Ford created a mountain we racers couldn't climb. The 66cc number was arrived at for the HR and later carried over to the MR. Problems arose because the early HR combustion chamber was delivered to customer at around 78/80cc from the factory and only required removal of about .060 to .070 to achieve the 66cc minimum. During late 64 when the MR was introduced (MY 65), the engineers saw fit to increase the as-delivered chamber volume to near 90/93cc. on both the MR and HR heads produced after that point. That meant that we had to mill 0.125 to 0.135 just to get to NHRA minimum cc's. I recall that the Ford blueprint spec for nominal finished deck thickness was in the 0.190 to 0.200 range. A deck .065 to .075 thick will crack at compression ratios well below the 14:1 blueprint spec for the HR and MR engines.