The A was produced from 1955 through 1966, the LA from 1964 on; the LA was a modified version, with the most obvious difference between the lightweight (hence "L"A) or thin-wall casting and heads. The difference, about 50 pounds overall, was significant enough; but the LA was also considerably smaller, so that the 273 (the first LA) could be stuffed into the small Valiant engine compartment. The A engines' poly heads were wider and heavier than the LA's wedge-shaped combusion chamber; the LA's valves were simpler too, saving money and size. In the end, the 273 was only about fifty pounds heavier than the slant six!
Bill Watson wrote: "The LA 273 used the poly (A) 318 crankshaft, bearings, bearing caps, vibration damper, and conecting rods. The 273 and 318 both had a 3.31" stroke. Pistons were different, though, as the LA bore was smaller; and camshafts could not be shared due to the different valve arrangements. They did share a timing chain."
