If you disconnect the dog bones at the swingarm you should not have to compress the shock at all to fit it to the relay arm. Just move the relay arm up to line up with the shock.

All you have to do then is raise/lower the swingarm to refit the dog bones to the swingarm, again the shock does not move during this, all you lifting/lowering is the weight of the swingarm (and wheel if attached).

By doing it this way you are using the lever effect of the swing arm to your advantage. A pry bar of length of wood might be handy to raise/lower the wheel if it's attached.

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