Folks tend to think that because the outside and major open inside areas of a block and head are clean when the come from a machine shop the parts are clean enough for assembly. That is not always true. Most machine shops do not have the wash equipment that really cleans and flushes the internal oil lines and blind bolt holes. To get those really clean, get a selection of nylon bottle and pencil brushes that are snug fit to the hole diameter and have a handle length that allows them to pass completely through the open holes from one end to the other. Then go at the oil lines and bolt holes with hot water and a good detergent like Dawn dish soap.