Trunk-type pistons are subject to forces such as gas pressure, side thrust, inertia, and friction. These forces, together with overheating and the presence of foreign matter, may cause troubles such as undue piston wear, crown and land dragging, cracks, piston seizure, clogged oil holes, and piston pin bushing wear.
Excessive Piston-to-Liner Clearance
Symptoms of excessive clearance between a piston and its cylinder are piston slap and excessive oil consumption. Piston slap occurs just after top dead center and bottom dead center, as the piston shifts its thrust from one side to the other. As the cylinder taper increases with wear, oil consumption increases. Since taper causes the rings to flex on each stroke of the piston, excess ring wear occurs, allowing lube oil to pass and be burned in the cylinder. This results in the accumulation of excessive carbon deposits.
Crown and Land Dragging
Pistons and liners may become sufficiently worn to permit the piston to cock over in the cylinder. This allows the crown and ring lands to drag on the cylinder wall. The results of dragging can be determined by visually inspecting the parts of the piston in question.
Although piston wear is normal in all engines, the amount and rate of piston wear depend on several controllable factors. (The causes of excessive piston wear, and crown and land dragging, are also the causes of other piston troubles.)
One of the controllable factors is LUBRICATION. An adequate supply of oil is essential to provide the film necessary to cushion the piston and other parts within the cylinder and prevent metal-to-metal contact. Inadequate lubrication will not only cause piston wear and crown and land dragging, but also may cause piston seizure, and piston pin busing wear.
Lack of lubrication is caused either by a lack of lube oil pressure or by restricted oil passages. The pressure-recording instruments usually give warning of low oil pressure before any great harm results. However, clogged passages offer no such warnings, and their discovery depends on the care that is exercised in inspecting and cleaning the piston and connecting rod assembly.
Another controllable factor that may be directly or indirectly responsible for many piston troubles is IMPROPER COOLING WATER TEMPERATURE. If an engine is not operated within the specified temperature limits, lubrication troubles will develop. High cylinder surface temperatures will reduce the viscosity of the oil. As the cylinder lubricant thins, it will run off the surfaces. The resulting lack of lubrication leads to excessive piston and liner wear. However, if temperatures are below those specified for operation, viscosity will be increased, and the oil will not readily reach the parts requiring lubrication.
Oil plays an important role in the cooling of the piston crown. If the oil flow to the underside of the crown is restricted, deposits caused by oxidation of the oil will accumulate, lowering the rate of heat transfer. Therefore, the underside of the piston crown should be thoroughly cleaned whenever pistons are removed While insufficient and uneven cooling may cause ring land failure, excessive temperatures may cause piston seizure; an increase in the rates of oxidation of the oil, resulting in clogged oil passages; or damage to piston pin bushings.