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How to Size and Drill Fuel Rails (Fuel Rail Worksheet)
Extrudabody offers fuel rails in many lengths with two options for injector mounting holes: predrilled or undrilled. Undrilled fuel rails are available in 3 standard lengths (8", 18", and 24") and are made for customers who can tackle hole drilling themselves and save some money. For an additional charge, we can custom drill the fuel rail and also cut it to a specific length (both are subject to extra charges - see www.extrudabody.biz for current pricing) Whichever way you go with fuel rails, you'll need to determine where to place your injectors on the rail and whether you'll need a custom length or use a standard length.
1. Determining Fuel Rail Length
Step 1. Measure the distance between the farthest two intake ports on your intake manifold. Measure from the center of each port (i.e., center-to-center distance).
Step 2. Add at least 2.5" to the result of Step 1. This result is your Minimum Total Rail Length.
Step 3. Choose an Extrudabody standard length that is at least as long as your Minimum Total Rail Length. Standard rail lengths are: 8", 18", and 24"
Step 4. Examine your engine bay to determine if you will need to custom cut the fuel rail to fit your application. You will need at least 1.5" of clearance on each end of the fuel rail to attach fuel fittings and hose.
For example, you may calculate that you need a minimum of a 22" rail. Therefore, you need to determine if the closed standard size rail will fit, in this case our 24" rail. Add 1.5" on each end for fuel fitting and hose clearance, and you will need 27" of space for your fuel rail. You may decide this would be too tight a fit and opt for a custom cut 22" rail. Luckily, if you buy a 24" rail and decide you should have gone with a 22" custom length, you can have just about any machine shop mill it down a couple inches for a minimal cost.
Step 5. Order a custom length rail(s) or a standard length fuel rail(s).
2. Determining Injector Hole Location
The easiest way determine injector hole location is to have your throttle bodies and fuel injectors mounted on your intake manifold, then just mark the fuel rail with a permanent marker right in the middle of each fuel injector. You need to be within a millimeter of each hole, so mark once, then do it again to make sure. Then continue on to Section 3 on this page titled Drilling the Fuel Rail.
Figure A: Injector Hole Locations

The next easiest way to determine injector hole location is to use you intake manifold (see Figure B below). Since the throttle bodies will be aligned with the intake manifold passages, the fuel injectors will be placed exactly in the middle of each passage. It's hard to measure to the center of an open hole, so measure the distance between each hole two ways: from the left side of each passageway to the adjacent passageway, and then from the right side of each passageway to the adjacent passageway's right side. If the distance between any two passageways differ, measure again. Most manifolds are symmetrical and passageways are evenly spaced, or pairs of passageways are evenly spaced.

Figure B: Measurement Points Using a Manifold:

If you don't have an intake manifold yet and want to order a pre-drilled rail from us, just drop us an email or better yet give us a call. We've probably done the same type of manifold before, or may have one around.
If you order a custom manifold, please make sure you have the info to fill in all the blanks below:
Your Total Desired Rail Length (T) ____________________ Total Number of Injectors Per Rail _____________________ .
Fuel Injector Hole Diameter _____________________ (generally 11 or 14mm for most applications)
Next, enter “center-to-center” measurements in positions below:
A. Distance from Rail End to 1st Injector = __________________ (Note: Minimum is 1.25”)
B. Distance from 1st Injector to 2nd Injector = _________________ (stop here for a 2-injector rail)
C. Measure from 2nd Injector to 3rd Injector = _________________ (stop here for 3-injector rail)
D. Measure from 3rd Injector to 4th Injector = __________________ (stop here for 4-injector rail)
E. Measure from 4th Injector to 5th Injector = __________________ (stop here for 5-injector rail)
F. Measure from 5th Injector to 6th Injector = Position F _____________
G. Distance from other Rail End to last injector is assumed to be the same as Distance A. If you are doing a custom rail, let us know if G is different from A.
If you are using the fuel rail to replace Weber-style carburetors, your bore spacing between each "pair" of throttle bodies is as follows:
DCOE = 90mm
IDF = 90mm
IDA = 120mm
Note that if you DCOE, IDF, or IDA manifold measures different than the above, then you manifold is most likely a little bit off. This is not uncommon due to variability in sand-cast aluminum manifolds.

3. Drilling the Fuel Rail
1. Use only moderate force to clamp the Fuel Rail for drilling. Over tightening can easily cause deformation of the the aluminum Fuel Rail slot.
2. Support the “T-Slot” with the End-Cap Mounting Nuts to keep the vise from crushing the T-Slot!
3. Do not over torque the End-Cap Bolt. The stainless steel bolts are stronger than the aluminum Rail and can mar the aluminum threads if tightened above 10 ft-lbs.
How to Drill:
A. Mark the Fuel rail to the desired injector Bore Spacing (calculate bore spacing using the instructions above). Below is an example of an inline 6 cylinder fuel rail.

B. Support the T-Slot edge as seen below with the T-Slot Nuts and DO NOT over clamp the rail in the vise! If you have a jagged vise use 2 pieces of scrap aluminum or rubber to protect the finish!
C. You will be drilling holes as pictured below. The lighter gray indicates fuel passages within the rail. You will be drilling through the rail to create the fuel injector holes that will connect with the pre-existing passage in the rail.

C. On your marks, drill 1 hole at a time. Start by using a ¼” drill bit and drill through to the inner fuel passage. Then use the 3/8" bit, and again to all the way through to the fuel passage. Use bits you know to be sharp and take your time. Let the drill cut the aluminum and do not force it through the Aluminum, especially when emerging into the inner rail - this will keep burrs to a minimum.

D. Now drill the hole 9/16” or 14mm x 8.5mm Deep if your using an “end mill” if you are using a normal drill bit, drill to a depth of 8mm. If you do not have a 14mm Drill Bit/End Mill, use 9/16” since it is close enough that the 14mm O-ring on the injector will still seal. If you will be be running a very high fuel pressure (over 45 lbs), make sure you use the exact 14mm bit.
E. Do one hole at a time and you will have great results! The key is to use a vise and to clamp the vise so it will not move on the drill press.
F. See below what a finished injector hole looks like. Note this was drilled with a 9/16” Drill bit also notice the Funnel shape at the base of the Hole! There is also a slight chamfer created on the outer rail by using a ¾” Drill bit. This chamfer helps ensure that the fuel injector o-ring will not become damaged during installation.

1. My seal area (side walls of the hole) is jagged or ruff. Is this OK?
ANSWER: No. Smooth the seal area by using a piece of 220 Grit sand Paper rolled into coil as shown below and Slowly twist the paper in the direction as to unroll the coil, this opens the paper in a uniform manner and exerts pressure on the seal area. Simply twist back and forth to polish the Bore to a smooth surface. Moving to finer grits will improve the seal area.

2. My Holes are off center.
ANSWER: It’s ok to not have them perfect, plus or minus a millimeter or so is no sweat because the O-ring is pretty forgiving. If you are in doubt take it to a professional machinist or call us about our 1-day Custom Drilled Manifold Service. Take extra time to pressure test you fuel system for leaks before starting your vehicle.
3. Can I make the rail shorter?
ANSWER: Yes, our fuel rail design uses end-caps so that just about any custom length is possible without having to drill and tap the ends. Technically, you can cut the rail using an angle grinder or cut-off saw, but you would need a metal planer to true the ends. It's better to have the end or ends on a professional milling machine to the desired length and de-burr the edges. Make sure you put a chamfer on the Fuel Rail Bore to help the O-ring slide in easily. A little bur can do a lot of damage to an o-ring.
Neat Rail Modifications:
1. Below is a Picture of a Wire Cover made using some stock “C” Channel form a home improvement Store, it’s 5/8" x 1/2" stock. The rail was milled 2mm deep x 5/8" wide to receive the “C” Channel. A couple of 6/32” button head screws finished off the cover. If you have a custom idea send it to us, we will post it for others to see.


7,399 Posts
Discussion Starter #5
reason y i posted this up is cuz when i upgraded to a 3"maf i need to upgrade the fuel injectors and pump n/a. so imagine if u put a 3" maf on a boosted car, "u might need to upgrade the fuel rail"

26,965 Posts
yea only reason you would need a fuel rail of that size is if youre putting out some serious power.. and those aluminum rails go for like $20 bucks on ebay.
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