new gaskets and sticky stuff - fuel tanks part 9

Busy couple of weeks at the shop since my last blog update.

After finding the fuel sensor assembly gasket dimensions weren't scaled correctly I redid the CAD drawings and cut new ones that have wider surface areas while maintaining the center hole diameter and the outside dimension of the sensor plate:

Original test gasket laid on top of new gasket.  This should improve hole edge tolerances.  It will shrink the access hole a bit in the wing tank skin, but I believe it is worth the trade off:

With the fuel sender mounting holes already drilled to full size for the well nuts, I needed to come up with a way to secure the new gasket and drill mounting holes at full size instead of upsizing them as I went.  I took some scrap plywood big enough to fully sandwich a sensor gasket and drilled an index hole in the plywood at opposite corners of the original gasket:

Opened the sandwich, put the new gasket in place using the index holes as a guide, then screwed the sandwhich down tight to hold the new gasket for drilling:

Placed the fuel sensor assembly on top and indexed it over the gasket using the same index holes:

Used the sensor plate as a guide, pilot drilled then right sized the holes in the gasket sandwich: 

Much cleaner result and the hole edge distances are now improved.

 
Tsped the gasket in place to confirm the holes do indeed match up on the skins and traced out the new access hole dimensions on the tank skin:

Removed any non-pertinent lines after corning drilling the new access cut-out:

Dremel tool with cut-off wheel to final shape the access hole, then filed the corners and final deburred the edges:

Took a quick peek at the truss head screws inside the tank that hold the drain bung and gasket.  This is where the fuel will drain from the tank for pre-flight testing and any future required fuel tank maintenance if necessary.  Very pleased at how the gasket I made fits, but I'll add some sealant on the screw heads as well:

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With all the cutting and fitting of access holes, filler necks and other outlets complete, I can finally move onto the dreaded sealing of the tank seams.  This process has been keeping me awake at night figuring out order of operations and how to do many things at once.

First, I fully cleaned the tank inside surfaces to remove any contaminants or markings and used Scotchbrite pads to scuff the joining surfaces on all seams inside and out where sealant will be applied.  This included a full wipe down with lacquer thinner to remove any other oils or residue that could impede the correct adhesion of the sealant or curing of the sealant once applied.

Next I hand placed all the A3 squeeze rivets in the bottom seam of the tank on the outboard side and used tape to hold them in place so I could flip the tank upright.

Tank on the bench with rivets placed in the lower seam, the back edge and front side of the tank.

Next up was preparing the ProSeal sealant.  It gets mixed 10 parts sealant (big can) to 1 part activator (small jar) - this is done by weight not volume so I used a gram electronic scale to measure out exact ratio of each and once in the same mixing container, stirred them together to start the sealing process.  Working time of this sealant is about 2 hours after mixing depending on ambient temperature with early cure time of 7 hours once applied.  Full cure time is suggested as 1 week before applying any pressure or liquids to the tank.

I was so wrapped up in applying the sealant to the outboard tank side lower seam I didn't take any pictures of the process.  Needless to say, it's messy and stinky stuff to work with and anyone using it can tell stories of it getting on everything so I wore old clothes I didn't mind throwing out if needed.

A peek inside the tank after it sat overnight shows that the sealant I applied in-between the joints did squueze out exactly as I planned into the inside seam of the tank - perfect!

Same with the sealant between the front upper edge and the folded angle I had to add:

I used the remainder of the sealant I had mixed up to cover the rivet tails on the tank fill neck flange and it cured up very nicely.  It doesn't look pretty, but it's inside the tank where no one sees it and is fuel tight which is the real goal.

With the outboard tank side started, I started on sealing the inboard tank side which included copious amounts of sealant on the fuel line outlet bung.  I figured to to do it first where I had 360 degree access to it before the tank side gets placed in the tank skin:

Here is a progress shot for the inboard tank side.  Same as the outboard side, hand placed rivets, secure them with tape on the outside of the tank.  To apply the sealant easily, I used some large bore disposable syringes filled with sealant - kind of like a miniture caulking gun and ran a bead of sealant on the inner side of the rivets as I want the majority of the sealant to move inwards as I squeeze the rivets:

As you can see in this picture, sealant squeezes out between the seams on the outside as well as coming up through the rivet holes.  A good complete seal on the seam and the rivets.  More sealant applied arounf the fuel outlet bung as well.

Extra sealant doesn't go to waste (it's way too expensive to throw away!) so I added some around the fuel drain bung screws as well.  Anything to improve the fuel tightness of the tank.  You can also see how the sealant squeezed properly into the inner seams of the tank:

While I let the inboard lower seams cure, I placed the right tank into the left wing and finished fitting the fuel sensor assembly and gasket for the tank in anticipation of bringing the other wing out of storage.

The new version of the gasket fits perfectly and I could drill the sensor plate and gasket out to full size mounting holes right on the tank this time instead of the plywood sandwich shown earlier:

I took the Dremel tool and a grinding wheel to both fuel sensor assemblies and carefully cleaned up the sharp weld edges where the fuel sensor wiring may come into contact to prevent chafing with vibration.  No issues any more.

Next I repeated the process for the top seams of the tank.  As there is little room to waggle the rib into place onto the rivets, this time I laid down the bead of sealant on the rib, folded down the top tank skin then hand placed the rivets into the holes.  Messy, but effective as I needed to do both inboard and outboard sides and the front edge as the top skin laid down.  This sealant gets on everything!

Once mixed it can't be stored, so to use up each batch I made (40 grams at a time) I started sealing the outside of the tank edges - rivet heads and rivet tails:

I purposely went sealant heavy on the inside corners to ensure the corner plates I added formed a fully tight corner - again, none of this gets seen inside the wing so it doesn't need to be pretty.

First section of the top seam, working forward from the back corner of the tank.  The sealant clearly is doing it's job at the rivet holes.

Forward edges done, sealant on top of squeezed rivets:

​Extra sealant on the front corners

All edges sealed and left to cure.  An inspection mirror inserted inside the access hole shows excellent sealant fillets on all the inside seams.

Picked up a couple of nylon NPT plugs for each of the outlet bungs.  These will tightened in place when I pressure/leak the tank.  Nylon seals well but won't damage the aluminum threads of the bungs.  Permanent thread sealant will be added once the final fittings are in place.

Got the final truss head screws I wanted from Fastenal and used them to seal the fuel sender plate with the gasket.  All part of sealing the tank for testing. 

Fuel sensor in place - the relief grinding I did makes a huge difference in helping the sensor wire to lay flat across the top of the tank and to prevent chafing.

Fuel tank test fit in the wing, first time completely riveted and sealed - nothing changed, tank shape still the same and sitting in proper position for fittings and filler neck - YES!

Next step is to add buffer strips to the sides of the tank to prevent tank movement in the wing especially when close to empty or less than full of fuel.  I have some high density foam that I secured to the tank sides with high strength contact cement.  Once dried I can literally lift the tank by the foam blocks.

Next close up the bungs with the nylon plugs and test the tank for leaks.  If it passes the leak test, back into the wing it goes where I'll add the top cork strips and connect the fuel lines.

​More to come :)