How to Build a Fuel Rail

This article is intended to guide you through the process of fabricating a low-cost fuel rail for engines with a perpendicular injector mounting from scratch. We will cover versions for both barb style injectors and domestic 14mm O-ring types. A sample O-ring style rail is shown here. You will need access to a lathe, drill press, vise, calipers and welding equipment, preferably a TIG welder. We recommend that the basic rail components be constructed from either mild steel or stainless steel rather than aluminum as steel gathers far less heat after a hot shutdown leading to less fuel boiling in the rail. Precision is important here as misaligned bosses, porous welds or wrongly dimensioned or finished pieces may lead to a fuel leak.


The main rail piece can be fabricated from a piece of 3/4 inch square tubing with a .050 to .063 wall section. Square tubing makes jigging other pieces far easier than round tubing. The rail must be long enough to span the distance between the injectors on one cylinder bank. Add inch to each end from the injector center lines to allow for welding of the boss and end caps.

Cut the tubing off straight with a hacksaw or bandsaw and file finish the ends to remove burrs.

Precisely measure the distance between each injector on your intake manifold and write these dimensions down. Scribe a centerline mark on the fuel rail from one end to the other and also scribe an intersecting line halfway down the rail. From this second mark, you can transpose your injector spacing dimensions onto the rail, ensuring that the end locations are centered on the rail.

You should measure again to be sure that you have accurately marked the proper injector spacing. You can now punch mark these locations. Using a 1/8 drill bit in a drill press, drill through one side of the tubing on your punch marks. Next, step up to 1/4 inch then drill to the final size of 5/16 inch. Radius the top edge with a 3/8 bit and deburr the inside of the tubing with a long, slender file. Be sure to remove all burrs and carefully flush out all filings.


O- Ring Bosses

For injectors with 14mm domestic O-rings, use 3/4 inch round bar stock. Cut off the number of pieces needed at around 9/16 inch lengths. Chuck these pieces in the lathe and face both ends until the total boss length is .450 inch. Center drill each boss, then use a 1/4 bit and finally a inch bit to bring the boss ID out to .500 inch.

Using a 3/16 or 1/4 inch tool, bore the boss ID in careful steps out to .540 to .545 inch. Chamfer both ends at 45 degrees to permit easy O-ring entry. The ID finish must be smooth to prevent fuel leakage so use a fine feed and an ultra sharp tool here. If the finish is not perfect, you can carefully polish the ID with 400 grit paper on a die grinder. Do not exceed .550 inch ID. O-ring crush must be .020 to .035 for proper sealing.

rail rings

If you are building rails for use with 10 or 11mm O-ring injectors, the same procedure as above can be used except the boss ID will be smaller. Carefully measure the injector O-ring OD without crushing it with the calipers. Subtract .020 to .025 from this dimension. This is what you should bore the boss ID to.

For injectors using an 8mm (5/16) barb connection, use an unplated 3/8 NF or NC nut for the boss. Drill these out to 23/64 in the lathe and thread with a 1/8 NPT tap. You can then screw standard 1/8 NPT to 5/16 brass hose barb fittings into these which will fit the 5/16 ID hose used to connect to the injectors.

rail to hose connection

End Caps

Rail end caps can be made from 3/16 to 1/4 plate stock. Finished dimensions should be 3/4 X 3/4 with rounded corners to match the ends of the tubing. Drill an 11/32 inch hole, dead center in both caps and thread with a 1/8 NPT tap. These threaded holes will be used to mount the fuel inlet fitting on one end of the rail and the fuel pressure regulator on the other end.


Mounting Tabs

Mounting tabs to hold the rail to the intake manifold will vary widely depending upon the space limitations and design of the manifold. We are showing a typical design with staggered tabs. These can be made from 1/8 plate stock. We are using 4 inch long 1/4 NC bolts to hold down the assembly to the intake manifold. You will also have to weld some small, threaded bosses to the manifold for the bolts to thread into.

rail tabs

Assembling the Pieces

Center the O-ring bosses over the fuel holes drilled in the rail. Clamp in position and tack. Weld all around, being sure that there are no pin holes. The next step is to weld the end caps in position. Be sure to face the larger threaded end outwards for proper fitting engagement. Finally, the mounting tabs can be welded on in the correct positions. Let the part air cool, then flush throughly with hot, soapy water and blow dry. If using mild steel, rub the part down with lacquer thinner and paint with a high quality engine enamel. Light colors are a better choice as they absorb less heat than black does for instance.

The upper end of the injector may either butt up against the rail itself or you may use C-clips fitted to the groove in the plastic upper body to sit against the O-ring boss itself.

rail assembled

Injector Bosses on the Intake Manifold

All Bosch style injectors use a lower body dimension of around .630 inch just below the solenoid casing. The injector can be sealed to the intake manifold by sliding a 5/8 ID- 3/4 OD .065 section O-ring over this area. The injector boss may then have a recess machined in its upper base .750 OD by .040 deep. This will capture the O-ring to prevent it from squishing out.

rail boss

Injector bosses can be made from either steel or aluminum 1 inch round bar stock depending on the material used for the intake manifold. If you plan to seal the injector to the boss as described above, bore the ID to around .645 inches. This will prevent metal to metal contact and reduce heat transfer and fuel boiling in the injector. Counter bore the O-ring capture as described above. The boss can be cut off at any angle required for proper mounting to the intake runners, 45 degrees is common.

To use the 14mm O-ring which is already in place to seal the injector, you will need to step bore the injector boss inside. Use the same .540 to .545 ID as was used on the O-ring bosses for the fuel rails in the lower end of the injector boss. The upper end of the boss can be bored to .645 for injector body clearance and the same O-ring can be slid over the body to provide a crush zone for better sealing and less heat transfer.

Injector depth is critical. It is important to ensure that the fuel spray does not impinge on the injector boss wall before entering the intake runner. It is generally OK to have the end of the pintle up to 1/4 inch recessed inside the boss from the runner wall to 3/8 inch projecting into the airstream.

When using gasoline as fuel, the injector can be placed from 1 to 4 inches from the port. When using alcohol, the injector should be placed further from the port to aid mixing. This will often increase power while hurting emissions slightly.

The injector bosses on O-ring style injectors must be precisely placed for depth and angle to ensure proper fit and sealing. This should be repeatedly checked using the whole rail with injectors pressed in. Clamp the bosses in place before welding.

For more details on building fuel rails:Link 1
Link 2