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The crankcase is secured by 7 bolts, circled in red in the above picture. They should be torqued to 9 lb ft (12 Newton-metres). Torque them all to just 3 lb ft first, working diagonally, then increase to 9 lb ft, then run around them circumferentially to ensure you haven't missed one. Pay attention to whether any of them is substantially looser than that, and report back please.

On the Honda mowers I'm used to (bear in mind that the Australian versions mostly have no clutch or brake) the engine is held to the mower base by 4 bolts, which go into tapped holes in the crankcase cover. I haven't seen one with belt drive to the transmission, but it sounds easier to deal with than the propellor shaft coming out of the crankcase cover that I am used to.