Chrysler's Poly-Head Engine - Mopar's Rodney Dangerfield Part 1

For those of us with Chrysler Corporation products built after 1966, the small-block family consists of engines derived from the "LA" 273 package introduced in 1964. These came in 273-, 318-, 340-, and 360-inch displacements. A majority of the speed parts now available for small-block Mopars fit this package, with the 318 LA engine being the most common and pedestrian of the group. However, for people owning cars from 1955 to 1966, there is another "A" engine, commonly known as the poly-head small-block.

This name came about due to the polyspherical nature of the head design; it's not a wedge design like most other non-Hemi engines. Instead, it used a canted-valve or semi-Hemi layout, much like race engines use today. Unfortunately, since the '60s, the poly engine line has been "dissed" as little more than scrap metal, something to stick behind the garage since it was too heavy to tote away. There were some reasons for that, however. The engine, based on the early Hemi hardware, is a good 70 pounds heavier than the small-block that succeeded it. Unlike the LA engine line's 340, there were few factory performance packages for the poly engine, none after 1962-the year Mopar's styling cues and performance era really began. It didn't help that the largest displacement the engine went to was 354 cid.

Nonetheless, with many still in service, this article will serve as an introduction to the engine line, helping you ascertain the parts that are available, interchangeability, and engine identification. Moreover, if you have the urge to "kick up" your poly a bit, there are some tricks that will let you get more thunder out of it.

Engine ID: A Poly Primer
Chrysler's poly-head engine is unmistakably unique from other Mopar engines. It is easily distinguished from the later LA counterparts by its wide stance in the engine bay, looking more like a big-block than a small-block (the distributor location is in the rear, however). The valve cover design differs from the LA motors (273, 318, 360) in that they attach with two bolts in the middle of the valve cover (three bolts for the '56 Plymouth 277-inch poly) rather than the five small screws around the outside of the later LA motors. Some early versions used a unique scalloped valve cover.

With a valve cover removed from the poly motor, the intake valves are on one side of the rocker shaft and the exhaust valves are on the other side; in the later LA motor, the valves are all to one side of the rocker shaft. As a result, the exhaust ports on the poly are evenly spaced, similar to the Hemi design, as can be seen when the exhaust manifold is removed. These poly-head exhaust manifolds run parallel to the heads on each side, like a log, with very little gap between the manifold and the head. Then they exit downward at a sharp angle at the end of the cylinder head-not high-flow by any stretch of the imagination.

As previously stated, there are some variances between the Dodge, Chrysler, and Plymouth poly motors. The earlier Plymouth poly motors have a closed valley cover which is sealed by the intake manifold itself, while the '55-'58 Dodge and Chrysler polys have a valley cover pan under the spider-like runners of the raised intake manifold (we call them air-gap manifolds now, but Mopar had them in the '50s on the poly). The timing-case cover on the earliest Dodge and Chrysler polys are similar to the early Hemi engines, while a different version in both appearance and fit can be found on the Plymouths.

The engine ID is on the front left bank of the block next to the timing-case cover below the cylinder-head face. The first letter and digits may help identify which year poly engine one may have, though the external design of the poly head is self-authenticating. Chart A is a list for basic identification of these powerplants.