Hi Joe,
When the piston ring is pinned on the center of the intake it can run on a bridge or in the open central port as long as the port has smooth edges. There is much less pressure on the ring at the bottom of the stroke because the cylinder pressure is down to atmospheric and cool unburnt mixture gasses spray past the ring ends and piston pin to stave off thermal stress.
Wiseco has offered the centered-pin feature to custom piston distributors since 1990. I've used it on my custom designs of 100,144,300cc pistons since 1994 and theres no abnormal failure rate. I've seen way more failures on pistons with off-set, especially like the type on late model 125s where the rear transfer ports are so wide that the ring rides the vertical edge of the port. That puts a lot of stress on the pin as the ring rattles from side to side. Typically the ring spin failure etches a rectangle pattern on the back sides of the rear transfer ports, it sort of looks like a chamfer pattern but its deeper and sharp. Its also common when the ports are widened too much with porting.
The Japanese manufacturers are already moving on updates to the OEM piston and cylinder design to include the centered ring design. Its standard on the 2002 YZ & RM125s
Happy 2002:)