Semi Hemi - 318 "Poly" A-Series Cylinder Head Deep Dive - Why It Exists, Swap Questions, And More

In the beginning, there was a Hemi. And it was good. Of course, because we're talking about Chrysler here, anything that costs money is too expensive, so engineers quickly set about figuring out a way to make the Hemi cheaper. Enter the original "poly" - a single rocker shaft cylinder head design that dropped right onto Dodge and Chrysler Hemi blocks starting in 1955. It borrowed many of the basic design characteristics of the dual rocker Hemi, but accomplished almost the same job with a single rocker shaft, smaller parts, and less material in general. For the following year, Plymouth would finally get their own V8 in the A engine - which would grow to the 318 cubic inch size we know and love in 1957. The A engine was based roughly on the dimensions of the DeSoto Hemi, existing in the Goldilocks zone between the diminutive Dodge and the much larger Chrysler engine. Further cost cutting soon followed, and before long the A engine would power basic vehicles from all of Chrysler's brands across the board. On today's episode, I'm tearing down one of the cylinder heads for my 1964 Dodge W200 Power Wagon. Along the way, we'll investigate the architecture of this cylinder head, compare it to the Hemi that came before and the LA wedge head that came after, examine some of the finer details of the valve and rocker arrangement, and answer one big question I've seen asked many times - can I put Poly heads on my LA engine block?