Cylinder wall damage?
They feel smooth but I can feel a very sloth difference with my finger.
Normally black marks indicate scuffing, that is the aluminum build up on the cylinder wall. In a racing motor its called the black death. For you it may have come from the hot tank. The straight line is usually caused by a ring. It usually means the ring was cracked or the motor overheated and the ring swelled and scored the wall. If the line is not too deep, say no more than 2-3 thousandths, its OK.
Good Luck.







