Why does a gas tank have a filler neck? It's not that you can leave the nozzle there and put gas without having to hold it.

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  • Can you add a photo of an example of a long filler neck?
    – HandyHowie
    Commented Oct 25, 2021 at 18:58
  • The length is not important, why does it have a neck at all? I will edit the question.
    – ronenfe
    Commented Oct 25, 2021 at 20:22
  • You need something for the cap to hinge or lock onto.
    – GdD
    Commented Oct 25, 2021 at 20:51
  • What might you propose as a better design?
    – jwh20
    Commented Oct 25, 2021 at 21:06
  • Agreeing with @HandyHowie here ... can you add an image or at least specify the year/make/model of the MC in question? Commented Oct 25, 2021 at 21:23

1 Answer 1


The neck extends downward from the actual tank lip so there's an air space above the fuel level. There are at least three reasons for the air space above the fuel level:

First, the space is necessary to accommodate expansion of the fuel when the tank warms. While a fuel overflow tube would in any event be present, the space allows some expansion while preventing fuel from spilling onto the ground.

Second, the space is also useful for the engine's emission control system. Fuel in the tank gives off damaging vapors. Air space above the fuel level allows easier plumbing to prevent these vapors from reaching the atmosphere, and, in some models, to capture the vapors and feed them into the motor's intake manifold.

Third, the design also provides some resistance to fuel slopping out of the filler hole due to rider inattention when filling the tank.

  • I don't see how it prevents overfilling. You can still fill it till the top if you want. The only thing I can think of is it gives you a reference when to stop filling the fuel. When it reaches the bottom of the neck.
    – ronenfe
    Commented Oct 25, 2021 at 23:27
  • @RonenFestinger I didn't say it prevents overfilling. A determined or inattentive rider can hold the trigger of the fuel nozzle down and fill past the bottom of the lip. Commented Oct 26, 2021 at 0:11

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