First thing in my opinion you should find a new mechanic cuz all he did was take you for a ride. Second he (the mechanic) should of told you the best thing to do was to replace the whole motor from the beginning. Bad thing is that now he has you over a barrel and is gonna take ya for a ride again. You're prolly gonna be charged $1400 plus for this lil job. i would find out what he is gonna charge yall for an engine and then do your own shoppin around cuz yall will prolly find a better deal than what he is gonna give. If he says it just needs an oil pump get a second opinion, if it was knockin then the engine is one its last toe.