Blow-by is the gasses that escape past the rings in a worn engine. A new OR overhauled engine will have next to no blow-by, because the rings, pistons, and cylinders are all new either way. The way you measure it is by observing how much blows out the crankcase breather tube. A new engine will have next to none, if any. And yes, you DO want to check this on anything you want to buy. Does this help you?
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