Shake Shake Shake

KB pistons are not stock weight. That's your problem if you didn't balance the crank. On a 4 banger, as long as the pistons are the same weight your good (balance each other out), a 120 degree crank 6 is somewhat self balancing with identical pistons and a 90 degree 'V' 8 require a formula to balance prior to dynamic balancing the package.