Next time your exhaust manifold is off the vehicle, hold a straight edge against the flange. I did this to a used, ported 2G manifold and was surprised to find that I had a huge high spot in the center of the flange!
In the picture below you can get a sense of the "hump" in the middle of my flange. It was hard to photograph, and came out more exaggerated than it really was, but it's there.
If you don't have a mill and fly cutter, and you don't want to spend the coin having it resurfaced, you can use some good old fashioned elbow grease and ingenuity. The only tricky part of this process is choosing a good surface on which to do it.
I had a large sample of a granite countertop lying around which is 3/4" thick, about 12" x 18" and it came with rubber feet on the bottom. It was left over from a remodeling project. You can probably get a nice, small piece of finished granite countertop from the scrap pile at a countertop or stone vendor. A thick piece of glass will do fine. Glass and granite are both finished to a very flat surface.
Next you will need some 220 grit WET/DRY sandpaper. You use the entire sheet when you do this, so you'll need at least 5 sheets if your flange is warped a lot. Changing the sandpaper often will make the process faster.
You'll also need some water. A 20 oz. bottle was enough. I tried lapping the flange dry at first, but the paper loaded up very quickly. Wet sanding is superior.
Okay time to begin the art of lapping. This will require patience. Resist the urge to use sandpaper coarser than 220 grit. At first, I taped the paper to the granite, but it didn't hold the paper very well. You could apply some sort of temporary tacky spray or something to the back of the paper, but I found that once the paper was wet on the backside, it held itself to the granite well. I bent an edge of the paper down and laid my hand flat on the paper, with my thumb holding the bent tab/edge of paper against the edge of the granite. This worked nicely.
Once your paper is situated, pour a little water under it, and then pour some on top. Assuming you're doing this on a surface where water dripping down doesn't matter, you can be generous with the application of water while you're sanding. It flushes the paper and lubricates the lapping process.
Set the manifold on the paper like so:
Let the weight of the manifold do the work for you. As long as you keep using fresh sandpaper and rinse it often, it will cut with reasonable efficiency. Using long, slow, smooth strokes, pull and push the manifold back and forth (this is the key to keeping the surface flat). Avoid pushing down unless you are holding the manifold from the collector area. I found the number 1 runner to be the most comfortable place to do reciprocate the manifold. I started doing strokes diagonally across the sheet of sandpaper, then gradually turned my strokes so that I was making diagonal strokes the other way. After a while you will want to turn the manifold around so you are pushing/pulling form the other side.
In the picture below, you can see where I'm starting to knock down the high spot. It took a couple of minutes to make this "clean spot" you can see in the middle of the flange.
Here you can see the cross-hatch pattern and nasty gunk being replaced by a smooth, shiny finish.
Now all you have to do is have the patience to finish the job. Keep going! Keep that sandpaper wet! Change it often! After about 45 minutes I had the middle two runners almost perfectly lapped. Here you can sort of see the a few cross-hatches at the edge of runner 3. The water will get black with iron filings and be very messy. It feels like you're really doing work.
I admit that I was tired, and really needed to get this done quickly, so once I had the middle 3/5ths of the flange looking really nice, I went a little further, then gave up. Please excuse the next picture because I did not clean the manifold surface before I photographed it and it does not look as amazing as it does in real life. With the exception of the last inch on the end, it is TOTALLY smooth and flat.
Keep in mind you can do this operation for other flanges as well! I also lapped the turbine flange!
The following photos were taken after I bead blasted the manifold myself, and applied some expensive Liquid Stainless Steel paint from a company called "Contronics". The paint is also not really that brilliant unless using a flash. The last pics is abmient lighting.
