Originally Posted by
Recycled Flyer
The reason why (as told to me by a very good Aussie engine builder) is that a used engine will collect carbon crystals in the combustion chambers rim between the contra and the cylinder wall.These are extremely hard and sharp and will easily score the liner.
The very good engine builder I know will tell you that the contra must be pressed in from the BOTTOM, carbon crystals notwithstanding. The carbon threat is dealt with by careful cleaning.