which was up does the piston go?

i am fitting a new cylinder kit to my aerox and have come to putting the new piston in, but am not sure which was up it goes. so does the solid side face upwards or the side with the two whole in?

cheers

 

Theres a little arrow on it... make SURE it points to the pipe, down on a aerox

 

i found it in the manual so all good, the 2 holes had to be at the top. i lost a circlip though attempting to fit one so need to find somewhere to get them now :msn1: