Internal/external balancing

When a crank is balances, its balanced to its rods and pistons as a unit. You put a neutral balance 318 crank in a 340 with its heavier pistons and rods and its not going to work. The 340 rods and pistons were heavier than than the 318's even though the 340 cranks were drilled to help a little. Its not like a 4 banger 180 crank where if the piston pack and rods weigh the same its balanced (as they counterbalance each other). a V8 has 2 banks at 90 and they are both above the crank centerline so they need crank counterweights to match the bobweight of the piston pack and rods. The cast cranks were lighter than the forged (less dense) so they had to add weight to the balancer to "Detroit" balance it as a unit while using the same rods and pistons. Its pretty easy to spin the assembly in the motor and then only balance the damper to the spinning motor to get a 'good enough' balance for a street motor. Going back to my first comment in Post #3, it cannot be "internally balanced" if it in fact needs the offset weight of the special "cast crank" damper that looks like an egg! As YR stated, anything can be made to be internally balanced with tungsten, but its like $50 a slug and sometimes you need a few! Far cheaper to just get the correct balancer and be a little over so your shop can take metal out of the balancer perimeter to get you to zero. All 318s were internally balanced, and all their balancers were neutral also. 318 and forged 340 balancers were compatible as they were both neutral.