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A fuel anti-siphon device (ASD), also called a fuel anti-syphon or anti-theft device is a perforated insert in a fuel tank filler neck, which allows fuel into the tank, but stops siphoning hoses from being inserted.

Usually, it is an aftermarket product, which can be retro-fitted on any fuel tank. It is sold at vehicle dealers and parts distributors. 

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There are many different designs on the market, but all anti-siphon devices can be divided into two main groups:

  • External these are installed on top of the filler neck and a fuel cap fits on the ASD;

  • Internal these are installed inside the filler neck and do not interfere with the fuel cap.

Thus, external models have the advantage of being a visual deterrent and can turn a fuel thief away from the vehicle. However, their heads might obstruct a fuel nozzle, especially on rigid trucks.​

Internal models, do not change the height of a filler neck, but also do not make a thief aware of them, which might mean a broken fuel cap is a fuel theft attempt is made.

The most important parameters of an anti-siphon device are:

  • Security - how good they protect fuel from siphoning;

  • Protection from "skimming" - separate part of security against siphoning the top layer of fuel, which can be siphoned from within an ASD;

  • Installation method - how easily it can be installed and whether any fuel tank alteration is needed;

  • Re-filling flow speed - how easy and safe it is to re-fill the fuel tank through an ASD;

  • Corrosion resistance - how long an ASD will last in challenging truck operation conditions;

  • Compatibility with OEM and aftermarket fuel caps.

In short, an efficient anti-siphon device needs to have a secure, simple installation which does not damage the fuel tank. It needs to protect all the fuel in the tank and allow high speed re-fills without any fuel splashing back or a nozzle cutting off.


When choosing the right anti-siphon device for your vehicle or fleet, it is important to consider:

  1. Installation method - there are many different installation methods, some with grub (set) screws, fixing with one-way screws, rivets, glue, bending tabs, spring tabs, etc. You might want to avoid the ones with screws or rivets, as they require drilling the fuel tank, which can affect tank sealing. They can be found on the cheapest products. Rivets are also quite easy to remove. Glue fitting is inconsistent and can be messy. Other methods' ease and security depend on their design for example large bending tabs can leave big holes in the body of an anti-siphon, which defeats the purpose of an ASD.

  2. Security consists of ASD strength - how well it resists break damage, size of the holes - whether they offer protection from smaller hoses, reliable fitting - whether it is possible to remove the whole ASD. Strength of ASD is important, but it's a matter of balance, as there is nothing unbreakable and it should be strong enough to make theft difficult, noisy and time consuming. This will deter most of the thieves, while a well-prepared thief or gang will find their way to the fuel anyway. The size of the holes should not exceed 6 mm; with larger holes, smaller hoses can be used to siphon fuel. In certain conditions, slow speed is acceptable. Fitting has to be secure and should not allow removal of the whole device.

  3. Protection from skimming. Skimming is the term used to describe siphoning the top layer of fuel from an over-filled fuel tank. This layer can be reached from inside an ASD and the longer the ASD is, the deeper it dips into the tank, the more fuel can be skimmed. As a general rule, the shortest device will work the best; however, there are other methods to stop skimming, e.g. a shut-off valve (float) which will close the ASD's chamber in a full tank. The idea is great, but in some designs, it can be hacked - a valve can be depressed or even jammed for easy siphoning. In such cases, an inspection will not show anything suspicious and the ASD will be considered secure.

  4. Refill speed varies greatly around the world. For example, diesel pumps in Europe are mostly limited to 30-40 litres per minute to avoid filling issues with a vast range of cars, vans, light trucks etc. HGV fuel stations have high-speed pumps - up to 120 l/min or even up to 150 l/min. High refilling speed is important to reduce time spent off-road, especially for high volume fuel tanks on long haul trucks. Any ASD is an obstacle for fuel and at high flow speeds it can bounce back, splashing out badly and also cutting off a fuel nozzle. To dissipate fuel at this flow rate, the construction of an ASD must be more than just some drilled holes in a cylindrical basket. To improve the flow, some ASDs have longer bodies, which allow a lot more skimming. Or they will direct a fuel nozzle in a specific way or limit acceptable nozzle types and sizes, which might make a simple refill problematic and inconvenient. A shut-off valve on some ASDs doesn't help as well - when fuel reaches the valve and starts pushing it up, the filling speed will drop down a lot and will require driver's attention and more time to finish it.

  5. Corrosion resistance. The most common ASD materials are aluminium alloys, steel or plastic. Aluminium and plastic have very good corrosion resistance, but steel has to be coated, and some cheap coatings don't last long; for example, an external ASD with a basic zinc coating will definitely rust in a couple of years in wet conditions, especially in a marine climate.

  6. Compatibility with existing fuel caps is another thing to look at. Most anti-siphons can work with OEM fuel caps, but some require a custom one, which could come at extra expense or be inconvenient to use.


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FuelDefend's basic principles of anti-siphon devices are simplicity, security and high speed refuelling at affordable price. We use our own original designs protected by international patents and offer two different ranges - premium and budget, targeting different markets. Both ranges tick all the boxes in the list of main parameters shown above, namely: 

ULTRAElite - Premium range

ULTRAStandard - Standard range


Installation method

Installation with internal grub screws, covered with security collar - our original concept, which works extremely well and doesn't require tank modifications.

Installation with snap-off grub screws - another simple but very efficient FuelDefend's design. No tank modification needed.


Strong aluminium alloy, thick walls, holes and slots not exceeding 6 mm, stainless steel mesh covering bottom of the anti-siphon - well-balanced security.

Steel body, hard nickel coating, holes and slots not exceeding 6 mm, stainless steel mesh - same level of security as premium range

Protection from skimming

Short basket - very little to no skimming possible

Short basket - very little to none skimming possible

Re-fill speed

Accept all fuel nozzles including extra-large ZVA32. Works without splash-back and cut-offs up to

125 l/min with ZVA25 nozzle and up to 

150 l/min with ZVA32 nozzle

Accept all fuel nozzles including extra-large ZVA32. Works without splash-back and cut-offs up to

125 l/min with ZVA25 nozzle and up to 

150 l/min with ZVA32 nozzle

Chemical resistance

Used aluminium alloy has great weather and diesel resistance, which doesn't degrade over time.

Hard nickel coating is superior to zinc galvanising and less hazardous than similar chrome coating.

Compatibility with fuel caps

External anti-siphons work perfectly with OEM and aftermarket fuel caps;

Internal anti-siphons don't interfere with fuel caps

External anti-siphons work perfectly with OEM and aftermarket fuel caps;

Internal anti-siphons don't interfere with fuel caps

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Anti-Siphon Review

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