Now that the holidays are over and I have some time to get back to the project...
The business of ending up with a higher c.r. Than desirable for one of the main objectives of rebuilding the engine (turbo), I obsessed with reducing it from 8.3:1 to 8:1
the problem was due to my inexperience and getting the horse before the cart with respect to getting those pistons mounted and not checking volumes and such first. Hopefully won't make
that Mistake again!
Here's the logic behind what I've done to fix that problem. I discussed the situation with the shop that balanced the rotating assembly. They offered to remove the pistons, mill out the relief to my specifications, then reinstall the pistons. They claimed a very high success rate in doing this without ruining the wrist pin fit ("you aren't the first one to get it backwards" was what he told me), but no guarantees, of course. So when I had to make that decision, I shrank back and thought better of it. But since it was clear that removing the same amount of material from the top of each piston would not upset the balance already put into each piston/rod assembly, I figured if I could very carefully grind out the necessary material, confirmed through weight removed as opposed to volume removed, then that could be a path forward. Today I finished doing just that. Here are the specs:
1 cc water = 1 gram. Since dense aluminum has a specific gravity of 2.8, 2.8 grams = 1 cc. So I needed to remove around 5.6 grams aluminum from the top of the piston. Consulting with silvolite catalogue's cross section of the piston, it was obvious the crown was thinnest at the bottom of the bootheel. So I would polish out the bootheel and extend the "d" to roughly complete the circle. Again, the machine shop would have done just this except with using an end mill, my guess was that careful grinding would be the same thing.
I have a triple-beam scale accurate to 0.1 gram. I first weighed each complete assembly (minus rings and bearings). The four piston/rod assemblies were within 0.4 grams of each other. My goal was to maintain this spread, and possibly improve upon it.
I selected the lightest assembly (1,961.0 grams) and using wide masking tape, taped it all up to keep out the grit. I weighed that assembly (adds about 9 grams!), and then began lightly grinding away at the heel's arc with a 3/4" drum sander mounted in the drill press, then switched to a 2" rotoloc disc (180 grit, iirc). Since the top of each piston has very fine concentric lines, it was easy to follow the desired contour. What resulted was actually a very nicely polished surface, much better than what was in the heel to begin with. Of course I'd grind, weigh, grind, weigh, grind, weigh, then do another, compare, weigh, compare, etc. Until I got them all down to about the same weights (within 0.3 gram), and paying particular attention to the pairs that would go up and down together. Got those to 0.2 g or better. When I finished, I cc'd the reliefs and confirmed they were now 14cc instead of the original 12cc. I now calculate a c.r. Of about 8.03:1. Close enough, no? This was a very time consuming process and took a whole day.
I slipped a digit somewhere, that's why I ask to confirm. Anyway, a recheck and you are right of course. Plugging that 6.75" in, I get: 6.35 dynamic, 8.51 w/5 lbs. Boost, and 9.82 w/7 lbs. Boost. Using 64 degrees abdc. What is not clear to me in the data point they are asking, is "degrees @ 0.050" + 15 degrees". I pulled 64 degrees from my grind sheet, and assumed that was when the valve was closed, not almost closed. 
please let me know what you think.
