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Those sound like the OEM ("Thompson Products" which later became TRW) pistons for the low compression 1963 415 HP M.W. motors. Before making a choice on using them check the skirts for collapse very carefully and the ring land clearances, if the pistons where used at all Also make sure the 413 block is thick enough(sonic test it) before having it bored out

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Dang Cab you know your stuff

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I ran a 1963 Plymouth Belevedere 415 HP in NHRA stockfor over 10 yrs and in SS for two yrs before that. Back then you weren't allow to use aftermarket pistons, just OEM stockers unlike today when you can run a SS CP piston with a another brands approve "stock" part # stamped into the top and get away with it Never again