To minimise the risk of breaking the piston or rod when you do the rope-in-the-cylinder trick, make sure you put in just the right amount of rope so that the piston is halfway up the stroke after the rope has compressed as much as it is going to. If you do the rope trick and end up with the piston near the top or bottom of the stroke, you are greatly magnifying the forces on the piston and rod, due to the crank and rod acting as an over-center mechanism.
