New engine has loss of compression

i'm glad you caught it when you did. that's the plus side to the story, along with the fact that you found it during the offseason :D

Don't put it back together until you are 100% sure everything is right.

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that is the key what triple R said^^^

you can never bolt on any heads without checking, partially because of manufacturing processes and partly because no two blocks are the same either. a set of heads with valves and geometry all set up for one short block might have piston/valve clearance issues on another short block.