i just carried my crank to the local machine shop to get it balanced they told me that they needed the connecting rods to do the job when i returned with them the machinist had already left my question is why does he need the rods to balance a crank

I'm not totally sure on that question, but I can tell you my machinist also wanted the rods, pistons, harm. balancer, flywheel, clutch setup, and I requested that it all be balance seperate incase I needed to replace something later down the road... like a new clutch.

I do know that my machinist clamped weights on to each journal to simulate the rod/piston weight when he was balancing the crank.

Rings and bearings also. At least one cylinder's worth. And, the rod and piston you take them HAS to be the lightest one. Then you make all the others match.

When you grid or lighten the rods, you need to balance the rod, it's not just grinding the rod to make it as light as the lightest one. In some cases you will grind on both ends, or just one end. I have seen the fixture that used for this, but have never done one.

Fred.

RK,
This is very simplified but ....The crankshaft is balanced to the weights of the parts attached to it. You have to weigh all the parts, pistons, rings, bearings, rods etc and then you make up a set of special weights that you attach to the crankshaft to simulate the weights of the parts. Then you put the crankshaft, with the weights attached, on a balancing machine, similar to a tire balancer, and balance it. Thats pretty simplified, but it would make it a lot more clear if you asked the guy to show you a crank while they weredoing it. Actually, I would suggest getting a couple of good books on performance engine building and reading them from front to back. You really should know the answer to this question already if you are planning to build your own engine.