know the types of Piston ring clearance

We have three types of clearance for piston rings.
  1. Butt clearance. 
  2. Axial clearance.
  3. Back clearance or radial clearance.

1. Butt clearance :
  • Is also known as ring end clearance or gap clearance. 
  • It is the distance between connecting faces of piston ring. ( or clearance between the end butt of the ring inside the liner.)

  • This clearance should be within the limits specified by the engine manufacturer.
  • The ring expands when the engine is hot. If the clearance is less than the minimum limit, ends touch due to this thermal expansion.
  • The ring expands against the liner. This increases the friction resulting in breakage of ring and Seizure of piston.
  • If the clearance is greater than the maximum limit, combustion gases blow by the rings and exhaust gases enter the crankcase.
  • This results in loss of compression pressure in the cylinder.
  • To measure butt clearance, insert the rings in the bottom of the liner and ensure that they are in line with the liner axis.
  • Measure the clearance between the faces of the piston ring using a feeler gauge.
  • Record two sets of readings, forward aft and port starboard.
2. Axial clearance 
  • Is a clearance from the top of the ring to the top of ring groove.
  • The combustion gases exert pressure behind the piston rings and push towards the liner for proper sealing.
  • If the clearance is less than the minimum limit, combustion gases cannot flow behind the rings.
  • To measure axial clearance, fit the ring in the respective groove such that it rests on lower surface of the groove.
  • Measure the clearance between piston ring and upper surface of the groove using a feeler gauge.
  • Record two sets of reading, forward aft and port starboard.
3. Back clearance
  • Is the clearance from inner diameter of the piston ring to inside diameter of the groove.
  • Thickness of the ring should be less than groove depth so that the outer ring face is inside the ring groove. 
  • If there is no back clearance, the rings take the side thrust. This results in high friction and possible piston seizure.
  • To measure back clearance, measure the depth of the piston ring groove by means of the depth gauge.
  • Take both these measurements in at least 6 different positions around the groove circumference.
  • The difference between the two readings will give the back clearance.
*Back clearance = Ring groove depth - Ring thickness.

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