Cylinders wear in taper and out of round.
Taper is a bore diameter difference between the top of the bore and the bottom. The top having the most wear as this is where operating pressures are high pressing the rings, especially the top ring hard against the cylinder wall at a point of maximum heat and minimum lubrication. This is complicated by the diameter immediately above the max ring travel having a ledge that is about the original bore diameter above that ledge. The ledge needs to be removed as new rings and the possibility of a new and dimensionally different piston can jam the top ring into that ledge busting the ring.
Out of round is an egging of the bore diameter caused by the thrust surfaces of the piston (the skirts) applying thrust force against the cylinder wall. This is wear at 90 degrees to the wrist pin it is generally greater at about the mid stroke point on the bore wall.
All of these have factory specifications on allowable wear generally it is .001 inch for taper and .002 for out of round. Beyond these values engine life is lower than than than the factory is shooting for.
Another issue that is more insidious is waviness in the wall, hard to detect unless you polish up the wall with a ridged hone. This will leave a polish on the high places and dull in the valleys. The down side of using a dingle ball is these areas go unnoticed as the dingle ball polishes all the surfaces but does not restore any dimensional correctness leading to a false sense of accomplishment because everything looks shiny though dimensionally incorrect.
Forged pistons come in two alloys, 2618 or 4032:
2618 is generally considered the stronger alloy but it is more malleable than 4032, however, being more malleable it distorts a lot but hangs together further into its failure zone but the distortion makes if functionally useless causing other parts to fail even if it technically hasn’t. This alloy has a greater ratio of thermal expansion which requires it run with looser cold clearances so it is unwise to pull power when the engine is cold, racers excepted where engine parts and engines are considered expendable in the interests of winning. For a street driver this usually isn’t considered a good trade so it’s a good idea to have the engine warmed up before showing the hundred thousand dollar import in the next lane at the traffic light what you got.
4032 is a high silicon alloy though not a hypereutectic it is pounding on the door, therefore, it behaves a lot like a hypereutectic in that it is very thermally stable allowing for tighter skirt clearances so it doesn’t require the same temperature respect of a 2618. As with hypereutectic cast pistons it is not very malleable, it fails in chunks rather than distorting like 2618. So if your running a hyper cast piston the 4032 alloy is a better match to the existing clearances.
Honing is used to set the final skirt clearance and to provide some bite for the rings to marry with the wall and provides an oil trap to keep the rings lubricated especially the top ring as it goes over TDC. The type of honing starts with the ring selection in general a chrome ring requires a course stone for an aggressive wall cut. Moly rings like a finer stone for a less aggressive cut. Get this wrong like a fine cut on chrome rings they will never marry to the wall and you will have high blow by and oil consumption forever. Put a course hone on a moly ring and it wears quickly leaving you in the high blow by and oil consumption miles too early in the engine life. Then of course is plateau honing where you start with a coarse stone and finish with a fine. This works quite well on moly rings as well where the coarse stone digs deeper channels in the bore while the finer final cut knocks off the sharp edges leaving a gentler ridding surface that is backed up with deeper oil holding grooves in the wall.
Bogie
