Hello, my 01 Sentra started to Idle rough when it started. The'service engine soon' light came on. I took it to the dealership and they said it was the head gasket as they looked into the cylinder and saw coolant on the top of the piston. They fixed the head gasket and when I got the bill it was twice as much as they had estimated. They claimed that they had to have the head planed down as it was warped. Now my question...Is this possible that the head could be warped even though the engine never even overheated? Also, how long on average should a head gasket job take (hrs)?

Thanks.