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I am in the process of doing a HCI build on my 06 LS2 GTO and want to make sure i'm doing this correctly.
I posted in the advanced section, but there hasn't been a mod in since the 18th and the heads are already down with Phil@AI.
I first did the straight edge/feeler gauge, was not cozy with that at all. I then went on to measuring the highest point of each piston, and still wasn't cozy.
After searching on here and a few other sites I then tried this:
Using a deck bridge and dial indicator
For the top and bottom of each piston, I measured the highest point, the lowest point, averaged the two taking into account negative and positive movent and then averaged the averages of the top and bottom
To get the averages for example
Piston #1
Top of the piston
.016 high point /.001 low point
There is .015 of "rock" at the top of the piston (.016-.001) /2 = .0075
Bottom of the piston
.015 high point / .001 low point
There is .014 of "rock" of the bottom of the piston (.015-.001)/2= .007
Average of top and bottom (.0075+.007)/2= .00725 out of hole for piston #1
Once your get the travel/rock figure and divide it by two, you have to take into account the starting point.
For example, if you measure .015" out of the hole and .001" in the hole, you get .0016" of rock. Divide by two and get .008" but the piston will actually be .007" out of the hole because you started at .001" in the hole or .015" out. The .008" figure is a distance from one of those starting points.
Don't forget that dry room temp is not hot with oil on the bores/skirts
Call it zero deck for compression ratios like GM since the aluminum deck grows .007"-.008". Also explains why to stop worrying about loose timing chains at room temp. Cam moves north and tightens the chain on aluminum blocks
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Piston-Deck Clearance, SBE LS2 
