Thorp Air Command - T18.net

Hey Gents,
Does anyone have one of these laying on the shelf? This was the standard fuel cap the came with KB's fuel tanks .. most of us have moved on from these. I'm hoping one of you has a spare on the shelf, I have a friend in need. Pic attached to post.
Thanks!
Lee

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

That appears to be the "OLD Style ACS cap ? They no longer make the rubber gasket if it is the cap in question . I got ALL the remaining rubber gaskets (free) from the ACS factory and mailed them out to folks that wanted one (this was several years ago) . I MIGHT have the metal portion of the old cap ( I would have to search and destroy ) to find it , I do not have the rubber portion . Why doesn't he want to go with the new style Vitron O-ring cap ?

RB

Thanks Rich,
This is a unique case he just needs a complete cap. Going with another type is not an option at this point. (Not a Thorp)
Lee

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

If it is an ACS cap it wouldn't hurt to contact the ACS factory in AZ to see if they have the WHOLE cap (with gasket) .

RB

I have one of the Spruce caps that I milled down 100 thou to fit my tank. I don't use it right now because I could smell fuel. If you want me to send it to try it out, I will.
Cubes

That's the cap that you sent me for my thorp Lee. It fits on mine and has been working well although I have a really hard time removing it. If anyone has a better fitting cap for the thorp I would gladly trade and send this back to Lee for his friend.

Your friend may want to consider converting to a more available cap. Check with Newton Equipment and let Mike Byrne know Dana LaBounty sent you. I've been friends with Mike since 1983 and if anyone can get you a cap that will work it would be Mike. I don't use anything but his products on any of my projects.

Bringing this back from the dead.

So that cap in the picture is the one I have from Lee. Recently the rubber portion swelled quite a bit and now is almost impossible to remove or replace. I'm not sure what happened with it that caused it to swell. It was difficult to remove before but the swelling was quite a bit. I had to actually trim the outer edge about 1/8th inch (portion that is outside of the tank) because it wouldn't fit down into the recess.

Since they no longer make them I need to find another solution. My old cap that I used prior to this has dry rot and will no longer work. Since my panel is back in and everything working as I want it I will not remove the tank to perform a retro fit. I currently don't have the time with my work load to perform a lengthy task.

I've heard I can mill down the new ACS cap and it will work. Truth? I think this is the part number on spruce: 05-01391 or 10455

The swelling came from being in contact with something it wasn't compatible with. Hard telling exactly what but alcohol was the first thing that came into my mind.

I think there is a post about milling down the cap you are referring to. The filler neck on mine was big (original to the drawings?) and about the only thing I could come up with that didn't require welding, excessive amounts of sealant or money was an old Willys Jeep filler neck and cap I had in the scrap pile. I had to expand the filler neck from the Willys to fit over the filler neck on the Thorp. A little sealant to fill what little gap there was and a rivet or 2 to make sure it stayed and I was done. There isn't a lot of room to get a hold of the cap but it works well. There are vendors on the internet that sell bits and pieces for building fuel tanks that may have something you can adapt.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I just realize what happened to the cap. My grease had worn off and the only thing I had access to was dielectric grease which is silicone based. I used a tiny amount in order to get my fuel cap in. That had to be the culprit. Having worked with alcohol fuels in the drag car I have found that the myth of ethanol damaging fuel safe Orings and rubber is false so even if I had contaminated fuel it wouldn’t have caused a problem, especially a 10% mix. I’ve used 90% on rubber oring systems for years.

Stupid silicone. I didn’t even think much of it because I had to get my fuel cap back on and it wasn’t going. What a shame.

Jeff,

FWIW, I used a small amount of silicone grease on my fuel cap gasket - same type - for years, with no adverse effects. Silicone grease will not harm (or cause swelling) of any rubber compound that I know of. Silicone rubber, on the other hand, will swell from contact with many hydrocarbon-based lubricants, fuels, etc. there is generally no permanent harm to the silicone rubber, and the swelling goes down after the hydrocarbon evaporates. I believe it has to do with the size & structure of the silicone molecules. You said the grease you used was “silicone based”... perhaps it contained something else that caused the swelling? Anyway, alcohol in fuel can very definitely cause swelling of certain compounds used in some o-rings. There are rubber o-ring compounds that are not affected by alcohol, but there are others that are, for sure. That may apply to the rubber that your gasket is made of.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

I am NOT a chemist , but I think the Viton O'ring used with my ACS fuel cap is compatible with "Moon Shine" based auto fuels . I also use a silicone spray on my o-ring from time to time to keep it happy . Bottoms up !

RB

Well, I guess that makes sense. I've used pump gas before without issues and I checked ethanol content with a hydro tester and didn't find any. Today when i pulled the fuel cap to fill it came out easy and most of the swelling is down. I have to use a flat head screwdriver to get the end tucked into the tank when I put it back in but other than that it comes out easy.

Not sure what could have caused the issue. It will continue to work for me now. I'll have to search for long term solutions. Although it will work I'm nervous that the oring ACS cap won't hold as strong in an accident like the large one I have that allows the rubber to expand under the rim instead of just against it.

Mine had a PA23 fuel cap on it. The problem I had was the opposite. It became hard enough that I couldn’t tighten the screw enough to generate enough friction to hold it securely in place. That is when I searched out an option that used tabs or threads to hold the cap on instead of squeezed rubber. I don’t think I would have ever known the cap wasn’t secure if I didn’t try to pick up the tank by the cap when it was out of the airplane.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I forgot to update. So going back to the plane the cap comes out really easy again. The rubber is a little "floppy" so I have a small flat head screwdriver to push around the edges to squeeze it back together in order to fit it in although it slides in easy.

So problem basically solved itself.