The vast majority of GM pistons were cast. As far as forged pistons that GM used in high performance motors they were made by TRW and do not have the vertical ribs inside the skirts. TRW replacements have these. Supposedly they are stronger with the extra material. There are sometime other differences as well. For example 69 Z28 forged pistons carrying the part number 3927172. I have had several sets of these and have noticed differences in some of them. Ring land sizing and spacing, oil control hole diameter and number of holes have sometimes been different on some sets, even though they had the same GM part number. I've got one of the above part number pistons in front of me, it doesn't have the ribs, uses the larger ring sizes and larger, but fewer oil control holes. The reason I never used this piston is they did a poor job machining one side of the wrist pin retainer grooves. A good machine shop can fix that.
I said they are supposedly stronger because I purchased a set from Yunick's that Smokey had modified extensively. They were the GM part 3927172. I would think if there was a significant difference in strength he would have selected the TRW replacement over the GM part. With the amount of material he had removed from these pistons it doesn't appear strength is a big concern.
I've run both types of piston and haven't had a problem with either style.
[ 07-24-2003, 10:01 AM: Message edited by: SY1 ]