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The one on the right side is important because it centers the swing arm, and yes, as Charlie says you have to pull the swing arm to measure it.
If I'm understanding Bazil correctly, he needs to know the dimensions of the spacers between the engine and the frame, at the front engine mounting bolt.
but it's hard to see how I'll get a couple of 12 to 20 ? mm thick spacers between engine & rails without a bit of persuasion