Oakum
Oakum is a preparation of tarred fibre used to seal gaps. Its main traditional applications were in shipbuilding, for caulking or packing the joints of timbers in wooden vessels and the deck planking of iron and steel ships; in plumbing, for sealing joints in cast iron pipe; and in log cabins for chinking. In ship caulking, it was forced between the seams using a hammer and a caulking iron, then sealed into place with hot pitch.
Break-in (mechanical run-in)
The goal of modern engine break-ins is the settling of piston rings into an engine's cylinder wall. A cylinder wall is not perfectly smooth but has a deliberate slight roughness to help oil adhesion. As the engine is powered up, the piston rings between the pistons and cylinder wall will begin to seal against the wall's small ridges. 
Additionally older design engines had a flat lifter that was pushed by the camshaft lobes. It needs to spin during operation to avoid excessive wear to the camshaft lobe. At idle speeds on a new engine poor machining tolerances could prevent the lifter from spinning and destroy the camshaft. After 20 minutes of wear, or "self machining" at higher engine speeds they would typically be able to spin freely.