I bought several sets of those new pistons off ebay and returned most of them. Problem with the new ones is they put a huge champher around the edges of them which lowers the compression. the next thing they do is reduce the compression height by about .020 compared to stock. Stock pistons in my 250 were .035 down in the hole. add the gasket of .040 and you get a squish of only .070. with the new pistons they are don another .020 to put you at about .090 squish which is really bad. Ideally you want it to be around .040....distance from piston to head. Improve power gas mileage and reduce detonation with crappy fuel. With a space of .090 you end up with a dog. So my solution was to have the blokc decked .020 and get those original GM pistons with a decent compression height and I will be getting a squish zone of around .038 which is real nice :-) What this does is give lots of turbulence to the mixture so you don't get detotanion with the cheap gas even though the compression will be higher than normal. To get the same thing with the new pistons I would have had to have .040 or more shaved off the block, which was not a good idea. Do a search on "piston squish clearance" to see how it works and how important it is for a good running engine. A tight squish allows high compression and you get more power and gas mileage.
