2002 Grand Prix crank balance gone wrong? (pics inside)

[crzydmnd72]

We are in the midst of rebuilding the engine for my son's 02 Grand Prix. Block required a .020 overbore, so he bought some Federal Mogul pistons. The crank was L67, rods are L32. We thought it wise to balance it along with all the other work. Machinist is a one man shop and semi-retired so he took his time which is fine. He called to tell us he was delayed because the crank needed tungsten added to it to balance it, then called several weeks later to tell us it was ready and did not need any tungsten added. We were surprised to see ALL THE HOLES drilled in it, and have doubts as to whether he knew what he was getting into. So I am posting here to get some opinions.

[crash93ssei]

Wouldn't this be part of the reason for the balance shaft?

[95naSTA]

The engine's rotating assembly is balanced by the weights in the harmonic balancer, flex plate, and crank counter weights.

The harmonic balancer is meant to cancel out firing order vibes created from using a 120* firing order in a 90* v6.

[MattStrike]

In order to balance the rotating assembly the machinist needs to have the harmonic balancer, the flexplate, at least one full piston and rod assembled, bearings, rings, bolts and the crank. Yes he would have to take out a lot of material from the crank to offset a small change on the piston. Worst case, you could have another shop check his work. Also note that engines are not 'zero balanced' from the factory. They are balanced just enough to run smooth in the normal operating range.

The balance shaft cancels out 3rd order harmonics associated with the 90 degree V6 at idle and low RPM cruising (due to the natural imbalance of having two pistons traveling up on one bank and only one on the other).

Here's a good read:
http://www.carcraft.com/howto/ccrp_0803 ... balancing/

[crzydmnd72]

Thats great info Matt and I was hoping you would chime in. He had all 6 pistons, rods, rings, bearings, bolts,, flexplate and HB. Thats interesting that they are not zero balanced from the factory also. By drilling the really thin weights, has that weakened anything?