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The A was produced from 1956 through 1966, the LA from 1964 on. The basic blocks are similarly sized and hard to tell apart from the outside, though the A's polyspherical heads are considerably larger than the LA's heads. Bill Watson wrote:
The A block was fractionally bigger in external dimensions than the LA block, although it was heavier due to the new casting techniques. The LA block 273 weighed 55 pounds less than the poly 318, both used 4.46" bore centres, and both were 21.4" in length, without water pump, crankshaft, etc.
The poly heads are much wider and heavier than the LA's wedge heads and that gives the A block engines the appearance of being bigger than the LA engines. The 273 heads weigh 12 pounds less than the poly 318 heads.
Chrysler was able to lower the height of the LA 273 over the poly 318 by using the wedge combustion chamber with its simplified valve arrangement plus a lower, and 12 pound lighter, intake manifold. But the headless blocks are virtually identical in height, length and width.
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