The head clearance on my C6 out of the box was only 0.020". I like to run around 0.028" with 30% O'Donnell Race Fuel. There was only 0.3mm of shims under the head, so I added another 0.2mm shim.
I used the Cyrul break-in method as it is very quick and has been kind to all my .21's. The engine was more than I expected for the price (~$400USD new). It was a tough weekend for tuning engines..... 95F and about 90% humidity. Everyone was running temps in the 230's just to get any performance. I took 0.1mm out and that really helped the engine since there was not much "air".
I would definitely recommend the C6 to anyone
