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Auto/Motor Designing a fuel tank, need advice

  1. Feb 13, 2019 at 5:57 AM #1
    Hello everyone,

    So, I want to make a fuel tank for a motorcycle with a glass window in it. I have seen a custom bike with a completely pyrex tank, so I know it's not impossible. My current questions are:

    Are there any important regulations for fuel tank construction which I will need to investigate and adhere to (UK) - I know there's a lot for boilers but I imagine that fuel tanks are ok as long as they don't leak and have the correct venting caps on them. Alternatively, will I have to commission this from someone licenced to produce fuel tanks?

    Secondly, and more importantly, will a glass window cause issues with the fuel warming in the sun on a hot day, or anything like that?

    If anyone has knowhow on this, that would be great!

  2. jcsd
  3. Feb 13, 2019 at 9:19 AM #2


    Staff: Mentor

  4. Feb 13, 2019 at 7:40 PM #3


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    Speaking as a person who once put a dent (it was just a little dent...) in a motorcycle gas tank, I am very concerned about any gas tank material that can shatter or be attacked by gasoline. Be aware that there are materials that stand up to 100% gasoline, but are attacked by 10% ethanol/90% gasoline. Also be careful about sealant or gasket material.

    Glass, plastics, and metal all have different thermal coefficients of expansion. This must be allowed for in the design.
  5. Feb 15, 2019 at 2:24 AM #4
    Thanks Anorlunda, I have seen these before but they aren't what I'm after, my design is more of an aesthetic one, I want a brass-rimmed porthole style window in both sides of a flat-sided fuel tank.

    I've a reasonably good design for fitting and sealing it, I will just need to source fuel-resistant O-rings. and I am also considering putting a plate inside the tank, about 10mm from the window, so that it stops the sunlight from flooding in and heating the fuel too much. the fuel would be able to go between the plate and the window so would still achieve the effect I'm after.

    jrmichler, I intend to use pyrex (heat treated glass) for it's superior strength. I do not believe that glass will be attacked by any ethanol or petrol mixture, but I will certainly research it before I start making! The seals will be petrol-specific, and I will make sure that they can cope with a mixture of petrol and ethanol, as I know this is in some fuels (apparently it plays havoc with plastic fuel tanks, some cheap bikes have to be filled at supermarkets, as BP and such have ethanol in their fuel, which makes the plastic brittle!)

    I have considered the thermal expansion of the parts, the construction I intend is overlapping layers, rather than snug fits - this should allow for any expansion without allowing any leaks.

    As for the potential for breaking the glass, I will have a look into alternative safety-glasses, which may yield a better glass to use, which might just "crackle" without losing integrity (probably some sort of composite glass made of layers). My original plan was to use a thick (6mm) sheet of pyrex in a machined brass surround, which would fit into a hole cut in the tank (all with seals etc. to make it work). The overall effect I want is to be quite chunky, so allows for a good amount of space for seals, bolts, and over-engineering.

    Thankyou for the replies so far, I will try to make some designs to post soon!
  6. Feb 15, 2019 at 7:03 AM #5


    Staff: Mentor

    We didn't have any specifics to help, but I think that is still the main question.

    If it was the USA, I would fear that my innovative design would not pass the mandatory annual vehicle inspection. Those inspectors have broad authority to accept/reject an out-of-the-ordinary things they see. They are not obligated to investigate small details like o-rings or materials before making a judgement.

    Perhaps you should ask the question to a shop in the UK that does motorcycle inspections.
  7. Feb 15, 2019 at 11:15 AM #6
    Yikes ! Your window is so going to 'wriggle' and 'weep' after a very little time on the road. And, in the event of a 'lay down' or 'T-bone', it is very likely to split..

    Please, do not do this.
  8. Feb 15, 2019 at 12:04 PM #7


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    In the USA, factory supplied fuel tanks have to pass NHTSA side/rear/front impact tests (whatever they entail). Custom builds (probably) don't have the same level of scrutiny, due to low numbers. I doubt anything with a deliberately brittle window in it would pass an impact test without pretty healthy armor and some luck. I'm not sure what the UK has for requirements, but I'd be surprised if impact test results weren't considered.

    I'd suggest a fake sight glass as a much safer alternative. It would provide the same level of aesthetics as a real one, except without the potential for a glass breakage fire. I'm thinking about a couple layers of glass/plastic, separated 1mm or so by an o-ring, with oil between. Make the back silver, or a mirror. Ring it with your porthole brass, glue/weld/braze to the fuel tank, and you have a "sight glass".

    There is certainly enough gasoline in a motorcycle to cause serious injury, or worse.
  9. Feb 16, 2019 at 2:00 AM #8


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    How do you make a seal that way?
    Or a permanent one at that on flat sheet steel.
    If the tank can take the ball peen hammer test without spilling a leak so should the assembly.

    I would be thinking of over-engineering this thing greatly. Nothing better than a leak on a trip.
    Wouldn't a welded flange to the sheet metal of the tank be advisable, onto which you either screw or bolt the assembly.

    And a double glass pane - one to hold the fluid in and the other to protect the inner.
    And maybe an outer mesh to withstand flying debris.

    Pause for thought.
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