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What happens if I have 5 cards in hand when Crankcase attacks but during the battle I go down to 4 or fewer cards in hand? Does Crankcase keep the attack bonus?
Yes. All that matters is that you had 5 or more cards in hand when Crankcase attacked. The attack bonus will last until the end of the battle, even if the number of cards in your hand changes.
Source: Wave 5 FAQ