Ausindo172P
The piston and ring damage you are seeing is almost certainly related to detonation ( a serious condition that will result in engine failure if left unchecked) within the cylinder caused by wrong octane levels or ethanol inclusion in the fuel. I am guessing these are not the same 172s you had in the 80s hence probably different engine models too (if these are higher horsepower engines = greater chance of detonation).
If you wish to continue operating auto fuel ( and you should be aware that ALL aircraft engine makers advise against this and hence insurance/the courts may not cover you in the event of failure) you should arrange a stable supply of the highest octane completely ethanol free fuel you can find.
This may require you going direct tot he wholesaler and or refinery to achieve supply and testing your own samples (relatively inexpensive ethanol test can be carried out by settling out a sample. Particularly if the supplier can not be trusted not make a quick buck by slipping some ethanol into the fuel.
Good luck but please be aware of the risk detonation poses to your aircraft.