OVERHAUL OF AN OIL COOLED MAIN ENGINE PISTON AND ITS TESTING

Procedure for overhaul

1. Preparation: Stop the engine, secure turning gear, drain the piston cooling oil system, and arrange lifting gear/ the piston lifting rig. Obtain the maker's overhaul manual.
2. Remove the piston unit: Lift the cylinder cover; disconnect the piston rod from the crosshead (remove the piston rod clamp) and lift the piston (with rod) out of the liner. Land it on a stand/ blocks.
3. Dismantle the piston:

• Remove the piston crown (top) - un-torque the crown retaining bolts/screws as per maker (these may be a ring of bolts accessed from the top).
• Remove the piston rings from the ring grooves using a ring expander/ with care; number them and record their positions.
• Separate the crown from the skirt (shroud) - the crown and skirt are usually bolted (the crown-to-skirt joint bolts), with the crown being the upper, the skirt the lower part.
• Remove the internal piston pin (gudgeon pin) bush and withdraw the pin where the design allows; in others the pin is carried by the connecting rod.

1. Clean all components using an approved (caustic-free or appropriate) solvent; remove carbon from the crown and ring grooves with a scraper/ soft brush, avoiding burring the grooves.
2. Inspect each part:

• Crown: for cracking (particularly around the combustion bowl rim and valve recesses), burning, erosion, and the condition of the fire ring/flame ring on its underside.
• Ring grooves and lands: for wear, step/ ridge, and the condition of the top ring groove insert (let-in ring carrier/ chrome ring) if provided.
• Rings: for wear, broken/free rings, wear height and side/radial clearances; check gaps (end gap in bore, side clearance in groove).
• Skirt/ shroud: for scoring, scuffing, cracks and the condition of the lower oil scraper grooves.
• Pin, pin bush and boss bores: for wear, pitting and correct clearance; check the pin bush bore.
• Oil-return/ scavenge passages and the piston cooling oil galleries: must be clear; clean any sludge.

1. Renew as necessary: Replace rings (typically with oversize or standard new rings as per cylinder bore condition), renew the crown-to-skirt joint bolts/gaskets, renew the pin bush if worn, and renew the scraper rings. Use the OEM parts and tools.
2. Reassemble: Fit new rings with correct end gap staggering (180 deg or as maker); refit the crown onto the skirt, torque the crown bolts in the correct sequence and to the correct value; fit the pin seals; check the ring-groove and piston assembly clearances.
3. Final checks before refit: Check the piston pin float/ motion, the ring side clearances and that the rings are free in the grooves, and that the crown-to-skirt joint is uniform.

OEM tools used for overhauling the piston

• Piston lifting rig/ lifting sling (with the ceramic/ nylon and steel support cradle) to lift and carry the piston.
• Ring expander/ ring plier to remove and fit rings without breaking them.
• Torque wrench and correct sockets/ extension for the crown bolts (e.g. angle torquer/ tension wrench).
• Pin drift/pin puller and bush drift to remove the pin.
• Feeler gauges, micrometers and telescopic gauges for clearance measurement.
• Cleaning/ scraping tools (carbon scraper, groove cleaner) and torque/micro-check gauges
• Target/ tension wrench and the maker's lifting beam inserts.

Testing of the oil cooled piston after overhaul

1. Bar-check/ engine turning: Fit the piston and bar the engine over several revolutions to confirm free movement and correct assembly.
2. Cooling oil flow/ pressure test: With the engine on barring gear, or by priming the piston cooling system, pressurise the piston cooling oil circuit and confirm oil flows to the piston crown (verify at the tell-tale/return) and that the oil return opens; check the cooling oil pressure is at the maker's value and that the cooling gallery pressurises without leakage.
3. Confirm no oil leaks at the piston/crown joint and the pin seal by running the pump and inspecting.
4. Compression/ leak test: With the engine running, check compression and firing pressures and confirm no excessive blow-by.
5. Operational test: run the engine at load and monitor piston/crown temperature (via the cooling oil outlet temperature), exhaust temperature and the overall condition; confirm normal operation.