arent the pistons in a pumps way smaller then those in the block of an engine? I would think all that mass may have something to do with it not being a viable option in an engine. Also if the head expanded from the block at all which mostly happens under a high load may life the head off and blow the sleeves apart since there is nothing for the sleves to hold back against since the would be no cylinder wall there.
I dunno just throwing out some things at ya.
edit: Also I think the gasket might be their as a point of failure, so when the engine is taking loads it can't handle it blows a gasket(in some cases) instead of going and going and going until it reachs the breaking point and disingrates the rods.
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