Continuing on with my effort to smooth the intake manifold, I decided to copy my "gasket delete" o-ring technique to the joint between the lower and upper manifold sections.

This is the joint in question.  One gasket seals all 4 runners.

I mounted a scrap cylinder head to the table of the mill, which served as a perfect fixture. The mounting flange and sealing surface are exactly 90º apart.  Once mounted up, I indicated the sealing surface parallel to the machine travel and found the center of each runner hole.

In order to maintain a consistent groove depth, I faced the manifold flat with a fly cutter.  The surface was only flat to about 0.007", so I removed 0.008" in small skim cuts.

Milled flat. The surface finish isn't the best I've seen, but not too bad for a casting.

Next I mounted a hand-ground HSS face grooving tool in my boring head and set it up (using scrap test pieces) to turn a groove.  I am using a standard Buna-N -132 o-ring in a 1.700 x 1.950 x 0.084" groove.  The runners are about 1.600" inner diameter, spaced approximately 2.010" apart, leaving 0.050-0.060" of land around each o-ring.

After grooving, I came back with the fly cutter and removed another 0.001" to make sure no burrs would be above the surface.

Success!! Again, not the most spectacular surface finish I've seen, but it will work just fine.

Before removing it from the machine, I test fit the upper half of the intake manifold.  I made sure the mating surface was in full contact since the o-rings apply sealing pressure around the hole, which is not exactly the same as the gasket contact pressure as originally designed.

The next step will be match-sanding the upper and lower halves so they appear seamless once bolted together.