This paper gives details of the lives of 188 sets of filled P.T.F.E. piston rings in a variety of reciprocating compressors covering a wide range of operating conditions. Although satisfactory performance has been obtained in most cases, there has been a considerable experience of inconsistent behavior. This had led to the adoption of certain standardized practices based on the experience obtained so far in an attempt to increase the consistency of performance. In particular, these cover ring formulations and design, and the avoidance of trace quantities of lubricant.