The Moving Parts — Liner, Piston, Gudgeon Pin and Connecting Rod Wear

The liner wears

The cylinder liner is designed to withstand gas pressure and the side thrust of the piston, and it is subject to continuous abrasive and corrosive attack. It therefore wears, and it wears neither evenly nor circularly. The liner is gauged at regular intervals as specified in the maintenance manual, and the records kept, because it is the rate of wear that matters as much as the absolute figure.

The gauging procedure, the wear rates to expect, the causes of wear, the pattern that points to the cause, and the limits at which a liner is condemned are in Chapter 12, which owns the measurement.

The piston in a four-stroke engine

The four-stroke trunk piston has a different job from a two-stroke piston, and it looks different.

• The crown forms the combustion chamber roof, so it must tolerate the flame. The shape of the crown is set by the combustion chamber design and by the injector's spray pattern.
• The top land — the strip of piston between the top of the top ring and the crown — is the hottest part and the part most prone to carbon build-up.
• The ring belt carries the compression rings and the oil scraper ring.
• The skirt transmits the side thrust, caused by the varying angularity of the connecting rod as it swings. This is a duty a two-stroke crosshead piston does not have, because the crosshead and its guides take the side thrust instead. The skirt is therefore a load-bearing and wearing surface, and it is the reason a trunk piston is longer and heavier than a crosshead piston of the same bore.

Because the skirt takes side thrust, the piston also has to be lubricated and cooled at the skirt, and the liner has to resist the resulting wear. This is the fundamental difference between the auxiliary engine and the main engine, and it is why the two have different lubricating arrangements.

Anti-polishing (carbon scraper) ring

Some medium-speed trunk-piston engines have a replaceable steel ring at the upper end of the liner, positioned where the piston top land passes at top dead centre. Its inside diameter is slightly smaller than the liner bore and is matched to the piston design. As the piston rises, the ring scrapes carbon from the top land before the deposit can bridge the running clearance and rub the liner.

That deposit can polish the liner surface, closing the fine pores that retain oil. Reduced oil retention increases friction and wear and may raise lubricating-oil consumption. The ring reduces this risk; it does not correct poor combustion, worn rings, or an incorrect liner/piston clearance. At overhaul, inspect its seating, condition, carbon damage and clearance using the maker's limits. Do not assume every auxiliary engine is fitted with one.

Piston rings

Piston rings are designed for maximum sealing effect and minimum wear rate. On a medium-speed auxiliary engine the set is normally:

• Three compression rings — the gas seal. The upper rings run hottest and wear fastest; the top ring may sit in a hardened carrier or a ring groove insert, because an aluminium alloy groove cannot take the load on its own.
• One oil scraper ring — the oil control ring. Its job is to scrape surplus oil off the liner and return it to the crankcase, and its condition decides how much oil the engine consumes.

A ring's performance depends on:

• Its fit in the groove — too much side clearance and the ring pumps oil; too little and it seizes when hot.
• Its end gap — set so that the ring can expand with temperature without butting.
• Its radial pressure against the liner — decided by its own elasticity and by the gas pressure behind it, which forces it out against the bore.
• The liner's surface — a worn or glazed liner will not let the rings bed in.
• Heat deformation — a distorted piston or a distorted liner will not let the rings seal all the way round.

What goes wrong at the connecting rod

Two failures account for most connecting rod trouble.

Bottom end bearing failure is usually a lubrication failure — loss of oil pressure, contaminated oil, a blocked oil passage, or a bearing fitted with the wrong clearance. The bearing wipes, the clearance opens up, and the knocking that follows is the warning. If it is not acted on, the crank pin is damaged and the crankshaft has to come out.

Bottom end bolt failure is more dangerous, because it is sudden. The bolts are subject to repeated stress and fluctuating stress every revolution, and they are tightened to a specified stretch, not a torque. Bolts that are re-used beyond their life, or tightened by feel, can fail and let the rod go through the crankcase. This is one of the classic causes of a crankcase explosion.

Big-end bearing shells and inspection

The connecting-rod big end runs on a replaceable pair of shell bearings around the crankpin. A common medium-speed-engine shell is a steel backing with a bearing-alloy lining and a thin overlay; the exact material stack varies by maker. The backing locates the shell and carries load into the rod and cap, while the softer working layers provide a conformable running surface and embed limited fine particles. They cannot compensate for dirty oil, blocked oilways or a lost oil film.

At overhaul, keep each shell identified to its position and orientation. Clean it and inspect for wiping, scoring, fatigue cracks, overlay loss, corrosion and signs of movement at the joint. Check the crankpin for scoring and measure journal diameter, ovality and taper by the maker's procedure. A shell's visual appearance alone does not establish clearance or fitness.

Measuring big-end bearing clearance

First secure the engine against starting, isolate starting air and fuel, and follow the ship's permit and turning-gear procedure. Clean the journal and bearing surfaces. Use the method and acceptance range specified for that engine.

1. Plastigage: place the correct grade of strip across the bearing crown on a clean, dry journal. Refit the cap in its original position and tighten the fasteners in the specified sequence and to the specified torque, hydraulic pressure or bolt elongation. Do not rotate the crankshaft while the strip is trapped.
2. Remove the cap and compare the flattened strip with its scale, recording the reading. An uneven width can indicate misalignment or a non-parallel condition; further dimensional checks are still needed to establish journal ovality and taper.
3. Lead wire: where approved, place soft lead at the measurement points, assemble and tighten by the maker's method, then measure the compressed wire with a micrometer. Use the specified wire diameter and locations.

Feeler gauges, bridge-gauge measurements, journal micrometers and dial lift checks may be required as complementary or alternative checks. Do not substitute a compressor or crosshead-bearing example's figures for an auxiliary-engine crankpin. Record readings against the maker's clearance limits and the previous inspection.

Bottom-end bolts: tightening, elongation and renewal

Connecting-rod bolts see repeated fluctuating loads as the piston and rod reverse direction. Excessive clearance, bearing distress, abnormal piston loading, incorrect assembly or reuse outside the maker's limit can add to bolt stress. A failed bolt can release the rod and cause catastrophic crankcase damage.

During overhaul, clean and inspect bolts and nuts for cracks, necking, thread or fillet damage, corrosion, twisting and evidence of fretting. Measure free length or installed elongation only at the maker's specified datum points with the specified gauge. Tighten by the designated method—often hydraulic tensioning with a measured extension, but some designs specify another controlled procedure—and compare readings between bolts as instructed. Record each measurement.

There is no universal operating-hour life or elongation limit for all auxiliary-engine rod bolts. Renewal intervals and discard criteria differ by engine and may also depend on a specified overload or incident. Use the engine manual and ship's maintenance record; replace at the prescribed interval or whenever the maker's inspection limit is exceeded. Never infer a safe reuse from appearance alone.

Top end bearings

Top end bearings — the bushes in the small end — and the gudgeon pin are checked for clearance and for surface condition at overhaul. A worn top end bush shows up as a knock and as a change in the piston's position, and it allows the piston to tilt in the bore, which shows up as abnormal liner wear.