Fuel tank plug-style cap failures

[GWMotley]

I thought I would post my experience with the Shaw Company pressure plugs that Sonex uses on the old style of fuel tanks which do not have a threaded cap. As many of you may know these plugs are made with Buna rubber which is suppose to withstand gasoline, at least for a while. My original plug lasted about three years of use. Note, I use auto fuel with alcohol about 98% of the time and have about 300 hours on my engine now and I have inspected my fuel lines without any noticeable rubber change. Last year though, I noted someone else say they had a fuel plug failure so I purchased a spare to keep in the plane at all times. About two weeks ago my original plug finally failed. The failure occurs right under the very large washer and just rips thereby losing a seal. So, luck me, I just took my spare out my bag and figured I would order a new spare once I had enough items to make the shipping cost worthwhile. Yesterday I noticed quite a bit of fuel fumes but figured that perhaps I had just not tightened the cap enough as this has happened before. I wanted to go flying again this morning and when inspecting the cap it was obvious that it was very very loose. Sure enough the new cap had split in the exact same way as my original, but again, only two weeks after put into service! I know that these caps may be made with neoprene rubber as well which is not appropriate for fuels. I'm not an expert but the rubber does appear to be the same as my last in that it is not a hollow cell type that I have used before with neoprene. Perhaps just a bad batch of rubber but I would advise everyone to start thinking about a more though check and possibly keeping spare with you as it would be very hard to fashion and emergency cap. However, if anyone does have a good idea for a make shift cap I'm sure we would be all ears.

[MichaelFarley56]

Sorry to hear that Gary. I guess I should probably check mine since it's been used for almost 3 years at this point as well. I only use 100LL so I wonder if that has an effect on the durability of the neoprene. Did you order your spare straight from the factory, or through a separate vendor? If they are prone to failing, I certainly wouldn't mind having a spare!

[vigilant104]

I'm not sure it helps, but after reading about trouble others have had with the rubber on these caps I try to avoid letting the cap "stew" in the liquid fuel between flights. So, I leave the tank a bit shy of absolutely full (this also reduces chances of splashing fuel on the windscreen during filling), or even elevate the nosegear slightly once I've got her in the hangar. 100LL exclusively for me.
It probably helps the rubber live a longer life if we avoid excessive tightening of the tensioning screw. On the other hand, if I ever find myself hanging in the straps in a flipped airplane, it would be some comfort to know the cap was on very securely.
I think I'll buy another cap to have on hand, thanks for the reminder.

[fastj22]

Gary,
When you buy your replacement, buy two and I'll reimburse you. Always good to have a spare.

[GWMotley]

I believe I ordered from Sonex. It was recently on an internet search I discovered there are two types of rubber caps from Shaw. I'm sure that the amount of tension needs to be kept to a minimum, which I try to do. Also, I don't fill my tanks to the point where the plug is bathed in fuel all the time while not in flight. This might of been just one of those things but wanted to bring it to your attention. And yes, John, I'll order an extra one for you. :-)

[n982sx]

I have had terrible luck with these caps. I am on my fourth or fifth one now in 250 hour of flying.

I do seem to have to tighten the cap more than I would like to make a good seal. Anyhow I order several at a time. I run 100LL exclusively. The first one went after the first winter when it sat with a full tank for a couple of months. It only had about 20 hours on it at that point.

[SonexN76ET]

Has anyone tried a thin coating of fuel proof lubricant (EZTurn Fuel lube) on the rubber on the fuel caps? This is the same type of lubricant that is used on the O-rings on the AeroInjector. The directions on the fuel proof lubricant state that it can be used to protect rubber from fuel damage and fuel aromatic damage. This is sold by Aircraft Spruce.

Thanks

[grevaly]

Hi, I am getting ready to start my fuel tank install. My tank came with the spin on Kelch type cap.
I would like to do some rolls and stuff and wonder if this type of cap is ok for limited inverted flight
or if I should go to the plug type cap

Mark

[SonexN76ET]

When I was at the Sonex Builders workshop a couple of years ago one of the participants asked Jeremy about the new fuel tank caps. He said they were an improvement all the way around and were fine for aerobatics. If someone else has some first hand experience please speak up.

Thanks,

Jake

[mike.smith]

Hi, I am getting ready to start my fuel tank install. My tank came with the spin on Kelch type cap.
I would like to do some rolls and stuff and wonder if this type of cap is ok for limited inverted flight
or if I should go to the plug type cap

Mark

I have the spin-on and fly loops, rolls and spins with no problem. I had the "17 gal, 100LL" engraved right into the cap which was a nice touch. The cap is so wide that it lightly bumps the vent fitting as I spin it on/off, but it's not been a problem. When I discovered that, I knew I could shave a little off the outside of the cap if I had to. Love the spin-on cap!