You can also feed a decent length of cotton cord in through the spark plug hole (leaving some hanging out so you can remove it) then use a wrench to crank the engine over, pushing the piston up. The piston compresses the cord againt the valves, so they will have no where to go.

Works like a charm.

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Those that say it cant be done should say so quietly, so as not to disturb those who are doing it.