1986+ Mercury 6/8/9.9/10/15HP 2-stroke Factory Service Manual, Page 174Get this manual
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4Water Marks: When bearing surfaces are subjected to water contamination, bearing surface, etching occursThis etching resembles the size of the bearing51853 51853
aPitting 5Spalling: Spalling is the loss of bearing surface, and it resembles flaking or chippingSpalling will be most evident on the thrust portion of the connecting rod in line with the beamGeneral bearing surface deterioration could be caused by or accelerated by improper lubrication6Chatter Marks: Chatter marks are the result of combination of low speedlow loadcold water temperature operation, aggravated by inadequate lubrication andor improper fuelUnder these conditions, the crankshaft journal is hammered by the connecting rodAs ignition occurs in the cylinder, the piston pushes the connecting rod with tremendous force, and this force is transferred to the connecting rod journalSince there is little or no load on the crankshaft, it bounces away from the connecting rodThe crankshaft then remains immobile for split second until the piston travel causes the connecting rod to catch up to the waiting crankshaft journal, then hammers itThe repetition of this action causes rough bearing surfaces which resembles tiny washboardIn some instances, the connecting rod crank pin bore becomes highly polishedDuring operation, the engine will emit whirr andor chirp sound when it is accelerated rapidly from idle speed to approximately 1500 RPM, then quickly returned to idleIf the preceding conditions are found, replace both the crankshaft and connecting rods