A 340 piston weighs in at 720 grams, the 360 piston at 571, IIRC. There's NO WAY the 0.040" difference in bore could make a 150 gram difference.

Basically if you calculate that weight increases by something like the square of the diameter change, the piston should have weighed another 10 or 20 grams, or something like a 318 piston, at around 590 grams.

BTW, the 318 Poly, the early LA 318 and the 273 used THE SAME FORGING for the connecting rod. It weighed something like 725 grams. The 496 rods used in the 340, and the 645 rods used in the 340,360 and later on the 318, weighed 758 grams.

Put those overweight rods together with the overweight pistons and you see that 340s had a very portly rotating assembly. It should be easy to take a half a pound out of each piston/rod combo.

R.