“Originally, the by-passage was somewhat small, little larger than the oil delivery pipe to the engine, which was about 3/16 inch bore, and the result was that the pressure when starting with the oil cold rose to about 25 pounds per square inch, and fell to about one pound per square inch with the oil hot and the engine running slow. It was possible, however, to bore out the by-pass passage and fit a larger pipe, about three times the area of the main delivery pipe, with the result that the oil, when cold, never rose above about 15 pounds per square inch, however fast the engine run. When thoroughly heated, the normal running pressure was about 6 pounds per square inch, falling to 2 pounds per square inch with the engine only just turning over, which brings up the question of the correct working pressure. This will vary very largely with the design of the engine, but, broadly speaking, the higher the pressure the better for the bearings. The limiting figure is determined by the tendency of the engine to throw out oil at the end of crankshaft bearings, and by the amount that gets past the piston rings. Obviously, an engine with new, tight bearings and new piston rings will stand a higher pressure without undue waste of oil or excess deposit in the cylinder head than will an old engine with worn bearings and slack rings. And, again, the question will be affected by the design of the pistons. For instance, where the trunk of the piston is bored for lightness, much more oil will get past the rings than in cases where a ‘solid’ trunk is employed. Roughly speaking, 8 to 15 pounds per square inch is a good figure for a new, high-speed engine. An old and worn engine, particularly if not of a high-speed type, may require no more than 2 to 6 pounds per square inch.” [Illustration: Brookes Gasoline-Electric Generating Units for Operating Search Lights. An Independent Unit is Used for Each Light. ] The writer recently encountered a rather curious difficulty in connection with obtaining a free by-pass. The return pipe from the by-pass led into the case carrying the gearwheels of the camshaft and magneto drive, and oil continually flooded out from the end of the camshaft and other bearings. The waste and mess were sufficiently serious to warrant investigation, and the cover plate over the gears was accordingly taken off. It was then noticed that the oil delivered to the gearwheel case had only two small holes by which to drain away to the crankcase. The flow from the by-pass was beyond the proper capacity of these holes, and so the whole gearwheel case became filled with oil under considerable pressure, quite possibly 2 or 3 pounds per square inch, and it was not surprising that oil exuded from the ends of the bearing. A few extra limber-holes, if one may borrow a nautical expression, were drilled through to the crankcase, and no further trouble was experienced. (124) Plug Oil Holes When Painting.