SL6 EFI Manifold & Fuel Rail Construction

by Doug Dutra

Let’s review our options to setup an SL6 intake manifold for EFI injectors. As many people know, there are a number of different intake manifolds available for the SL6 and this boss mounting process will work for all of them. Please note that the newer Clifford SL6 intakes have "bosses" already cast into the ends of each runner. If you have one of these you can simply machine the correct sized o-ring hole (.540) right into the manifold and skip the separate boss "insets" installation process as outlined here. Also note that the Die-Cast 2 piece & "E-Beam Welded" SL6 intakes have really thin walls so these are not a good choice for this modification. This insert process will work for manifolds made in Cast Iron or Aluminum with .18 or greater wall thickness.

In general we want to get the injectors mounted into the SL6 manifold and then create a fuel rail to "feed" them. I will list a number of options for building the manifold and fuel rail(s) but will spend the most time on a "low dollar, do-it-yourself" procedure using junkyard parts. Another way to get this done is to simply send your manifold out to an FI shop and have them set it up for you. See or for more info and pricing. There are many web sites on Fuel Injection so take a look but expect to spend $500 to $1000 to have manifold and fuel rail(s) made by a shop. Complete EFI systems can run $2000 to $3000 and these will include all new parts (not used wrecking yard stuff!).

ManifoldI have outlined a way to get injector bosses into a common Cast Iron SL6 1 BBL intake. The "missing link" part that I ended up making myself were the screw-in bosses. I did this because the ones I found were either "weld-in" (3/4" OD "Tubes", 1.5 inches long) or expensive "screw-in" type, with a ¾" -16 UNF thread ( or, listed under instructions - fuel management). What I did was modify a plumbing supply ½" NPT OD x 1/8" NPT ID x 1" long "adapter bushing", machined into "screw-in bosses" ( ½" NPT OD) and a stepped, smooth-wall ID, .437 through & a .540 to .550 (14mm) counter-bore (~ 9/16"). (**See Sketch**) Another good option for raw boss material is a ½" "flair plug". These are a bit longer (1 ¼") and made of brass. The bosses are installed into 45º angled tapped holes and sealed by using "liquid steel" type epoxy.

The tools I used to do this work included a ½" chuck drill motor, a 3/8", 14mm "burr" & .700 diameter drill bits, a ½" NPT tap, a tape measure, center punch and other common hand tools. I started by marking the positions of each hole to be drilled. Use the center manifold mounting flange hole as your "zero point" and work out to both ends marking at 1", 5" and 9" inch points (4" spacing). Locate your pilot hole right where the mounting flange tab slope goes completely flat onto the top surface of the manifold. Drill a 3/8" pilot hole straight down. This is done to give the bigger drill bit a "path" to follow. I also drilled straight down with the larger drill, then slanted the holes buy pulling the drill sideways after it was through. In order to help me get the correct angle and be consistent hole to hole, I made-up a wooden block with a 45º angle cut on the end to act as a visual guide. Use a small carpenter’s square to keep the holes perpendicular to the front mounting surface. Re-check the "squareness" as you tap out the holes. Doing this will maintain the 4" spacing between the installed injectors. Note that the middle two injectors are spaced two inches apart, the rest are on the 4" spacing.
(O -4"- O -4"- O -2"- O -4"- O -4"- O)

Injectors, Adapters & ToolsThe actual "bosses" (bungs) are modified "off the shelf" plumbing fittings that cost about $1.30 each. It is best to do the re-machining on a tool room lathe but to be honest, this can also be done by hand with a drill motor. The first step is to open up the through hole to either 7/16" or ½" inch size. It is best to drill in from both ends in order to keep the hole "centered". I was able to find a "rotary file" (die grinding "burr") at the needed 14 mm size, so I used this to generate the slightly larger counter-bore (.545 diameter x .550 deep on a 1" long bushing). Using this tool to make the counter bore took a little longer but left a really nice surface finish. I used a 45º countersink to put a "lead-in" at the entrance to the counter-bore and "blended-in" all the sharp edges to reduce the chance of a cut or nicked o-ring.

The next step was to "fit-check" the bosses into the manifold. I ground a small dip into the manifold’s top runner surface to allow clearance for the bushing’s hex. The goal is to screw the 1" long bushing in as far as possible. (The 1 ¼" long "flair plug" is less of a problem here but it has a less common 7/8"-14 UNF thread size to contend with.) Once the bushings are installed as intended, mark the part of the bushing sticking down into the runner passage with dye or paint, then number each bushing to keep track of where they go. This is a good time to install some injectors and to measure the spacing and the height between them all. If you are using smooth sided bosses to be epoxied in, it is best to make up your fuel rail before doing the gluing in the bosses so the rail can hold and position all the bosses during the gluing step. This is your last chance to adjust the boss’ position(s) so doing this final fit check and making any adjustment(s) now will save you a lot of time later. Once everything is in the correct position, remove the "bosses" and grind off all that excess material you saw hanging into the runner passageways. Doing this saves a lot of "inside the port" grinding work after gluing.

GluingMake sure everything is clean and dry, and we are ready to glue! Mix the epoxy and apply it generously to both the boss and the threaded manifold hole, then screw the boss in tight. I worked quickly to get all the bosses installed first, then went back to wipe up all the extra epoxy. I left a "fillet" of epoxy under the hex of each boss to ensure a good seal. The same basic process can be done for screw-in or smooth sided bosses but use the fuel rail as a holding fixture if you are using smooth sided type. Once the epoxy is cured, you should finish grind the inside of the runner to remove any excess epoxy or protruding boss material. Congratulations, you now have a Fuel Injection Manifold for your Slant Six! My total "out of pocket" expense was $11.33 (bushings and some epoxy). Now let’s make-up a fuel rail for it.

To be Continued...

Copyright © 2001 by Doug Dutra, All Rights Reserved