Salty, if you have a piece of shaft about 9-12" long and the same diameter as the crankshaft extension (or PTO), you can slide it right through your old clutch half and rest the protruding part of the shaft on both sides on two pieces of flat steel that are exactly level. When you let go of the clutch half, it will rotate until its heaviest part is at the bottom. When it is balanced, it won't have a heaviest part, and will not rotate by itself - it will stay where you put it. You should also do this test on the new clutch half, with the other half of the clutch bolted to it. You will soon find out how well it is balanced. You'll find that the three screws holding the clutch-halves together need to be identical. You may also find that the grub screws disturb the balance just slightly, because unlike the short Allen type (hexagonal recess) grub screws, they stick out above the clutch boss. However the grub screws are close to the axis, and have less effect than those three bolts holding the clutch halves together.
It is a good idea to get into the habit of checking the balance of any largish object on your mower that rotates at full engine speed, like that clutch. A well-balanced mower is nicer to use than a rough-and-shaking one.
