Things you need to know.

Max340 said

Also, what if the locks and the retainer are stuck, and it requires a bit of persuasion? Tapping against air pressure won't work so well.

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Air will work better than rope, the rope will compress and if the engine is not right at tdc it will roll the engine over. With air you have 400 or so pounds of force pushing on the valve to smack against.

Just had to replace a broken spring a few weeks back and a quick smack is all it took.