Origenal Piston
 

Anyone know where I can find an NOS STD piston for a 390 / 427 ?

Chances are Federal Mogul still offers OE replacement pistons in the Sealed Power or Speed Pro brands. Go to napaonline.com, input your vehicle information and see what comes up. Also, suggest you google "Keith Black pistons" and see what they offer in their hypereutectic line. Either piston can be bought from many retail parts sources.

Beware of the stated CR values. They usually assume nominal deck clearance and a thick gasket, but the actual thickness is not always specified. That's why you need to calculate the actual CR for each specific engine, starting with your deck clearance measurement that you measured BEFORE YOU DISASSEMBLE the block. Also, at least on small blocks, KB pistons have .003 greater compression height.

If you have 93 PON fuel I suggest a maximum, true measured CR of 10:1 on a 427/390 and reduce 0.1 for each lower octane point, so for California 91 PON fuel, limit CR to 9.7. Going conservative on CR to allow an aggressive spark advance map is vastly superior in terms of across the range power output compared to too high compression that requires a sub-optimal spark advance map to keep the engine out of detonation.

Duke