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Not only should they weigh the same, but they should be paired by "moment weight". That's how we paired up sets of 2 or 3 jet engine fan blades at a time. Once paired, you could safely replace an opposing pair or 120* triad blades in the field and not affect the original engine balance. Moment weighing consists of balancing the pistons on a beam, then getting the beam level when the pistons are sticking out from the center, from a common distance from center point. You would generally locate them in the position that they would be running. The everyday explanation is that you have a 2 pound baseball bat, and a two pound wooden cylinder of the same diameter. They would weigh the same on a balance scale, but it you mounted them butt end toward the center, far end pointing out, the cylinder would not have a tapered handle, nor would it be as long, so more of its mass is closer to the butt end, but the bat has a center of gravity that is farther out, so the scale would say the bat is "heavier per inch from the fulcrum", or over that moment. So they might weigh the same, but in motion, the bat takes more force to swing. In an assembly of a rod and a piston, it can make a considerable difference, of course not as nmuch in a 1200 rpm engine, but every bit helps. Using a static balancing method like this can virtually replace a dynamic method that most shops don't possess. But anyway, if you feel the need to balance, static gross weight is a good place to start, but if you can moment weigh them, you'll get them so close that the engine will not be able to sense any difference. Just think about what happens if you moment weigh a 2 pound, 6' long broom stick versus the basball bat? I helped a local youngster build a unit and he won his local science fair a couple of years ago. It's a cute trick, especially when you start sticking things on two identical objects so that you can see how little it takes to throw things off. Frank
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