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FIRST the machine shop weighs each piston finding the lightest one , then the others are drilled to remove the weight necessary for all to match. Then each rod is weighed big end and small end on a special scale and the same process employed. The rods have balance pads on them for grinding off material top and bottom. Now this weight of rod, piston assembly is attached as a bob weight to each throw on the crank which is spun and balanced as one would balance a tire. Cranks are either drilled or heavy metal slugs added to compensate for weight.
I have heard engine builders discuss balancing and disagree on the need. Our experience started in 78 when we started having 7080 engines breaking cranks.
The reasons were many - most poor quality control out of Harvey is my diagnostic. \We found bad viscous dampeners, loose front pulleys, probably bad metal in cranks, bad timing - too advanced - AND after ordering a rod and measuring it just fof kicks , terrible balancing. We tood a motor to Tuckers at Murray Ky and then Jerry Don was helping us diagnose the failures. He measured a rod and said he had never seen so much weight difference on an engine. I told him AC bragged about precision engine balance of components. He showed me a finished engine he had for me. It looked like a drill and chop saw demonstration site. I reported this to AC - they would not warranty the balancing but did allow us to warranty a rod - claiming out of balance and not making us return it.... the field guys way of fixing a problem . Anyway when we returned the engines to the owners over a 4 year period each owner noticed and reported "it sounds and feels like a different tractor" We never had another issue with the engines we rebuilt, some are still running. We did also check the timing, backing off a degree, and the advance of the RM pumps - some were off advancing way too soon, as the aforementioned front pulley and balancer. ( Never had a flywheel issue except for my stint in Jeanerette La where one of the mechanics decided to cure torque limiter slip by bolting a piece of metal to the flywheel to prevent the pad from slipping by - I lie not - I was ask to go down and see why all the 7000 were falling apart... I actually have a picture somewhere. When I ask "did you not know this would throw the engine out of balance/" " Ah I gonna told ya they say balance don't no matter on low rpm diesels"
SO does it matter? I guess peace of mind that something is done right has some psychological therapeutic value....
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