engine make, model, and detailed explanation of work done and parts replaced would be optimum for troubleshooting via interwebs..
for what's posted, my swag would be to verify con rods/caps mated and proper size bearings installed correctly..
then torque with plastigage one at a time and verify bearing clearances..
http://www.plastigaugeusa.com/how.html
this is per what is previously stated to imply crank and pistons fitted and installed correctly..
good luck!