Based on a bit of googling, youtubing and responses on another forum, it seems the arrow points (down) to the exhaust
port, and "UP" goes right-way-up at the top. This keeps the ring ends away from the exhaust
The interesting thing is that the piston was the other way round when I stripped it down, which might explain the broken ring and piston that got me to this point. I bought this motor
in close to new condition, and I believe it had not been stripped down at that point. So maybe this is one of Mr Yamaha's rare goofs.