Unlike a common rail injector which uses a solenoid to open a valve, a Cummins PT injector is a mechanical pump. The barrel and plunger are the components that create the immense pressure required to inject diesel fuel.

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  The Mechanism: The plunger is driven downward by the camshaft via the rocker arm. As it descends inside the barrel, it traps fuel and forces it through the small orifices in the injector tip at pressures exceeding 20,000 PSI.

  
  


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  Why "Matched": The clearance between the barrel and plunger is measured in millionths of an inch (0.000040" to 0.000080"). This is not a clearance you can measure with a feeler gauge. Manufacturers "hone" each barrel to fit a specific plunger.

  
  


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  The Consequence: You cannot mix and match. If you take a plunger from Injector A and put it into Barrel from Injector B, the clearance will be wrong. Too tight = seizure. Too loose = internal leak and low power.