James...did you check the ring gaps? That's the only thing you haven't covered....you looked at every thing else. I have seen this before...the motor I saw that was like this was set up too tight on the clearances ( Rod and Main Bearing along with ring gap) and what I remember looked like your pics.

it is posible that the assembler did not oil the cyl walls or piston skirts,and the piston pins,the damage would have hapened right away.what prompted you to take them apart,loss of power and oil use?

One was making a noise, decided to double key the cranks, and machine shop insisted on detonation, they also said .004 was fine so I said well both motors are tuned and run the same way lets see what the not noisy one looks like

28 degrees is a baseline starting point depending on many variables , i would say that 30 to 32 degrees is where you'll end up, but be cautious as the pistons you're using aren't "my" first choice lol
True blower pistons are designed to withstand the higher temps associated with blower engines, I'll call you and we can discuss this in greater detail...

Adding 420 blowers with chillers, reason for double keyway on cranks

How are the exhaust valves? 28* coupled with a lean afr, baby blowers=tons of heat in the cylinder.

and the pistons are not for a supercharged engine,all that heat may have caused too much piston expansion,once the piston started scraping the cylinder wall and scuffed the skirts the drag coupled with the heat oblonged the piston pin holes.

i remember when hawk was building those .they started out with a base 420 merc i belived they changed the cam and bolted on the b&m blower .they had a lot of problems ,dentonation!!! i have a question did the motor set for a long time? any heat signs of heat on the bottom of piston under the dome?

fixx
 

reason for asking,i had a customer that scuffed all the outside's of his pistons..he sucked up sand and all the sediment would not allow the water flow properly and the cylinder wall expanded and did the same thing your did..sea strainers cured his problem after a bore and a set of pistons..

Thanks fix, I have monster strainers, and there is no sediment I can find in the block.
I think mike and joe may be on to something.
I think the pistons were just too tight for the application. I know the pistons aren't designed specifically for a blower, and I don't want to put a bunch of $$ into these 454's right now. I plan on building a pair of 540's in a year or 2, so to put $1500 worth of pistons in these to only use for a short time just doesn't make a lot of sense. ESP when I have 2 sets of these pistons for short money. I'll get the bores measured up and see where they are at.