates match the fixed nut plates that were originally uses so I just needed to remove the old nut plates, open up the hole where ether AN3 bolt goes through so that it could move around a bit and then mount the new nut plates.

Issue #2 was that the factory installing bracket on the leading edge of the spar was put in the incorrect place. It needed to be moved one set of holes outward (towards the wing tip). TAF said that I really didn’t need to move the bracket and I could just use a longer AN3 bolt to secure the tank to the spar, they just need to be long enough to reach across the extra space between the incorrectly positioned mounted bracket on the spar and the one on the tank. I chose to take the difficult route and actually move the bracket over. To do this I needed to remove the solid rivets from the holes that the bracket needed to be moved to, install the bracket in the new location and then install new rivets (or AN3 bolts) in the now open holes.

I ordered some MS20470AD6-22 rivets from Military Fasteners. These are 3/16″ 2117 hard aluminum AN407 style rivets. You can identify that they are hard, not soft rivets because they have a small dent in the head. I calculated the length based on the thickness of the spar (about 1.1″) and then some overage that is used to form the shop head of the rivet. I also had to purchase a rivet squeezer with a 2″ gap, most only have a 1.25″ gap which isn’t wide enough given the rivet length and 2 fittings that need to be installed in the squeezer. Once I got the rivets and checking the amount of overage, I found that I should have actually purchased -20 (1.25″) length rivets so I ended up shortening the rivets I did purchase because they were about an 1/8″ too long. Installation wa pretty easy actually, The squeezer works well and it seems like the shop head formed well. I’ll have to use my borescope to check it tomorrow.

It was  bit of a nightmare getting the other solid rivets out though. I drilled off the factory head and tried to knock out the rivet with a center punch, like I do with pop rivets. Well this thing didn’t budge. I ended up having to very carefully drill down with a 3/16″ bit about half way and then with an 1/8″ bit a little further (not all the way through, but almost). I then used a long nail (like a punch) to knock the rivet out.

In addition to fixing the mount brackets I removed the gaskets from the fuel level senders to check if they compressed OK. It looks like they did, but I decided to use a 9mm leather punch to make the holes bigger so they would fit around the heads of the rivnuts rather then compress on top of them. I probably should have uses a 10mm punch to give a little more space, but it seems like the gaskets still fit well. I also use a very small bit of RTV to get the gasket to stick in position when I screwed in the fuel level sender mounting plate.

Finally got the solid rivets out. What a pain. The holes seem OK, they are a little oversized already so that gave a little wiggle room when using the 3/16″ bit to drill out the rivet shaft.

Yay the bracket is moved and the rivets are installed. I’ll check the shop heads on the rivets tomorrow. I can always squeeze them a little more if needed. Notice the small dimple in the head. That signifies that it’s a 2117 hard aluminum rivet, not a 1100 soft aluminum rivet.

I found this 2″ torque extension on Amazon. This and some masking tape made it much easier to remove and install the nuts and washers on AN bolts for the bracket.

Floating AN3 nut plates installed. They are a direct replacement for the fixed ones, rivet mounting holes line up exactly the same. I just had to make the large hole on the bracket that the AN bolt goes through a bit bigger so that the bolt can move around from side to side.


Argghhhh! Fuel Tank Test Fit

Time: 1.5hrs

Yesterday my kids helped me move my wings around so I could mount them with the leading edge facing up… thanks kids 🙂 This will make mounting the fuel tanks a lot easier. I’m just a bit concerned about leaving the wings like this because of the winds that we get from time to time. Let’s just hope the weather cooperates for a few more weeks and my make shift wing mounts are secure.

So now with the wings in the proper orientation I can continue with the fuel tanks. I test fit the fuel tanks in preparation for sealing them up and was hoping that all would go along and be uneventful. Well I wasn’t so lucky. Issue #1. The inner most mounting brackets (WG-BKT-010-X-A) mount holes don’t exactly line up with the holes in the spar. I measured and the spar hole are about 75mm center to center while the holes on the bracket are around 76-77mm. So it’s close, but you can’t get an AN3 bolt in there. The top hole seems to be lined up when I test fit the tank so it seems like the bottom hole is off. I wonder if they make floating AN3 anchor plates then I’d be able to open up the mount holes in the bracket and the nut would be able to move around a little. Now on to issue #2, which is a bit more of a problem. The bracket that is mounted to the spar at the factory is in the incorrect position. It’s one hole off (toward the wing root). Moving the bracket isn’t so much the issue, though it won’t be fun getting the nut and washer onto the bolt through the small hole sin the spar, but what do I do with the holes after I move the bracket? The correct fix would be to use a 3/16″ hammered rivet, but I don’t know how to rivet :-), maybe a rivet squeezer will fit. Anyways I need to wait for TAF to get back to me on the recommended fixes. It might be possible to use a stainless steel pulled rivet or an AN3 bolt. Until then I’m a bit dead in the water on the fuel tanks for now.

No workie… see how the threaded stud slants in. After measuring seems like the spar holes are about 75mm center to center while the holes on the bracket are around 76-77mm.

Oops thanks TAF guy for putting my bracket in the wrong holes… Moving it should be fun. I wish I checked this before I built the wings, but totally just slipped my mind.

At least all the rivet holes seem to line up OK so no real issues there.

Fuel Sender Adjusting and Other Misc Fuel Tank Stuff

Time: 2.5 hrs

Continuing work on the fuel tanks today. I torqued up the fuel supply and overflow/vent AN fittings and also installed the fuel drains on both tanks. Torquing the AN fittings for the pickup and overflow/vent were a bit more challenging then I expected. It’s difficult to hold the fittings and seemed to be hard to get them to fully torque. I was able to get the 4AN fitting to around 10 ft lbs (should be 11 ft lbs) and the 6AN to around 13 ft lbs (should be 16 ft lbs). I didn’t want to push it much more in fear of damaging something. The fuel drain also didn’t go exactly as planned. I use the thread lock and got them to around 30 ft lbs. The install instructions say they should be torqued to 80 ft lbs. I was worried that I may twist the drain receptacle and break the rivets holding it onto the skin. If the drain leaks I can always retorque it fairly easily, so I rather take it a bit safer and not damage something.

I also fit the fuel senders and adjusted the travel of the floats. The adjustment is made by bending small tabs on the sender to stop the travel of the float. I adjusted them just shy of touching the inside of the tank skin, taking into account that the skin bows out slightly because the rear channel isn’t installed. I also filed a small flat spot on the float arm so that the screw will hold a little better. I put some tape on the screw so I remember to put some sealant on it before I close up the back. The sealant is just use to keep the screw from coming loose.

Lastly I test fit and made some notes on how to install the mounting brackets. It’s a little confusing which way the brackets get installed so I want to make sure I have that all figured out before I actually install the rear channel and brackets.

A bit hard to see but I filed a small flat spot on the float arm where the screw holds it to try to prevent the arm from rotating and so the screw holds it a bit better. Seems to have helped a little.

Tank full float position adjusted.

Tank empty float position. The Viton gasket seems to work pretty well. I’ll have to check that it’s compressing enough over the rivet heads. If not I’ll just use my leather hole taps to cut bigger holes.

Since I’m using closed-end M4 rivnuts for the sender mounting plates I had to get some shorter 14mm M4 hex bolts (TAF supplies 16mm). Also I won’t need to use the felt washers they provided so I’ll use standard washers as well as some lock washers on the mounting bolts.

Fuel drain is in. I used permatex 56521 thread seal on it and was able to torque to about 30 ft lbs (supposed to be 80 ft lbs.) before I felt like I didn’t want to push my luck any more. If I could hold the drain receptacle with a wrench then I’d feel more comfortable applying more torque to it, but there’s no way to hold the receptacle. I’ll have to see if it leaks, if so I can try to torque it a bit more.

All the brackets fit well and I added notes on the direction to install them. Also the ones on the ends won’t need sealant under them because the rivets don’t go into the tank and normal,  non closed-end rivets can be used as well.

So next step I think will be to get my wings situated for the tanks to be installed. I want to turn them so the leading edge is pointing up so the tanks will be easier to install. Once that’s done I’ll test fit the tanks to the wings and make sure everything is OK.  I also need to bend the tubing for the tank overflow/vents. Oh and I need to see how my viton gasket is compressing. If it’s able to compress enough over the rivnut heads then I may not have to make the holes in it larger. I’ll have to remove the fuel senders to check the gasket.

Fuel Tank Fittings

Time: 6.5hrs

Yesterday the weather kind of cooperated (clear but very windy) and so I was able to install the fittings to the right fuel tank. The fittings are AN type fittings for the fuel return line, Fuel supply line and overflow/breather tube. I also installed the fuel pickup that connects the the AN6 right angle fitting for the fuel supply line as well as test fitted the fuel drain and overflow lines.

The install was pretty straight forward, the only changes I made from the manual was that I used washers on both sides of the AN4 fittings (for the fuel return and overflow) which means I have to purchase more because the AN4 fittings only came with one washer each while the AN6 came with 2. I did this because for one I thought it would give a better seal and two because I don’t like tightening the nut down onto the raw aluminum of the rib, its fine to do it that way, but I just felt like having a washer behind the nut was better. I also turned around the fuel return fitting so that the fitting nut (that is part of the fitting itself) is on the outside so that when I attach the fuel return line to it I can hold the fitting with a wrench while I tighten down the line. I wasn’t able to do this with the fuel supply fitting because it’s a 90 degree fitting and the bend goes inside the tank which leaves the nut that you tighten on the outside. I’m hoping though that because it attaches to the fuel pickup line and the it secured to the rib then that should keep it from twisting the fitting when the line is tightened down. AN6 fitting are torqued to around 20ft lbs so I think that line and clamp should be OK to withstand that. Also I used Loctite 577 on all the threads that the nuts screwed onto on the AN fittings. I won’t be using any Loctite on the tapered AN fittings since you don’t need it there.

The only oddity I noticed is that TAF supplies 2 fiber washers to be used on the fuel drain and they don’t fit over the threads of the drain. Even when you get the washer to fit onto the drain threads and screw it into the drain flange (riveted to the tank skin) it looks like the washer will just kind of bends as the drain seats with the flange. So I did a little research and found that stamped onto the face of the fuel drain it says “Curtis 2600”. Looking that up I was able to find the install instructions from Curtis. Here is a snippet of what the manual says about installing the drain.

So it appears that you don’t use any kind of washer since the drain should seat itself into the standard 1/4 NTP fitting. It does say though that some form of pipe sealant should be used. I ordered the Permatex from Aircraft Spruce, they didn’t have the exact number sealant but the one they had looks to be very similar, if not better than the 80632 sealant and it also has PTFE like the 80632 sealant does. I don’t know if this drain is supplied with all Sling 4 kits or if they changed the drain and now the washer is not needed.

Inside of the 101 rib. The filter of the fuel pickup just fits above that rivet for the fuel drain. I used a little longer closed end rivet for it.

Outside of the 101 rib

105 rib with the fuel breaker test fit to see how it will fit in, seems to clear the fuel cap ok. There is a long tube that attaches to the outside end of the 90 degree fitting that will need to be bent to go around the rear of the tank and through a hole in the bottom of the tank skin. Pascal L had some good drawing of the bends so I’ll check those out.

Next is to do the fittings on the left tank after I get my washers from Aircraft Spruce… hopefully Tuesday. I also need to get the fuel level senders attached and adjusted and bend the overflow tube to fit.

Update 4/1/2020: Yesterday I received the washers needed for the AN4 fittings. Apparently Aircraft Spruce is shipping on time even with the pandemic going on. I guess you can’t buy toilet paper, but airplane parts are easy to get… thanks God :-). So today I installed all the fittings on the left tank. Nothing exciting to report on that front.

This is what I ordered from Aircraft Spruce. I ordered both the regular AN960PD-716 aluminum washers as well as the thinner AN960PD-716L ones which I didn’t end up using. I chose to us the aluminum ones because I thought they might seal up a little better than the steel ones and wasn’t too worried about corrosion since they’re covered in sealant. Also shown is the Permatex I got for the fuel drains.

Note: the actual AN4 fittings above are the ones supplied by TAF and weren’t ordered from Aircraft Spruce.

Just a little tip. I bought the above at Harbor Freight for around $5. They have been invaluable in applying the sealant. I’ve used them to apply the mixed sealant to parts as well as shaping the fillets and other areas. Since they are all metal clean up is very easy.

Fuel Tank Ribs

Time: 15.5 hrs

Over the past weeks or so I made some progress on the fuel tanks. Since I’m a bit limited on space I wanted to work on the fuel tanks outside on a small table, but the weather hasn’t been cooperating so it’s been difficult to get some time to do work on the tanks. Fortunately we did have a few sunny days so I’m able to make a little progress. Both tank skins and all the ribs have been prepped and cleaned and are ready to be installed. Today (Monday 3/23) I got all the ribs installed into the left tank. I was hoping to get the right tank done as well, but it took a bit longer than I had planned to do the one left tank. I’m hoping the work on the right tank on Thursday and will update this page when it’s done.

One issue I was having was with the closed end rivets. When I was installing the reinforcing channel I had a few rivets that the mandrel broke off or pulled completely out of the rivet. I was even using the manual puller because my pneumatic puller didn’t seem to work at all. Prior to doing the ribs I figured I should see if I could determine what the issue was with the rivets. I was able to figure out how to get my pneumatic puller to at least reliably pull the 3.2mm rivets. One issue was I had something stuck in my rivet puller so while it could still pull the normal non close ended rivets fine I guess the smaller diameter of the closed end rivet mandrels did work in it. After clearing the stuck piece out I was able to pull the closed end rivets, but the mandrels would sometimes break or pull all the way out. To remedy that I found that using the 2.4mm tip on the rivet puller seemed to fix that issue. The only remaining minor issue was that the mandrels were a little tight in the tip so I had to use some pliers to get the mandrel out through the front of the puller, but it’s quick and not a big deal.

The ribs went it fairly easily and it came out OK. I’m not super concerned with making it look nice. I guess I just don’t have the skill to do that so I’m just making sure things are cleaned well and there’s enough sealant where it needs to be. In prepping that parts I cleaned everything with scotch-brite and my aircraft simple green cleaner. On the contact surfaces I uses some 300 grit paper and lightly used it to scuff up the surfaces for better adhesion. For further cleaning I used a MEK replacement thinner called “Pro Thinner” that I found at Lowes and acetone. I cleaned all the contact surfaces and rivets multiple times. The rivets I soaked in a container about 4 times switching out the dirty acetone each time. The contact surfaces I cleaned once with acetone and then with the Pro Thinner until there was no more black coming off the metal onto the rag. Finally I wiped everything down with a dry lint free rag.

Update (3/27/2020): Today I installed the ribs on the right fuel tank. All went just like the left. Next step will be to install the various AN fittings for the fuel pickup, return and over flow.

I cleaned all the ribs and contact surfaces on the skins and masked off all the areas that I didn’t want too much mess. I also made sure everything fit well so I wouldn’t be messing with it with wet sealant. I didn’t mask much off on the inside ribs since I’m not too concerned about a little mess on the inside. I just put down some reference tape so I know how wide to make the fillets (about 1/4″ wide).

For this stage of the fuel tank build I decided to use the 3oz tubes. I also decided to use the B-4 sealant to give me 4 hours of working time rather that the B-2 which is only 2 hours. Even though it will take a few more days for the sealant to cure I think it was a good idea so that I was able to use one 3oz tube to do all the ribs on one tank rather then having to mix up 2 tubes of the 2 hour and having to throw some away. The tubes are pretty easy to mix and it’s much faster not to have to mix the sealant in a container and load it into a syringe.


The end ribs don’t use closed end rivets. just the regular ones so it goes a little faster (no dipping the rivets) and with a little less mess.

A few photos of the installed ribs. To install them I put a bead of sealant on the contact surfaces of the ribs and the skin and then clecoed the ribs in place. I then dipped the closed end rivets in the sealant and riveted in place.

So next I’ll get the right tank to the same stage and then I need to install the pick up, return, and over flow fittings in each tank. After that will be time to put the back plates on and temporarily mount the tanks onto the wings.

More Tank Sealant Fun

Time: 5.5hrs

Didn’t do much work on the plane over the holidays. I’m hoping to get back on it in the next few days. I was hoping that the right fuel tank skin and wing tip was going to arrive at Torrance TAF before the end of the year, but it turns out it wasn’t in the shipment. They did provide me with a REV0 tank skin (my left one is REV1). So far the only difference I see is the row of rivets that attach the tank skin to the leading edge are 4mm on the REV0 skin and 3.2mm on the REV1. So if I decide to use this skin I’ll have to ream all those holes out and put in the larger rivets. I also need to check the fit of all the ribs and the skin on the soar to make sure nothing else has changed. If I don’t go with this skin then I have to wait until mod to late February when the REV1 skin should hopefully be here.

In the mean time I did attach the reinforcing stringer to the left tank skin as well as the right backing plate and rivnuts in rib 101 and assembled the brackets that hold the fuel level senders into the mounting plates. I think I’m getting a little better at using the sealant, but I still don’t like it. Actually the worst part is these closed end rivets don’t work so well. I’ve had a few where the mandrel pulls right through the rivet (pulling it manually). Some I’ve had the mandrels break off so then I have to cut them back with the dremel and the 3.2mm I can’t even pull with my pneumatic rivet puller so I need to pull all those by hand. Of the mandrels get stuck in the puller every so often which requires disassembly of the rivet puller to get the mandrel out. This all when you’re stressed about the sealant drying in 2 hours. In any event it seems to be coming out OK.

I think though I may have reached a stopping point on the wings until I get the tank skin and the left wing tip. I may move to finishing up the interior and doing some avionics instead and come back to the wings in a bit.

Stringer is in on the left fuel tank. I’m pretty happy with how it came out. I had to pull all the rivets manually since the pneumatic puller won’t pull them for some reason.

Fuel sender backing plate and rivnuts installed in the right 101 rib. I ran out of 4.8mm rivet so I was able to get a few off Amazon. The 4.8mm rivets are used to fill the manufacturing holes in the end ribs along with an M5 washer on the inside and a lot of sealant.

The brackets for the fuel senders are installed. I used a bolt to tighten down the area where the sender goes through so that I could get a good seal.