Now lets turn to overall wear on rings, piston and cylinder.
1. Abrasives wear
Fig 1: Abrasive wear causes dull gray vertical scratches on ring faces’.
When the ring faces are covered with dull gray vertical scratches and have excessive ring gap (preferably checked in new cylinder), the rings have worn by abrasives. Other indication of abrasives present in the engine are dull grey vertical scratches on piston skirts, scratched cylinder bore, loose piston fit and main bearing.
Badly worn compression rings can be identified by their reduced radial wall. Fig 2 compares reduced radial wall of the worn ring on the left that of a new ring on the right.
Fig 2: Radial wall wear on worn compression rings
We know what causes abrasive wear, but do the abrasives enter the engine? Possibly dirt left in the engine during the last engine overhaul. Through the air intake, crankcase breather or fuel systems. Or from scuffing and scoring of parts as they wear.
2. Scuffing and scoring
Scuffing is caused by too much heat. Where two metals parts rub and the the heat builds up to the melting point, a small deposit or ‘hot spots’ of melted metal is pulled out and deposited on the cooler surface. (Fig 3)
Fig 3: How scuffing and scoring occur
Scuffing leaves discolored areas on the surface of rings, pistons, and cylinder walls. Under a magnifying glass, the metal in the centre of these areas is burnished and smeared in the direction of motion of the part.
Scuffing starts as a very small surface disturbance, which may be difficult to see and identify. If they are not removed, scuffing spreads to adjacent areas and become noticeable and more severe, at which time it is called scoring. (Fig 4)
Fig 4: Severe Piston Scoring
Any engine condition which heats rubbing parts to the melting point, or which prevents the transfer of heat from these surfaces, has an influence on scuffing.
The Following is a List of Possible Causes of Scuffing and Scoring:
1. Improper warm up (fast speeds or big loads too soon).
2. Lubricating system not functioning.
3 .Cooling system plugged.
4. Combustion, knock and pre ignition.
5. Lugging or overloading.
6. Corrosive wear.
3.Corrosive wear
The third cause of overall wear on pistons and cylinders is corrosion/leaking coolant can corrode pistons. Cold engine operation or the wrong lubricant oils can also deposit chemicals in the crankcase which will corrode parts.
Severe corrosion will show up as a mottled, grayish pitted surface on pistons or cylinder walls (Fig 5) .The corrosion is caused by acids from products of combustion.
Fig 5: Corroded pistons has mottled grayish pitted appearance
Other corrosive may be harder to find. If excessive wear is found, and scuffing and scoring are eliminated as possible causes, suspect corrosive wear.
Physical damage to pistons
1. Connecting rod out of alignment.
2. Crankshaft has too much endplay.
3. Crankshaft journal has too much taper.
4. Cylinder bore out of alignment.
5. Piston pin locks wrongly installed.
6. Piston pin locks are faulty.
7 Ring groove scratched while trying to clean out carbon.
8. Piston handled carelessly or dropped.
Combustion knock (detonation)
Knocking occurs when combustion of fuel in the cylinder is too early, too rapid, or uneven (Fig 6)
Fig 6: How Combustion Knock or Detonation Occurs
Causes of combustion knock (detonation)
a) Lean fuel mixture.
b) Fuel octane too low.
c) Ignition timing too much advanced.
d) Cooling system not working.
Preignition (Petrol Engine)
Preignition is when the fuel ignites before spark occurs. As a result, part of the fuel burns while the piston is still coming up on its compression stroke. The burning fuel is compressed and overheated by the piston and by further combustion. The heat can get so high that engine parts are melted. (Fig 7)
Fig 7: Preignition burned a hole through the head of the piston
Causes of preignition
a) Carbon deposits that remain incandescent and ignite fuel easily.
b) Valve operation too hot because of excessive guide clearance or bad seats.
c) Hot spots caused by damaged rings.
d) Spark plug of wrong heat range.
e) Spark plug loose (Means hotter plug)