[Jac Mac]

I wonder if the rotation might occur mainly in the warm-up phase of operating the motor, when the piston crown has not expanded to its full diameter at operating temp, during this phase with the flame front on many motors tending to have a toroidal/circular pattern due to the spark plug placement & chamber design the gas pressure increase must tend to flow around the piston top land area therefore inducing ring rotation rather than just over the edge of the piston crown.