Sounds like your problem is the ring groove depth .

What year/era were these pistons you bought from fscreenname ?

Joe, everything should be MKIV. The stamped part numbers on the pistons are GM 3999295. They are "supposed" to use a 5/64 5/64 3/16 standard ring pack. Looks like I could order Hastings 2M4718 and get the right set to bastardize things, but that just seems rather jerry rigged to me.

My wonder is why GM would use the same part number but change the design.

Unknown. We were both pretty sure of the fact that with the part numbers the same that this wouldn't have been an issue. So far, I'm stumped as to the difference.

Yes, something had to change for merc to change part numbers for the replacement pistons from one 454 std bore to another , even though the same dome ...I think the slotted piston is a later style , not for sure though

Knock offs?

Not knock offs.

1970 as best we can figure

Couldnt find trw book (better ) but gm book says 4 different id pistons ,4032 9295 2972 9382 "id number " all fall under the same part number for 9 to1 1971 ls6 piston 6262976

The better book , once i again have possesion of it has all and i mean all spec measurements ect of about every v8 piston in the known galaxy. Hence my earlier reference of hugging . Because there is indeed some strange stuff going on out there

Well, I got into a whole bunch of ring catalogs. And they reference both shallow and standard rings across the line of 454 engines. It's a crap shoot.

The pistons I have were only available in later engines. Somewhere GM made a change. Oh well, my issue to deal with.

The expanders are the issue. I measured the oil rings. They're exactly the same.
Perhaps I can reuse the expanders and go from there. I'll do a drop test in the morning to see if they work.