You do get a ridge at the top of a cylinder but that is from wear in the cylinder walls caused by the rings. The ridge is at the top of the stroke & does not shorten the stroke. If it did it would break the top ring. The only way to shorten the stroke is with a different crank or a shorter rod.

Janan I agree. The ridge wouldn't form if there were something metal rubbing on it.

You'll notice a black ring around the top of the cylinder. That is carbon buildup. that will extend down as far as the top ring comes up during it's travel. Underneath the carbon, once it's removed, you may still find a ridge. That is wear from the rings, riding iron to iron or moly to iron, what ever the rings are made of. The rings rock just a little in the ring grooves and will wear the sharp edges of the rings down so they and the ridge have a small amount of radius to them. The ridge is removed because the new rings will once again have sharp edges, should fit tighter in the ring grooves and will want to travel those few thousandths that it didn't when they were worn out, thus the potential for broken rings, ring lands on the pistons, scoring of the cylinder walls. Trying to remove pistons with a cylinder ridge is like running the rings up against a wall. They may pass by the ridge with no ill effect or you can break things.

Stuck engines may have a coating of rust on the cylinder wall. When the cylinder travels up, it will scrape some of the rust off. With a ridge, it may trap some of that rust under the ridge, effectively lowering the distance the rings can travel without damage, again, potential for piston and ring damage.

You will not break a rod from the ridge on the cylinder wall! You break the rod from the bearing seizing on the crank and stuffing the rod into the cylinder wall. If if it is really spectacular it stuffs the rod out through the side of the block!