bad luck. sorry to hear it.

not to be too simple but you didnt mention checking the oil level.

the 560 and 420 should be physically the same from the outside so they should interchange. were they ever offered the same years? i always like to swap the same year engines if possible to minimize electrical differences.

good luck in whatever you do.

the only two things i can think of that would disentigrate a piston are as noted above, a dropped valve or very lean condition on the fuel mix.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.