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Actually, saying there is the same torsional load on the crankshaft from front to rear is not correct. This is a simple dynamics problem with combined loadings from the flywheel and the respective pistons. Obviously without any load from the drivetrain, the engine would rev free and there would be negligible torsional load on the crankshaft compared to the drivetrain loading (just friction, the compression stroke, and rotational inertia). All of the load from the drivetrain is transmitted to the crank through the flywheel. Think of the flywheel being one point load, and then the crank lobes being 6 other small point loads, summing to a torsional force that is equal and opposite to the flywheel point load (assuming the system is statically determinate, which simplifies this example). From the point of the flywheel forward each consecutive piston adds to the torsional displacement (angle of twist) on the crankshaft during it's combustion stroke. So if piston #6 causes 1 degree (just an easy number for example) of twist between the #6 crank journal and the flywheel, the entire crankshaft from that point forward will be indexed by that 1 degree since the front of the crankshaft is not loaded by the drivetrain. Then piston #5 adds another degree of displacement. From journal 5 forward, the entire crank is now torsionally strained 2 degrees relative to the flywheel. So on and so forth. The larger the torsional displacement from neutral of the respective pistons, the larger the vibrational amplitudes due to imbalance (and a few other things that would take too long to get into). The compression stroke does cause some load in the opposite direction, this adds to the cyclic mode of the torsional loads, but is nowhere near as large as the net angle of twist caused by the drivetrain loads.
Here is a pic that I am sure some of you will recognize that can help illustrate what I am talking about.
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