Sonex Fuel Tank & Oops Fittings

[Bryan Cotton]

David's tool is essentially a backwards sanding disk. You use some wire to fish it through the filler neck and have the part that goes in the drill chuck to pop through the hole. Then you sand the inside surface flat, so the O ring will seal. You can drill your hole ahead of time.

[BRS]

David, I suppose if you shot me a picture of the tool, there probably is plenty of stuff in my shop to coble one together.

Q. How smooth does the inside need to be for that O-ring to seal up? I'm guessing quite smooth.

Today I managed to drill out my 3/8" insert with the stuck AN fitting inside. As Bryan said, the 1" hole saw did just fine and only took off the corners of the AN fitting. The thought of pulling the tank, as some have done, did not set well with me. I managed to drill it in place without doing other surgery on the plane (or me). Wrapping several turns of blue tape around the bit got it to snug up in the ID of the fitting. At one point I seemed to not be making any progress. Turned out the insert and fitting were spinning which was preventing the bit from cutting. Just had to wait for the fitting to cool down (too hot to handle from all that spinning) then pulled it out and finished drilling.

Next I cleaned out the vaccuum and reversed the hose to the output. Put it in the filler neck and blew out most of the contamination.

[Bryan Cotton]

Nice! You can definitely recreate the tool especially if you have a lathe. Starts out as a bolt. Drill a hole through to use a piece of wire to pull it into place. Turn off the threads at the end of the bolt. Make a threaded nut with two flats. Plus you need a washer and some sort of abrasive cut to fit the washer.

David oops tool 1.png (248.26 KiB) Viewed 13934 times

Here it has been placed and ready for the drill.

Oops tool in place.png (426.27 KiB) Viewed 13934 times

Sanding the back surface. You pull on the drill while it spins.

Oops tool in action.png (782.82 KiB) Viewed 13934 times

Oops fittings:

Oops fittings installed.png (519.75 KiB) Viewed 13934 times

[Bryan Cotton]

Also, I made a wrench for each oops fitting size. I don't think the nuts are metric or SAE.

I also made big aluminum washers for the outside. I think the oops fitting directions call for an o-ring on either side I believe in prior discussion on this thread it was mentioned there was no way to hold the outside o-ring captive, and it would not prevent fuel from going through the external oops fitting threads. Based on this I went with a washer for stability and the inside o-ring does all the sealing. It is nicely captive in its groove.

[DCASonex]

Yes, hold off, kit has proper sized step drills.

David A.

[DCASonex]

When I installed mine, I used O rings both inside and out. However< I undercut the outside nut to create a groove for retaining that O ring. The outside O ring does nothing for the seal, so can be replaced with a tin fiber washer as many have done. One cation, when using the rigid washer, the inside and outside surfaces must be parallel to put even pressure all the way around the o ring. An outside O ring makes the assembly a bit more forgiving. Keeping that in mind, I see no reason not to use the fiber washer outside with reasonable care.

The step drill in the kit have been supplied by others and have been modified slightly to bore the exact diameter needed. The original fittings of the tank were molded in during manufacturing. One thing to note, the rotational molding process leaves the tanks a bit like snowflakes. No two are exactly the same. The size of the bulge that surrounds the original fittings can vary. The tool are used first to simply create nice flat surfaces for the o ring to seal against but some have found they need to grind the bump down further to get the oops fitting to protrude enough to get the nut on. Use of the fiber washer in place of the outside O ring is a big help for this.

Job can be done while tank is in the plane. That is how I did my A series. But would be easier with tank removed. Set up plenty of ventilation for the tank, to reduce possibly of igniting fumes, and for yourself so you do not pass out while upside down under the panel. A shop vac set up to blow - not suck, works well for this If replacing one fitting, best to do all. Install the large one first, The big hole for it comes in handy when working on the two smaller adjacent holes.

David A.

[BRS]

Thanks David,
While I don't plan on replacing any of the other fittings at this point, I have wondered if I should install an extra one on the bottom for something like a fuel-sump-drain. It has always bothered me to not have a sump drain and not have a gascolator (inline filter instead). Seems though that no one feels a sump is needed since the unsuable is so little. What do you think?

You can see from the pic below how the 3/8 insert seems to have been seeping for some time while the others are all perfect with natural coloration around the fitting. Seems it hard for the 3/8 insert to not eventually leak as there is always some sort of load on it from the shut-off valve and hose.

[DCASonex]

Your working around the area with the other fittings may flex the plastic enough to cause another one to start leaking. If you are in an area with wide temperature swings, the plastic has a much higher coefficient of expansion than the metal and I think that is what caused mine to fail after first winter.
Any bottom drain will protrude up into the sump and thus never truly drain all from the sump negating its value. Wish it were otherwise, would like to be able to flush ALL out at annual.

David A.

[BRS]

... wide temperature swings, the plastic has a much higher coefficient of expansion...
Any bottom drain will protrude up into the sump ....

David A.

Thanks David,
I once had the realization about the sump protruding but forgot - the reminder was needed. Perhaps a 3/8th (or 1/4") hose could be used (via filler neck) ocationally to siphon out any possible water/sedement.

Hanger here is heated, so only gets down to about 45 degF at night. So temp swings arn't too bad.

[BRS]

@DCASonex
David, PM sent.