I started with a scrap piece of 1/2" 6061 aluminum, chucked it in the vise in my CNC machine and milled the center hole, then milled the outside diameter offset by .070". The center hole was milled all the way through, but the outside edge was only milled to a depth of .400". After that I flipped the piece over to cut .100" off the top and drop the part out, but it took several passes to do this because as the material got thinner and thinner the part wanted to shake, and it was too small to really stabilize with some kind of a clamp. I ended up cutting most of the way through, then breaking the part out of the material by hand and finishing the back side with a file to remove the final amount of aluminum. Also the OD and ID are very slightly egg shaped, probably due to compression from the vise warping the material when it was clamped in place. Minimum OD was 1.220", and max was 1.225", measured 90 degrees from each other. It should reform into a circle when it is pressed into the hole in the manifold. Also due to the hand finishing the final thickness came in around .390" rather than .400". Again, it shouldn't matter in the application.
What I'm most concerned about is the press into the hole in the manifold. If your measurement of the hole at 1.223" is accurate, it should press in just fine, but if not it could be loose or tight. Since there's not really any load on the piece, if it turns out to be loose you could just use some Right Stuff or Motorcraft TA-31 to glue it in position, or even put in a set screw from the front of the manifold casting.
Anyway, let me know how it works out - Jay
