The Crossflow featured a change in combustion chamber design, using a Heron type combustion chamber in the top of the piston rather than in the head. The head itself was flat with each engine capacity () featuring different pistons with different sized bowls in 681F and 711M blocks. The 691M block had the stronger 'square' bearing caps later used in the 711M, and small combustion chambers in the near-flat head (the bulk of the volume being in the piston bowl). In 1970, the new A711 block for and A711M block for were introduced with thicker block wall, square main bearing caps, large diameter cam followers and wider cam lobes, with the latter block having a 7/16" taller deck height, together with a return to the flat head. These changes represented a significant improvement in the reliability of the engines, and the blocks are commonly referred to as '711M' blocks.