Cylinder liner gauging Cylinder liners should be gauged internally at fixed intervals during overhaul ( hours) to measure accurately the increase in bore. Continuous records of gaugings should be kept for each cylinder. The liner must be cleaned and inspected. General appearance of the surface may show whether lubrication has been adequate. A careful examination must be made for surface cracks, particularly around the combustion space.

The liner is now gauged with a micrometer and extension bar which has been calibrated against a master gauge. The liner should preferably be cold, but if this is not possible the gauge must be at the same temperature as the liner to cancel expansion effects. Gaugings are taken at a number of vertical positions (4 to 6) over the area swept by the piston rings. Readings are taken in fore and aft and in athwartships directions. To ensure readings are taken at corresponding points, a template may be used.

Gauging figures are noted as total wear from original and mean rate of wear since the last recording was made. The pattern of wear over the length of the liner will differ according to engine type but in a single acting, two-stroke engines it tends to be greatest at the top of the stroke adjacent to the combustion space, where pressure and temperature are greatest. This reduces towards the lower end of the stroke, but will increase at the exhaust and scavenge ports when relative pressure on port bars is increased and blowpast may remove lubricating oil film. The rate of wear varies over the life of the liner. It is high during the initial running-in period after which it should reduce to an almost constant rate for most useful life of the liner. Finally the rate will progressively increase as wear becomes excessive.

Normal wear rates differ but an approximate figure of 0.1 mm per 1000 hours is acceptable for large engines. Wear is increased if the engine is overloaded. Maximum wear before renewal is usually limited to % of original bore diameter, or less when the manufacturers advise. The time when this figure will be reached can be anticipated from wear records for advance ordering of replacements. After gauging, any ridges on the liner should be ground off. These may be evident at the top of the ring travel and at port bars. Ridges may be due to broken piston rings or where the piston has been raised to readjust compression. If new piston rings are fitted the liner should be deglazed. To allow running-in, cylinder lubrication should be increased temporarily and maximum load limited. Ports must be cleaned, sharp edges removed, lubricators tested and a close inspection made before putting back into service. If the cylinder is operated with excessive wear, the rate of wear will rapidly increase. Gas blowpast may remove the lubricating oil film, piston rings may distort and break, piston slap may cause scuffing. Compression is reduced causing incorrect combustion with fouling of the exhaust system. Unburned oil may be blown into scavenge spaces giving rise to scavenge fires.