I am rebuilding my corolla 1ZZFE engine because it was burning oil. Got my new pistons yesterday and wanted to compare them with the old ones to see if there really was a difference between the two .
Three of the old pistons had stuck oil control rings. They had carbon between the two oil control rings and a build up of carbon behind the rings of almost 1/16 of an inch. Also all four of the oil drillings that go from behind the oil control rings to the under side of the piston crown were totally blocked. The carbon went completley around the piston oil grove but was a quite a bit thinner over the blocked holes. The forth piston did not have stuck oil rings and even though it had quite a bit of carbon beneath the rings, the oil drillings were open ( There was no carbon within an inch of the holes) .
I cleaned up one old piston and compared it with the new one from Toyota.
At first glance they looked exactly the same, but I could see differences in the casting marks and numbers, same overall design though. Same weight( a couple of grams lighter), same diameter. There is a small difference in the cutouts for the valves though. The biggest difference is the oil drillings / holes behind the oil control rings. Instead of two small holes on each side over the skirt ( about 1/16 of an inch dia.) now there are four holes on each side almost double in size ( about an 1/8 of an inch dia. ). The holes are also drilled lower in the ring grove( actually about half of the hole is in the grove, and about half is below the grove in the piston body). You can see the bottom half of the holes even when the oil rings are in place.
The old oil control rings had very little wear compared to the new rings, again same overall design also.
Apparently when the oil holes become blocked , oil is no longer directed from between the rings to the underside of the piston and then to the crankcase, but is instead burned/carbonized onto the piston causing the carbon build up ,stuck rings and burning oil.
This is what Toyota says is the fix to the oil burning problem.
Look here to see a Toyota tech document, scroll down to attachment : http://www.toyotaownersclub.com/forums/index.php?showtopic=75536
You will have to register to read the attachment, but it is worth it.