Inspection and Replacement of Piston Rings
Well, it was easy to see why the combustion sparks were setting fire to the scavenge oil: a small piece of one of the bottom rings was missing and the rest of the rings were “sticky" in their grooves and showed little sign of lubrication.
This indicated poor fitting between the rings and the grooves in the piston allowing “blow by."
Sulzer engines piston rings are machined slightly oval, so when running at normal temperatures would transform to a circular form, pressing firmly against the cylinder walls. To reduce wear, the piston ring grooves are chrome plated on their lower landings.
The new rings were fitted using a special ring expander and the ring clearances were checked between the piston grooves and the rings and the liner checked for wear and ovality, and all were within the Sulzer recommended tolerances. The piston was then rigged for refitting and lifted off the maintenance stand.
A tapered conical ring-piece is set into the top of the liner and this compresses the rings as the piston is lowered into the liner, preventing any rings from catching on the entrance to the liner edge and breaking.
I was in the crosshead bearing again after guiding the piston rod through the scavenge gland, I now guided it through the crosshead bearing, screwed on the nut and hydraulically tensioned it, witnessed by the Chief Engineer.
The piston cooling pipes and couplings were reconnected and as the rest of the engineers fitted the insert and overhauled cylinder head, whilst I renewed the scraper rings in the piston rod gland. These were in sections, held together with springs and consisted of a set of top rings preventing scavenge oil/air from entering the sump and a bottom set of rings preventing lube-oil splashing up from the sump area, entering the scavenge space.
The scavenge space was given a final inspection and I refitted the door and opened up the drain valves to the dirty-oil tank.
Up top, the rest of the boys had refitted the cylinder head and piping so we opened and bled the fuel valve tidied away the tools whilst the Chief and Second went down below to start the engine, leaving me up top to ensure all was well with the repaired components.
I heard the engine room telegraph ringing, then the Chief gave her a good blow through on air and I shut the indicator cocks.
The next blast of air was accompanied by a safety valve lifting, then the engine settled down to a comfortable half speed, the Chief gradually increasing the revs and the Fourth and I watching exhaust and cylinder head cooling temperatures.
After about half an hour, everything had settled down and the Second and I went up for a breather and a shower, before I went down to take over from the Chief.
We completed the voyage without any further scavenge fires, and had a telegram form head office congratulating us on one of the fastest piston-ring changes and recommending we check the rest of the pistons whilst in ports up in the Great Lakes, and the Captain sent down a case of beer between us.