360 cheap-o build

The crank's counterweights are there to counter balance the weights of the pistons, rods, pins, etc. that are flying around them. These counterweights have to be of the proper weight to do that. So in the balance process, the weights of the crank counterweights are modified.

They don't actually use the pistons, rods, etc. flying around the crank in the actual balance process. A 'bobweight' is computed and that is what is actually physically attached the crank throws (rod journals) in the balance process. The computation of the bobweight is just math, based on carefully measured weights of the piston, pin, rings, locks, rod bearing, and both the big end and small end weights of the rods weighed separately. An assumption is made on how the rods' end weights distribute into the reciprocating and rotating weight components; 50% is the usual resulting math factor that gets plugged into the formula. You can find that bobweight formula on the Eagle site and elsewhere.

With the bobweights attached, the crank is spun on a machine that tells the operator what need to be added or removed where and on what crank counterweights. (Some weight is also removed at the factory inside of the outer crank throws in the case of the 340.)

For a 360's and the '73 340, part of the balance in the crank counterweights is contained in the damper and another part in the flex plate or TC, or in the flywheel. These are known as 'external balanced' motors, which is a misnomer, as it really is a 'partially externally balanced" engine design. So for those engine, you have to supply these external crank parts to the shop for the balance work too.

As an aside, Most folks will just hand all of this off the shop. If you are good at measuring and math, you can do all this bobweight computation. With well known rod and pistons weight numbers, I have done it myself and just given the bobweight number to the shop to work with; it saves considerable $$ on the balance work.

That is one beauty of good aftermarket rods; they come already weight matched AND with the big and small end weights already measured for you, and with weight matched pistons like KB's or the good SpeedPro's, you can compute the bobweight, let the shop do just the crank balance, and match up the pistons and rods yourself with a scale. That trick pays for a good portion of the new rods. With new lightweight KB pistons, I'd seriously look at this trick (but only IF you are comfortable with the math!) as a way to pay for a large portion of new lighter rods like SCAT's. Much weight and stress are taken off the crank, so that is another bonus, and makes the rebalance $$ spent even more productive if you are going to hammer on your engine at all.

BTW your dad sounds too smart or your own good ....LOL