The exhaust manifold's machining steps were similar to those used on the intake manifold. Differences in its size and shape, though, made things a little more challenging and at times confusing. As the exhaust manifold took shape, it would have become more and more difficult to fixture it as a standalone part. So, unlike the intake manifold, all its machining was done through five faces of the workpiece. The manifold wasn't released from the workpiece until all its machining was completed.
The manifold's ribbed outer surface was machined first. Since the top surfaces of all six runners were also accessible in the same setup, they were machined as well. A long stick-out 1/4" end mill was used for roughing, but the filleted surfaces were finished with smaller diameter ball mills. Special long reach 1/16" cutters running on a high speed 5-axis machine would have been needed to finish a faithfully scaled manifold. Mine was designed around an 1/8" ball cutter running on a Tormach.
Chatter from the long roughing tool was so annoying that I'd normally have left the shop while it was running. As luck would have it though, my Micro-drop coolant dispenser died shortly after starting, and I had to manually spray coolant and blow away chips during the entire two hour run. Fortunately, surface finish wasn't an issue since the operation was set up to leave .007" excess stock for the finishing tools. Total machining time through the first face of the workpiece was about five hours.
The trough left around the semifinished part in the first setup was filled with Devcon 5 minute epoxy in order to keep the part safely suspended inside the workpiece for the rest of its machining. The runners were designed so the counterbore locations needed for the mounting bolt heads would be accessible in this same setup so the special tool used on the intake manifold wouldn't be needed. Their exact depth was critical for the shared mounting bolt scheme used by Ford to attach the manifolds to the head. These were plunge milled once the epoxy had cured.
The manifold's much simpler rear surface was machined in the second setup. This step left the semi-finished manifold securely suspended inside the workpiece by only the epoxy. The mounting flanges and bolt holes were completed in the third setup.
The filleted surfaces immediately below the runners were machined through the fourth face of the workpiece using a long reach ball cutter. This simple finishing operation was done on my Enco mill so I could continually adjust the tool's feed rate and minimize chatter. A portion of the workpiece had to be milled away in order to get the spindle as close as possible to the runners for minimum tool stick-out.
The heat riser cavity was opened up in the fifth and final setup. Another chunk of workpiece had to removed for its access. A mounting recess, machined for the add-on exhaust pipe flange, was shaped to avoid any awkward intersections with the embedded pins and plugs in this rather busy area. The manifold's total machining time worked out to about ten hours.
During the original preparation of the workpiece, I decided on aluminum pins to backup the Loctite'd plugs used to block off the ends of the numerous internal passages. After noticing a black spot that showed up during the heat riser's machining, I thought one of the pins had not been inserted far enough. The spot turned out to be a steel pin that had been mistakenly used in this particular location.
A trial fit of both manifolds to the head without the use of the flange gasket showed the mounting screws shared between them appeared clamp both flanges equally. The flange thicknesses may be fine tuned later after more careful measurements. Both water and compressed air were used to sanity check the continuity of the exhaust passages through the heat riser.
The extended exhaust pipe flange was machined from 12L14 and permanently attached to the bottom of the heat riser. In addition to a pair of mounting screws, the mating surfaces were coated with Loctite 620 for a sealed bond which should stand up to the engine's exhaust heat.
After temporarily plugging all its openings, the entire manifold assembly was glass beaded to give its surface a cast iron appearance. However, the aluminum's natural color is wrong for an iron manifold. I have some POR-15 dark gray paint intended for exhaust manifolds, but it's gotten pretty thick over the years. I've ordered the special solvent this paint requires, and after some testing I may air brush the manifold with it.- Terry
