Gauging a liner is carried out for two reasons: To establish the wear rate of the liner, and to predict if and when the liner will require changing.
Although on a 2 stroke engine the condition of the liner can be established by inspection through the scavenge ports (evidence of blowby, scuffing etc.), the liner is gauged during the routine unit overhaul (15000 hrs), or if the unit has to be opened up for any reason
A liner is gauged by measuring the diameter of the liner at fixed points down its length. It is measured from port to stbd (athwartships) and fwd to aft. An internal micrometer is used because of its accuracy (within 0.01mm). To ensure that the liner is always measured in the same place, so that accurate comparisons may be made, a flat bar is hung down the side of the liner with holes drilled through where the measurements are to be taken.
Gauging a liner on a large bore RTA engine.
(Thanks to Emyr Davies)
Measurements are taken at more frequent intervals at the top of the liner where wear rate is expected to be highest.
To ensure accuracy, the micrometer gauge is checked against a standard, and the liner and micrometer should be at ambient temperature. If the temperature is higher then a correction factor can be applied. To ensure micrometer and liner are at the same temperature, lay the micrometer on the entablature for a few minutes before starting.
The readings can be recorded in tabular form, and from the data obtained the wear rate/1000 hours can be calculated. Wear rate varies, but on a large 2 stroke crosshead engine ideally should be about 0.05mm/1000 hours. On a medium speed trunk piston engine where the procedure for gauging is similar, the wear rate is around 0.015mm/1000 hours.
Figures are for illustration only.
Manufacturers quote max wear for a cylinder liner at about 0.8% of original diameter. If the wear rate is kept to a minimum, then the liner may last the life of the engine.