I understand your apprehension, But Id give them one more chance. Im sure they will get it right this time, their in the spotlight. Get it in writing that they wont hold your motor hostage for more money, unless its something YOU want to add.
JMO , and I also understand your frustation at this point. We all want it done right the first time, but that doesn't always happen with any shop IMO. These engines can all have their issues, from any shop.
