Put it in writing and make it totally clear what you want doing or look out for a much larger bill than feared. Friend put his disco in for service, told them to call him if any more work needed and they found something else but did not ask him for ok first - near twice the expected bill. Seemed to think that kind of behaviour is ok. He will not use them again.
I do all my own work so only relaying what I have been told. My only personal dealings with them was trying to get a valve spring to replace broken one before I put head back on. They did not call back despite several calls but probably not worth their while.