MARINE ENGINES.
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way the weight of the piston is transmitted through rigid blocks tothe packing ring, which bears on the internal surface of the cylinder.Short springs of a bow shape are placed all round the packing ring,to keep it well up to the cylinder surface. In some cases thesesprings are of a U shape, let into recesses in the body of the piston.The main body of the piston is strengthened with ribs radiatingfrom the centre, and has the necessary bosses, which are bored outfor the reception of the piston and air-pump rods; the holes aresometimes tapered to receive the ends of the rods, and in otherexamples they are quite parallel; in the former case, the piston rodis screwed tightly against the cone, and in the latter against ashoulder left on the rod itself. Holes are cast in the body of thepiston for the purpose of extracting the cores; these are accurately
bored out with a slight cone, andplugged up with cast-iron plugs,having a thin coating of red leadto make the joint perfectlysteam-tight. They are further securedby boring holes on the circum-ferential line of the joint, whichare tapped for the reception ofbrass or wrought-iron screws,firmly screwed in, one-half of thescrew being in the body of thepiston and the other half in theplug; these screws are cut off flush with the surface of the casting.The junk ring is held down by bolts screwed into the cast-ironpiston, or brass or wrought-iron nuts are fitted, recessed into it,having a thickness of metal all round; the bolts are kept fromturning by a screwed stud recessed in the head, and tapped intothe junk ring. The ring is accurately turned, as also the surfacesbearing on the piston and the spring ring, which are then scrapedto a true surface, and made perfectly steam-tight. After the pack-ing ring is turned, an oblong hole is cut out at the centre at thatpart where the ring is cut through, and a brass piece with a flangeall round is fitted into the hole, filling it up, while the flanges makethe spring joint steam-tight; the end of the brass filling-in pieceis secured to the packing ring at one end with screwed studs, anda wrought-iron bridle is placed over the tongue, and secured like-wise at one end to the packing ring. The object of this arrangement
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