Fuel Tank Fit Problems

[NWade]

Hello All,

I've already reached out to the Sonex Tech Support team, but I was curious if any experienced builders would take a look at these photos and let me know what they think of my fuel tank fitment... I can't quite get the tank to slide forward far enough to get the straps to fit between the molded-in ridges on the bottom of the tank. The tank is hitting the vertical firewall stiffener before sliding forward the last ~1/8" that it needs to.

The photos in this album have descriptions/notes at the bottom:
https://www.flickr.com/photos/noel_wade/albums/72157679545531185

Any thoughts or suggestions?

Thanks,

--Noel
Sonex #1339

[SonexN76ET]

Noel,

Check out the Sonex website for the Fuel Tank Installation Instruction.

Note the following:

"c. Front fuel tank straps do not fit perfectly within the ridges in the fuel tank."

Jake

[Corby202]

Mine was the same, from memory it was more then 1/8" and I had to drill another hole to get it to line up. I also ground the straps some so they were a better fit in the grooves.

Phil

[NWade]

Note the following:
"c. Front fuel tank straps do not fit perfectly within the ridges in the fuel tank."

Thanks Jake, but I believe they are referring to the fact that the straps are a smaller size than the space between the ridges (so there's "wiggle room"). In my current installation the straps wind up sitting on top of the front ridge, not in-between the ridges. Its clearly not an acceptable installation as-is; but I appreciate the pointer!

--Noel

[jowens]

Hi noel
My measurements from the front of the tank back to the first ridge is 1 1/8" and the second ridge starts at 11 1/4". Just a crazy chance your tank measures are not standard?

[SonexN76ET]

Hi Wade,

Please let us know what the factory says. I am pretty sure I ran into this same issue when I was installing my fuel tank. I am sure I was told that it was not critical for the fuel tank straps to fit inside of the ridges. I am also quite certain you will not want to drill additional holes in the angle in what is a critical portion of the forward fuselage.

Jake

[NWade]

Hi Wade,

Please let us know what the factory says.

For the record: The term the factory used when describing the molding process for the tank is "non-precision part" and that it may not fit as exactly as other (metal) parts.

In my email to the factory, I proposed a fix by notching the firewall stiffener and then reinforcing it with some Aluminum plates. They said that was acceptable. They also said the tank stops can also be moved, once the tank is "securely on the fuselage cross-tie". Not sure if they meant drilling new holes for the straps in addition to the tank-stop angles; but for my plane the new holes and the old holes would overlap; so that's not an option for me.

I've posted a quick sketch of my intended fix here:
https://flic.kr/p/QZTbqb

Take care,

--Noel

[NWade]

UPDATE:
Well I'm glad I decided to measure things before I cut the firewall stiffener! Turns out the forward tank straps fit just fine; it was only the aft straps that didn't. After careful measurement, I found that the longerons could accommodate revised hole locations for the aft tank straps after all. The edge margin between the new & old holes isn't ideal, but it's still between 1.5 & 2 x D... any weakness is in shear if the strap tries to move forward. I'm more concerned about making sure it doesn't move aft (into my lap) so I should be A-OK. Phew!

--Noel

[Area 51%]

Before cutting the vertical stiffener, if you find you still have clearance issues, consider warming-up the tank at the contact point with a heat gun (low heat and very carefully so as not to burn the plastic) and displacing the tank face for clearance. That material gets very pliable when threatened with BTUs.

When my tank was giving me fits, this was one of the options given to me by the "powers" at Sonex.

During construction of an aluminum replacement tank, I took that stiffener into account, as it was one of several issues I was experiencing with the factory plastic unit. Because I angled the front face of the tank to match the angle of the stiffener, I was able to move the tank ahead far enough to eliminate the need to clearance the windshield strap for the vent.

If you need confirmation that these "non-precision" items are actually the same, just universally wrong, you can have the turd that came in my kit.