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Hot Rods Moon Tank Fuel Cap Questions- tank in trunk

Discussion in 'The Hokey Ass Message Board' started by Deyomatic, Sep 26, 2021.

  1. Deyomatic
    Joined: Apr 17, 2002
    Posts: 3,102

    Deyomatic
    Member
    from CT

    Well, after 20 years in this hobby I finally have a real hot rod. 1930 Model A Coupe that I picked up yesterday.
    Now I'm running down all of the things that the internet has made me paranoid about...the first item is that the tank is in the trunk with a vented gas cap. I am assuming the cap is vented because there aren't any other tubes or hoses coming off of it and it doesn't stall when you drive it. Seller (also a HAMBer) said he never smelled gas in the trunk and I haven't in the past day either.

    Anyway, here is a photo of the cap. The Moon customer service guy said they have two size caps- 2.25" and 3.5". I am guessing this is the 3.5". The caliper measured 2.625" on the bung under the cap.

    1. Is it possible it only vents "in" - which would account for no gas smell in the tank?

    2. I plan to run a vent with tip over valve through the trunk- anyone know how I would find a non-vented cap? Moon said they don't have them in the 3.5" size.

    3. Any good tutorials on how they run these cool flip top caps up through the body so it wouldn't matter anyway? Like the photo stolen from Rodders Journal below?

    Thanks in advance.
     

    Attached Files:

  2. flatheadgary
    Joined: Jul 17, 2007
    Posts: 878

    flatheadgary
    Member
    from boron,ca

    i have one of those caps on my henry j gasser. my car doesn't have a trunk and i ran a filler hose from the tank under it to the outside and installed the cap on the body. i couldn't have ran any kind of vent hose on it. i did run a hose at the top of the tank to the filler neck. i bought it from a boat shop 20 years ago. never had a problem when i lived in the los angeles area but when i moved to the high desert where it will be over a 100 degrees for weeks on end i started having vapor lock. mine is not 3.00" though. i am pretty sure it was not vented too. i just drilled a very small hole in the cap and have never had that problem again. except when it is hot out and you go by the car you will smell gas. so no smoking around it.
     
  3. thefabulousnip
    Joined: Sep 8, 2021
    Posts: 7

    thefabulousnip
    Member

    Forgive me if I'm just a stupid redneck, but what about the vented gas cap makes you paranoid?

    I've driven VWs my whole life and I'm just used to the gas smell after fill up and on hot days. I've also got an old shovelhead with two vented caps right between my legs and have never given it any thought.
     
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  4. Deyomatic
    Joined: Apr 17, 2002
    Posts: 3,102

    Deyomatic
    Member
    from CT

    The fact that it’s in the trunk and could fill the enclosed trunk with the explosive vapors so that if the combo was right something could ignite it, was the concern. The battery is about 3” away, so there IS a possible ignition source.

    Maybe the aluminum and the air in the trunk act as insulators and keep the gas cool?
     

  5. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 2,451

    Elcohaulic
    Member

    That tank needs to be vented to the outside air.. It doesn't matter how many years its been this way and nothing has happened..
     
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  6. Deyomatic
    Joined: Apr 17, 2002
    Posts: 3,102

    Deyomatic
    Member
    from CT

    Thanks Elco (awesome name), that was my thought.

    Any ideas on where/ how to get an unvented cap?
     
    Elcohaulic likes this.
  7. 1934coupe
    Joined: Feb 22, 2007
    Posts: 4,648

    1934coupe
    Member

    Deyo I don't blame you for wanting the gas cap/tank vented to the outside . I was never a fan of a fill being in the trunk. I have a flip cap from a British car you can have, just let me go look for it and I'll give you the details.

    Pat
     
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  8. 1934coupe
    Joined: Feb 22, 2007
    Posts: 4,648

    1934coupe
    Member

    Deyo I found it, the cap is approx 4" across, the fill tube is 2" OD it's about 3" tall total. There is a round, flat, cork gasket that is dried up and broke. I had one of these on my 36 Willys sedan and it was great. Let me know.

    Pat
     

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  9. 31Apickup
    Joined: Nov 8, 2005
    Posts: 2,762

    31Apickup
    ALLIANCE MEMBER

    That cap came on many British sports cars in the 60’s, last one I got came off a Triumph.
     
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  10. Deyomatic
    Joined: Apr 17, 2002
    Posts: 3,102

    Deyomatic
    Member
    from CT

    1934Coupe- isn't the one I have just like the one you have? There's a spring that pushes back against the rubber gasket.

    31APU- were you saying that what I have is British Sports car, or what 34Coupe has?

    I think I'm going to take off the sending unit cap on the other side and tap it for my tip over/vent valve. When I do that, I'm going to figure out a way to pressurize the tank- even if I blow into it. I am wondering if the spring in that cap keeps it from "off gassing."

    I am also wondering if I can't figure out the threads to find a viable non-vented cap, I may have to modify this one. (?) so it won't vent?

    On the other hand- if I have a separate vent tapped in somewhere that is essentially always open, won't that mean that this cap will never see any pressure anyway and therefore won't vent- because of the spring?
     
  11. 1934coupe
    Joined: Feb 22, 2007
    Posts: 4,648

    1934coupe
    Member

    Deyo I think we are on different pages. Yes the one I have is a British car I think a TR something. I thought you wanted to go through the body and eliminate the fill in the trunk. That is what mine is designed for, an external fill.

    Pat
     
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  12. PotvinV8
    Joined: Mar 30, 2009
    Posts: 154

    PotvinV8
    Member

    That little hole in the plate that has "England" stamped into it is the vent hole. Weld/braze/bubble gum that hole plugged and you won't have a vented cap any longer.

    The vent is there for a reason however, and it's not to release pressure so much as it is to equalize the pressure inside the tank with the atmosphere as the fuel is depleted. You don't want to create a vacuum inside the tank, hence the vent.

    If your tank has return bungs at the top like most Moon tanks do, you could plug the cap as mentioned and run a line from one of the return bungs and out your trunk to act as a vent. This would avoid the vapors in the trunk. Just make sure the end is higher than the fuel level in the tank. It wouldn't be a bad idea to put a check valve in the end so no fuel can spill out. Just a couple thoughts.
     
  13. Deyomatic
    Joined: Apr 17, 2002
    Posts: 3,102

    Deyomatic
    Member
    from CT

    Now we're on the same page, Pat. I was wondering HOW to do that...the more I look at my coupe, I don't know if I have room to do it and make it look alright. It seems to be as easy as drilling a hole and tightening some things up...probably isn't.

    Potvin- I have one of those check valves already and planned on tapping it into the flat plate where the fuel sender is and running it out the floor of the trunk. I won't be surprised if I take off that plate and find out there isn't enough room. I think it's an older Moon tank because the only holes in it are in the bottom for the fuel to come out, the filler and sender.
     
  14. PotvinV8
    Joined: Mar 30, 2009
    Posts: 154

    PotvinV8
    Member

    If it has a sending unit, it's probably not as old as you think.

    If you do as mentioned, just be careful that you're not dumping fuel with a lower-mounted hose. With the check valve in place, you just might be OK. Perhaps put the check valve as close to the tank outlet as possible, that way fuel can't fill the line? Just a thought...

    What size is the fuel tank and is there a decal on it?
     
  15. Deyomatic
    Joined: Apr 17, 2002
    Posts: 3,102

    Deyomatic
    Member
    from CT

    I don’t see any logos on this tank at all. I haven’t really inspected it closely, and one whole side is completely hidden. All I did was send a photo to moonrise and one of their customer service guy said it was theirs.

    What if I found a rubber hose 2 1/2 inches ID, and some clamps- the hose would probably fit fine over a 2 5/8 threaded neck which is what this fliptop cap is threaded onto. The clamps would probably make the hose bite into those threads and seal pretty nicely.Then I could just use some kind of an adapter pipe 2.5 down to whatever that has a sealed cap and thread my check valve into that adapter.
     
  16. Deyomatic
    Joined: Apr 17, 2002
    Posts: 3,102

    Deyomatic
    Member
    from CT

    Well I figured I'd follow up with my fix...after buying a few different tip over valves, I found exactly what I wish I had found in the beginning.
    I WAS able to tap into the sending unit housing/cover on the tank with a 1/8" NPT to 3/8" hose barb and (ultimately) run this little inline tipover valve almost right out of the barb. With a 90* elbow that tipover valve stays vertical.
    This thing is 3/8 hose barb on each end, has the check valve inside of it, and is about an inch or so long. Made by Fuel Safe and sold by a bunch of online retailers.
     

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