How a diesel injection line is made: from steel tube to finished part

A high-pressure diesel injection line starts as seamless precision steel tube. The tube is cut to length, deburred and cleaned, given a formed sealing cone at each end and then bent. Lines for the highest pressures are also pre-stressed from the inside before they go on the engine. The flow here draws on two sources: the German steel tube maker Benteler for the raw tube, and Bosch’s diesel reference book for the ends, the bends and the pre-stressing.

What are the steps in making an injection line?

An injection line is made by cutting seamless steel tube, forming its ends and bending it. The sources do not set these steps out as one sequence; the order below, including whether the ends or the bends come first, is our inference. It rests on this: the raw tube comes first, and an end can only be formed on tube that has been cut. Pre-stressing is placed last because Bosch describes it as done before the line is installed on the engine.

1. Raw tube. Seamless tube is produced hot rolled or cold drawn; precision tube comes out of the drawing shop (Benteler).
2. Cutting and end preparation. The tube is sawn to length, deburred and cleaned; its ends are faced and chamfered (Benteler).
3. End forming. A sealing cone is pressed onto the end, shaped from the tube’s own material (Bosch).
4. Bending. The line is bent without sharp bends, to a radius of no less than 50 mm (Bosch).
5. Pre-stressing. Lines for applications of 1,400 bar and over are pre-stressed with internal pressure (Bosch).

The first two steps can be done at the tube mill: Benteler says its cutting centres saw tube to length, then deburr and clean it.

What kind of steel tube is an injection line made from?

Injection lines are made from seamless steel tube because the line has to withstand the system’s maximum pressure and very large pressure fluctuations. Bosch’s Diesel Engine Management says: “The lines are seamless precision-made steel tubing in killed cast steel which has a particularly consistent microstructure.” Its chapter on unit pump systems says the same: “High-tensile, seamless steel tubes are therefore used.”

What decides the strength is the side of the tube nobody sees:

“The primary factor determining the high-pressure line’s compression-pulsating fatigue strength is the surface quality of the inner walls of the lines, as defined by material and peak-to-valley height.”

Bosch, Diesel Engine Management, p. 170

Fatigue cracks that start from vibration are covered in our article on why injector lines crack.

The raw tube itself is made at a tube mill. Benteler names high pressure resistance, load capacity and uniform shape as the advantages of seamless tube. Its precision tubes come out of a drawing shop that, the company says, works to tight tolerances and in sizes from 4 mm outside diameter. At its hot rolling mill a three-dimensional inspection system scans the surface of the tube for defects along its full length, without stopping the line. An automotive brochure the company issued in 2016 lists diesel injection tubes among the applications it supplies tube for. Benteler is among the steel tube makers whose tubes RSS Kumru uses.

How is the cone formed, and when does the nut go on?

In the joint Bosch describes, the cone is not a separate part: the sealing cone can be shaped from the base material. The union nut presses the cone into the connection fitting to form the seal. Some versions add a thrust washer, which spreads the force from the nut more evenly over the cone. One rule comes with it: “The cone’s open diameter should not be restricted, as this would obstruct fuel flow.” Such cones, the book says, are generally made in conformity with DIN 73 365.

None of these sources says when the nut goes on. Our inference from Bosch’s description: the nut bears on the back of the cone, so the cone cannot pass through it, and the nut must be on the tube before the second end is formed. How a finished cone is measured and described is covered in a separate article.

What rule applies to the bends?

Bosch’s rule is short: “All high-pressure delivery lines are routed to avoid sharp bends. The bend radius should not be less than 50 mm.” The bends, the book adds, make up for the different distances between the pump or rail outlets and the cylinders, which is how every cylinder can be fed by a line of the same length.

Which lines are autofrettaged?

Autofrettage, which means pre-stressing a line with internal pressure, is not applied to every line. Bosch speaks of “prestressed high-pressure delivery lines” that meet especially demanding requirements, “for applications of 1,400 bar and over”, and describes the treatment:

“Before installation on the engine, these customized lines are subjected to extremely high pressures (up to 3,800 bar). Then pressure is suddenly relieved. The process compresses the material on the inner walls of the lines to provide increased internal strength.”

Bosch, Diesel Engine Management, p. 170

Why is a line coated and cleaned?

The coating on the outside is there against rust, and cleaning keeps particles out of the injection system. Benteler says on its tube processing page that it galvanizes the outside of its tubes and can add a sealant or an organic topcoat over a coating free of chromium VI, for corrosion protection.

Bosch gives the reason for cleanliness where its book describes the high-pressure components of the common-rail system: as performance has risen, machining has had to become extremely precise, with ever tighter tolerances and fits. Bosch, explaining the fuel filter, states the consequence: particles cause premature wear in the high-precision injection components.

On what manufacturing itself leaves behind, the book is blunter: “Particle residue from the production process may lead to increased wear, or even the total failure of the assembly.” The permitted particle size, it adds, keeps being reduced, and the cleanliness of components is measured during production by light-microscope image analysis.

The same reasoning applies in the workshop. In dealer service instructions for vehicles with a 6.7 litre diesel engine, revised in June 2024, FCA US notes that injectors, high-pressure fuel lines and the injection pump are made to very tight tolerances, and that dirt can cause rapid wear and plug the injector nozzle holes.

What does this mean when you fit a new line?

Four things follow.

1. Cap every open end. The same FCA US instructions say to cap or cover any open part after disassembly and, before assembly, to check each part for dirt and grease and clean it if needed.
2. Do not shorten a line or bend it to make it reach. Bosch’s rule: “Tubing of other dimensions should never be installed during service and repairs.”
3. Look at the bore of the cone. By Bosch’s rule, a restricted opening obstructs fuel flow.
4. Order by part number or by full dimensions. Bosch notes that length, diameter and wall thickness all affect injection, and that the prescribed dimensions must be strictly observed.