Sonex Fuel Tank & Oops Fittings

[Bryan Cotton]

As I get ready to undertake this endeavor I come up with a few questions:
Why a step drill? Is my HF step drill good enough? It looks shorter and fatter than some.
If I could pilot a holesaw would that be a good technique?

[Waiex 49]

Based on my "oops" experiences, the step drill does a good job of removing the original molded-in fitting. I think if you tried to drill out the fitting with a twist drill it could grab on the fitting and cause it to spin in the plastic. That could damage the plastic tank beyond the specified diameter of the hole for the replacement fitting.

If you are replacing the fitting for the main fuel line (the 3/8" NPT fitting) you might find that the step drill and 1" twist drill will require the use of a 1/2" drive drill motor. I replaced all three sump fittings and had to purchase 2 step drills and 2 twist drills. I also had to buy a 1/2" drill motor. It starts to add up. If you would pay shipping both ways I would lend you my step drills and twist drills for this job.

Personnaly, I would not use a hole saw. I don't think the finished hole would be as accurate as what you could get with twist drills.

Are you going with the "oops" fittings or have you considered using brass reducer fittings?

Good luck,

Don Bowen
N49YX
KLHM

[wlarson861]

I used a dowel on the hole saw pilot bit to drill out the main fuel fitting. I screwed up the clean up of the inside of the tank so the "oops fitting" wouldn't work. The hole saw was the only way I could drill out the fitting in the plane.

[Waiex 49]

Just my opinion, but I believe the best way to go about the fitting replacement is to remove the fuel tank. Removing and reinstalling the tank is no fun, but you can do a better job on replacing the fittings and you can do a much better job of cleaning out all the junk that gets into the tank from the drilling.

[fjdoug]

i used a hole saw, it neatly removed the leaky fitting and left a perfectly sized hole for the oops fitting.
before cutting the hole i screwed a brass plug into the leaky fitting with a 1/4" hole drilled into it to guide the hole saw.
big drill bits are not ideal for making round holes.

[Bryan Cotton]

Does anybody know offhand what size threads both stock fittings are? Same size as the oops fitting? I am a big fan of holesaw technology.

[SonexN76ET]

With all due respect, you can NOT use most step drill bits to install the oops fittings! The plastic on the tank is too thick. A step drill will bevel the hole where the outside will be with a wider hole then what makes it through to the inside of the tank. In other words, you will have a stepped hole in your tank that the oops fittings will not go through and seal properly.

Secondly, I need to state that it is wrong and just plain dangerous advice some have given on using reducers simply screwed into the plastic tank instead of using the oops fittings. The reducers will not work by just screwing them into the plastic. They will work out from vibration and side loads on the lines and fittings attached to them. The only way to make the reducers work long term would be to put washers on both sides of the tank with O rings under the washers and a nut on the inside of the tank. I tried screwing in a reducer without washers, O rings, and nuts and wiggled the reducer and it loosened up pretty quickly. You have to have compression with an o ring on either a reducer or oops fitting in order for it to seal long term. I don't know how you could apply torque to a nut inside of the tank on a reducer, so I believe the oops fitting is the superior method.

The threads on the inside of the stock fittings are 3/8 NPT for the main fuel line/finger strainer and 1/8 NPT for all others. The Oops fittings come in both sizes. The outside diameters are specified in the Sonex instruction sheet.

Jake

[Waiex 49]

Jake,

With all due respect, the step drills are used to REMOVE the molded in fittings. This is clearly stated in the Sonex instruction sheet. Sonex specifies using a twist drill to make the final hole. Others have used hole saws with varying degrees of success.

Thanks for sharing your experiences with reducer fittings. Very good info!

I am not a huge fan of the "oops" fittings. They are made of a lightweight material and don't impress me as being either strong or durable. I am even less of a fan of the stock, molded in fittings. These little buggers are prone to leaking and spinning when any torque is applied. I would love to see Sonex / Aero Conversions come out with some nice fittings (red anodized, of course) that would really solve this issue.

Don

[radfordc]

Secondly, I need to state that it is wrong and just plain dangerous advice some have given on using reducers simply screwed into the plastic tank instead of using the oops fittings. The reducers will not work by just screwing them into the plastic. They will work out from vibration and side loads on the lines and fittings attached to them. The only way to make the reducers work long term would be to put washers on both sides of the tank with O rings under the washers and a nut on the inside of the tank. I tried screwing in a reducer without washers, O rings, and nuts and wiggled the reducer and it loosened up pretty quickly. You have to have compression with an o ring on either a reducer or oops fitting in order for it to seal long term. I don't know how you could apply torque to a nut inside of the tank on a reducer, so I believe the oops fitting is the superior method.

Jake, I assume you have tried the reducers and shown them to be dangerous and ineffective. Dumb old me for putting them in my leaking tank 6 years ago and never a drop leaked since. I guess I better call the guy who bought my plane and tell him to ground it until he replaces the tank with a new one.

I think you need to be just a little less judgmental about this issue unless you have some evidence to back up your advice.

[WaiexN143NM]

Thanks don, and good to meet at rio Linda. I'll take a good look inside my tank tomorrow and feel and look to see how much plastic is in there. Not sure what way is best to go about the fittings. Will take a good look at the oops fittings and reducer fittings. Thanks to all who have shared ideas, any other builders with different ways to fix leaky fittings please share!!
WaiexN143NM
Michael Radtke