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timing could be off and is leaving the valves open, double check your timing belt and make sure all the timing marks line up, if they do, than maybe somebody bullshitted you and sold you a blown motor. hope not though.
i thought all rebuilt motors had weak compression until they got broken in....i could be wrong
the rings do have to seat themselves yes. but from his first post i am gathering that the motor didnt start and so he did a compression test and found low compression so thats why i suggested timing may be off.
Make sure you did not wash the cylinder with fuel. Did you rotate the engine with the distributor disconected to build oil pressure first? Did you perform a wet test to verify if it was a ring issue? Did you perform a valve adjustment?