Mike, all o.e.replacement pistons will have the 4 valve reliefs; the difference is in the design around the circumference of the piston above the top ring. Most of the replacement pistons now have a chamfer around that circumference. The pistons previously available did not have that chamfer (true flattop) and produced slightly higher compression ratio.It would seem that the chamfer would also increase the crevase volume a bit also(which usually results in greater unburned h/c).Can't explain the logic behind the change except that reduced comp. reduces oxides of nitrogen; maybe that was the intended purpose.All I can get in my area is chamfered pistons so thats what I use.........fats
