Minimize Upfit Frustrations with Transfer Flow’s All-in-one Solution


The filler neck—a critical component of completing a work truck—has long been an element of vague frustration for upfitters. We spoke to both engineers and upfitters to understand the importance and benefits of using safe, legal, and reliable filler neck kits.


A filler neck is the fuel system component that provides a pathway from the external fuel cap to the OEM fuel tank. While filler necks exist on all vehicles, filler neck kits are specifically for upfitting incomplete work trucks. Filler neck kits comprise various tubes, clamps, and hoses that are manipulated into custom pathways to safely transfer fuel around truck frames, tool boxes, or equipment on upfitted work trucks while filling at 10-plus gallons per minute.

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“These are commonly on what the OEMs Ford, Ram, and GM call an ‘incomplete vehicle,’ which is simply a truck cab with the bare frame rails,” says Ben Winter, director of business development at Transfer Flow.

Incomplete vehicles are upfitted to function as a variety of work trucks, including: flat beds to haul hay, bricks, or rocks; utility boxes to carry tools on all sides of the bed; or dump trucks where the bed needs the freedom to articulate. The location and routing of the filler neck can vary significantly from application to application.

When incomplete vehicles come off the line, they are equipped with some hose on the frame and a bag of filler neck parts called a Dunnage Kit. The upfitter uses the accompanying parts from the chassis to piece together a filler neck; however in many instances, the OEM may not provide enough components for the custom application. That’s when Transfer Flow’s complete filler neck kits come in handy.


A typical filler neck kit consists of a bezel, straight or formed filler neck with fuel cap, straight or formed transfer tube(s), various double-walled rubber hoses, clamps, and installation hardware. Each part included in a filler neck kit must meet Society of Automotive Engineers (SAE) standards.

“We aim to deliver a premier, out-of-the-bag solution for these upfitters,” explains Michael Larocco, manager of engineering at Transfer Flow. “By adhering to the most stringent testing standards outlined by CARB (California Air Resources Board), we are able to provide the executive orders that recertify the legality of that vehicle’s fill portion in all US states, which ends up saving the upfitter time, money, and effort.”

Most of the parts included in the filler neck kits are engineered and manufactured at the Transfer Flow production facility in Chico, California. “Although the shapes and components of each filler neck may vary from application to application, the production path involves many of the same stations,” says Brian Smith, mechanical engineer at Transfer Flow. “Filler neck parts pass through the horizontal band saw, the swaging machine, the hydraulic beading machine, small parts welding, and pressure testing to ensure a premier product. We work with materials of all sizes, so we have engineered the necessary tooling to allow us to be agile in our production capabilities.”


Upfitters are responsible for building out the body and completing the fill portion of the vehicle using CARB-certified filler neck parts. “CARB regulates aftermarket filler neck kits by mandating that the consumer must be able to safely pump gasoline into the vehicle at 10 gallons or more per minute,” Winter adds. “That’s why we run multiple fill tests at 10 gallons per minute on all of our filler neck kits. As a rule, we hold our diesel filler neck kits to the same standards.”

Installing a Transfer Flow filler neck kit gives the upfitter confidence they can complete the job legally and efficiently while providing the customer with a safe and reliable work truck.

“We’ve been buying filler necks from Transfer Flow for years,” says Gary Stromer, parts manager at LCL Truck Equipment in Hastings, Nebraska. “Without their complete filler neck kits, we would have to purchase hose, have pipe bent, and hope we can put together something that works. It’s a pain. With Transfer Flow filler necks, none of that’s a concern. We’re saving time, and time is money.”


There are other instances when the upfitter only needs one or two additional components, maybe a 90-degree elbow or a few more feet of hose, to use in conjunction with the OEM-supplied filler neck parts to complete the finish work. Transfer Flow manufactures and sells filler neck parts individually. However on gasoline applications, the upfitter is responsible for running the fill tests and completing the necessary paperwork for recertifying the vehicle for compliance, as Transfer Flow’s executive orders only apply to complete filler neck kits.


Transfer Flow is a leading manufacturer of premier American-made fuel systems and vehicle accessories. All Transfer Flow products are engineered and manufactured in Chico, California, and backed by 35 years of industry-leading craftsmanship and engineering excellence.

Transfer Flow offers a variety of OEM-quality retrofit filler neck kits for Ford, Ram, and GM/Chevy pickups and cab chassis. Whether your application is a pickup box delete, a pickup converted to a utility body, or a cab chassis work truck, Transfer Flow has you covered. All filler neck kits are engineered and designed for specific vehicles to ensure a perfect fit and easy installation. All kits come complete with everything needed for installation, including detailed instructions. Find out more, visit


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