https://www.americanrifle.com/shop/cocking-piece-m3-160?category=21#attr=82,259 The pieces have numbers on them either marked on the rear or the top of the piece. I think the Archimedes originally came with a 055 piece; the new ones come with a 035 piece.
You can replace them without replacing any other part. You choose the one that best works well with your trigger by measuring the depth of the cocking piece position within the bolt shroud with the bolt handle in the closed position and the upward open position. If the firing pin drops forward onto the trigger sear when you close the bolt handle (decock on close/negative cock on close), you go with a longer cocking piece. If the cocking piece moves backwards as you close the handle you choose a shorter piece (cock on close/bump on close). The pieces come in 0.010" increments (035, 045, 055, 065). You can also choose a longer piece and carefully hand fit the length of the piece. Ideal is 0 movement of the piece in bolt open or closed positions.
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