The Poly Head:

In 1950 Chrysler consisted of four divisions; Chrysler, DeSoto, Dodge, and Plymouth. The pecking order or hierarchy between divisions being just about in that order. Of the four divisions, lowly Plymouth was the only one never to produce the early hemi. Their V8 production consisted of the "Poly" head (twisted wedge head) design. This is significant because the '55 and '56 Plymouth Poly head engines were based on the Dodge hemi block and these motors can be a very good source of parts for the early hemi enthusiast. Chrysler and Dodge produced poly head engines as well. Chrysler's 301, 331, and 354 polys and Dodge's 270, 315 and 325 polys can easily be converted to hemis by adding the proper heads, intake and exhaust manifolds. It is important to understand that polys can be converted to hemis only within an engine family. Chrysler polys accept Chrysler hemi heads, Dodge and early Plymouth polys will accept Dodge hemi heads. DeSoto never produced a poly head motor so DeSoto hemi heads won't work on anything but a DeSoto hemi. (The 325 ci. Firesweep Poly head motor installed in some 1957 DeSotos were actually Dodge motors built in the Dodge engine plant.)

Drawing curtesy Motors Auto Repair 1959

One special note here. In 1956 Plymouth began production of a new series of wedge head motors. Although still referred to as Polys or new generation Polys these "A" series motors are actually more closely related to the later "LA" series. The "A" and "LA" motors share many parts between them but have little in common with the earlier version.

Drawing curtesy Motors Auto Repair 1959

Plymouth eventually produced 301, 303 and 318 ci. "A" motors while in 1959 Dodge began production of their own 318 and 326 ci versions. The latter being produced for only one year. "A" series motors should not be confused with the earlier Plymouth and Dodge poly head engines or the Chrysler Polys of similar displacement. Parts between the early Poly/Hemi design and new generation Poly generally will not interchange. Be very careful when searching for spare parts in the local wrecking yard and learn how to distinguish between the two. All Polys can be easily recognized by the scalloped lower edge of the rocker covers, however in the new generation "A" series Polys the scalloping is much less dramatic. More like a wavy line than a true scallop.

Note that in the Hemi design the valves are placed on opposite sides of the cylinder. The early Poly still has the valves more or less placed that way, however they have been moved much closer to the centerline of the cylinder. On the "A" series Poly head the valves are positioned on one side of the cylinder the same as the later "LA" series.