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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



Dang Cab you know your stuff