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Piston Protrusion

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Piston protrusion is a term that refers to how high a piston crown travels above the cylinder deck. There are several reasons for checking this specification but here we want to identify bent or twisted connecting rods. Pictured is a 6.4L PSD engine that has performance issues and low compression in two cylinders related to a failed common rail fuel injection system. The injectors in this engine have been known to stick open allowing excessive amounts of diesel fuel to enter the combustion chamber which can cause bent or twisted connecting rods that sometimes may not be easy to identify by visual inspection alone.Bent or twisted connecting rods can cause low compression and piston and cylinder wall damage. During the disassembly of any engine it is important to identify the cause of a failure as well as related damage. In this era where it is more common to repair damaged engines with remanufactured assemblies than it is to repair them with parts and make measurements, many technicians no longer acquire the necessary measuring tools. This test however requires a dial indicator which is still a commonly used measuring tool.With the cylinder heads off of the engine the cylinder block deck and the piston crowns need to be cleaned according to the proper procedures outlined in the service manual. For each piston, two measurements will be taken along the wrist pin axis and then averaged to obtain the actual piston protrusion.

Here is the procedure as outlined in the workshop manual:

  • Zero the Dial Indicator Gauge with Holding Fixture on the crankcase deck surface.
  • Position the Dial Indicator Gauge tip on the piston head at the 3 or 9 o'clock position.
  • Rotate the crankshaft to measure the maximum piston protrusion and record.
  • Reposition the Dial Indicator Gauge tip onto the piston head at the opposite position.
  • Rotate the crankshaft to measure the maximum piston protrusion and record.
  • Average the 2 readings.

If the average reading is lower than the specification, the piston is lower in the bore than it should be. This indicates a bent or twisted connecting rod.


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