Poly Engine what was good/bad about it and why is the LA better?

cudamark said
The cranks are not the "same". You can interchange them with some caveats. The convertor register is a different diameter, so, you can put the 318 crank into the 273 if you use a reducer bushing for automatic applications, and you can put a 273 crank in a poly 318 application if you machine the crank register to accept the 727 converter button. Manual trans isn't an issue unless you're going from auto to manual on the swap. Then you need to check to see if the pilot hole got the final machining for the bushing, or, you'll need to use the magnum bearing or make a custom bushing as the case may be. Also, the crank swaps should require a rebalance due to the difference in piston weights. This also only applies to the '62-7 polys, as the '61 and older had an extended 8 bolt crank flange.

Oops, forgot about the smaller register of the early 273's. One interesting factoid: When I swapped a later 225 into my 65 Valiant, I had to open up the pilot hole in the flywheel to fit over the later crank. But when I swapped a 340 into my 65 Barracuda, the 273 flywheel fit perfectly as is. I haven't had a 273 crank to eyeball in almost fifty years, but I assume from the foregoing that the early 273 crank register has the same outside diameter as the later ones, but a smaller inside diameter?? So it could be machined to clear a later torque converter?

Anyway, as to balance, according the Hot Rod's December 63 article about the new 273, since the 273 crank and rods were the same as the 318, but the pistons were lighter, "an extremely heavy-walled piston pin is used in the 273 to bring total reciprocating weight up to that of the 318. This eliminates the need for special balancing. . . ."