The request was a huge surprise for Cosworth, and the original brief for a 320 bhp engine based on the 136 bhp Mercedes M102 2.3-litre SOHC 4-cylinder engine was duly passed to Mike Hall, who "drew the famed DFV and BDA engine". Designed around the existing M102 head bolt pattern, the new twin cam, 16-valve, pentroof head, had its valves set at 45° included angle, rather than the 40° angle of the BDA. The valves were the biggest that could be fitted into the combustion chamber. Flat top pistons delivered the 10.5:1 compression ratio. The new Cosworth WAA engine also was Cosworth's first one-piece head, i.e. the camshaft carrier was cast integral with the head itself. Again the constraints of the existing head-bolt pattern meant that Hall had to shift the camshaft bearings from outside each pair of camlobes as in the BDA to in between each cylinder's pair of cam lobes. The upside being that this configuration made for less flex at high rpm.