Head Gasket and Piston Problem
 

My 1986 190D just experienced a blown head gasket. After replacement of the head gasket, the mechanic now is reporting a knocking sound which he suspects is a blown piston. Is it common for a piston to go out when the head gasket blows or can a blown piston blow out the head gasket? Any info would help me decide if it's worth saving my engine. It wasn't showing any signs of this amount of serious damage.

98 sl 500 head gasket replacement
 

I have replaced my head gaskets and am concerned that cylinder no 3 both the intake and exhaust open at the same time. Could I have the cams switched from one side to the other or what. I need help desperately

At the top of the exhaust stroke, (ending exhaust-begining intake) there is an overlap built into the opening and closing of the valves, to help clear out the exhaust gasses. Roll the engine by hand ONLY in the direction of rotation, and see if the exhaust valve closes, and the intake opens further. This will happen in every cylinder at that point.