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LO 7-9 Outline diagnostic techniques used for evaluating the condition of a piston ring-to-cylinder wall seal

1. Cylinder Component Service

1.8. Ring-to-Cylinder Wall Seal Testing

LO 7-09: Outline diagnostic techniques used for evaluating the condition of a piston ring-to-cylinder wall seal.

 

Ring-to-Cylinder Wall Seal Testing

Evaluating the piston ring-to-cylinder wall seal is an important indicator of engine life. Poor sealing means the engine is worn out or has internal damage. Three methods can be used to evaluate the condition of piston ring-to-cylinder wall seal:

  • compression testing
  • cylinder leak-down testing
  • crankcase pressure testing

Cylinder Compression Testing
Compression testing is a measure of the maximum pressure of the engine cylinders when the engine is cranking Figure 69. It is rarely used on diesel engines due to the small clearance volume of the cylinders and the prospect of hydraulic locking of the piston with oil when wet testing. Wet testing is a procedure used to check for worn rings. It is performed by squirting oil into the cylinders. If compression pressures significantly increase after adding more cylinder wall lubricant, it usually indicates worn rings. In addition to potentially locking the engine, the other problem with this method is that oil may also ignite during compression testing.

   Fig07_69

FIGURE 07-69: Diesel compression testing gauge with a variety of glow plug and injector adapters.

 

Large variations between compression pressure measurements in an engine’s cylinders at cranking speed can be the result of keystone ring design, which requires high cylinder pressure to help rings seal effectively. Furthermore, it is cumbersome and time consuming to remove an injector to install a compression pressure gauge into a cylinder. Smaller, light-duty (LD) diesel OEMs may recommend the removal of glow plugs to measure cylinder compression pressure. Other methods to evaluate cylinder sealing are more effective. Dynamic cylinder balance testing to identify weak, underperforming cylinders is a better place to start this type of diagnostic work.

 

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