I'm hand-lapping valves on the LS1.
This is probably a silly question, but I just wanted to confirm that I have to wipe off the lapping compound prior to the hydro test.
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Not only should it be cleaned off, you want to ensure you don't get/leave any on the valve stems. You'll wear out the valve guide in short order if you don't. Remember in engine building, cleanliness is next to godliness. You want to ensure everything remains as clean as possible and this means removing all of the lapping compound. Clean it all and don't spare the towels!! :o)