This is the only way I can think of:
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Step 1: take out a spark plug
Step 2: put in a length of dowel lightly in to the hole so that it sits on top of the piston
Step 3: wind the engine over by hand until the dowel goes as low as it can before it starts to come up again.
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Step 4: put a mark on the stick with a pen
Step 5: wind the engine over again so the dowel comes up.
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Step 6: mark the dowel when it won't go any higher again.
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Step 7: measure the distance between the two marks
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3S stroke
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(2.0): 86mm
5S stroke
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(2.2): 87.1mm
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Good luck though, not much difference between the two!