Ring Groove Wear/Side Clearance Check
The top ring groove wears the most. Rings with excessive side clearance can break. Also, the old ring wears into the groove. New rings cannot seal against a worn ring groove.
The cast iron piston ring often wears more than the aluminum piston.
- Before cleaning the piston, check the top ring groove for excessive wear.
Checking a ring groove for wear. Courtesy of General Motors Corporation, Service Technology Group.
- Normal ring-to-groove side clearance is from 0.002" to 0.004".
- A new ring is placed in the groove. if a 0.006" feeler gauge can be inserted under it, the groove is worn excessively.
- This check also ensures that a ring of the proper width and depth is used.
- A worn ring can be used for this test if the ring is held in the groove.
- Only part of the ring wears so the unworn part is good enough for performing this measurement.
When ring wear is evident, it is found on part of the bottom edge.
Engines that use low-tension rings usually clean up at 0.020" oversize when boring. They do not wear the cylinder wall as much as standard tension piston rings because they do not push as hard against it.
Measuring a Piston
The place to measure a piston varies among manufacturers, so check the service manual. The figure below shows where most manufacturers recommend a piston skirt be measured.
Measuring maximum piston skirt diameter. Courtesy of General Motors Corporation, Service Technology Group.
To compensate for worn piston skirts and cylinder walls, piston skirt diameter can be enlarged by knurling.