K Series pistons page
This page shows what happens to the standard K pistons when they are subjected to higher RPM and BHP, these are form Tor Atles Caterham Superlite 1.6, producing between 190-200BHP, rev limited at 8000RPM. The ring land between the top and second ring has become completely detached , the next stage of disintegration is shown below, the top land breaks away exposing the ring and pressurising the crankcase. The damage is always sustained on the inlet side of the piston where the thermal shock is highest, this side of the piston is cooled by the incoming charge, then heated by combustion, it is also the thrust side of the piston so it takes the highest thrust loads. Further below are some pictures of the Omega forged replacement piston.
Generally speaking the stock pistons is good for around 160BHP and 7200 maximum RPM, even at this level some will fail, a better prospect is the Trophy 160 piston from late 160VVC engines which has wider ring lands and smaller piston pockets, this seems capable of handling 180-190BHP and 7800RPM. These can be identified by the presence of the number 160 engraved on the crown and by the smaller piston pockets. Because of these smaller pockets it is less suitable for 1600 installations where the piston is 1.4mm higher in the bore
When fitted to 1600 engines, Omega pistons need
to have the crowns machined by 30 thou to prevent the piston hitting the head as the crown
will protrude from the liner top considerably.
This damage then progresses up through the top of the piston as shown below.
And here is the forged replacement from Omega, the ring lands still do not look very substantial, but at least the material is much tougher, they sit around 1mm higher in the bore and raise the CR by around 1 ratio, they also have large recesses to clear the valves. These recesses need to be enlarged if the valves you are using are larger than VVC valves (31.5mm on the inlet and 27.3mm on the exhaust).