Bring the engine to top dead center and find a way to lock the engine in that position. When you lift the oil pump into position, you'll notice the the oil pump drive gear rotates when it engages the pinion gear. You have to turn the pump backwards so that when it rotates while you lift it up, the hole comes into alignment with the slot when fully seated against the case. Just like timing the distributor or magneto.
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Bring the engine to top dead center and find a way to lock the engine in that position. When you lift the oil pump into position, you'll notice the the oil pump drive gear rotates when it engages the pinion gear. You have to turn the pump backwards so that when it rotates while you lift it up, the hole comes into alignment with the slot when fully seated against the case. Just like timing the distributor or magneto.