Be absolutely sure that the bolt head is smaller than the hole thru the piston. When you tighten the nut, it will pull the bolt thru and you don't want it to scrape the wall of the piston's hole.
To install the gudgeon, you do the same setup again except without the socket. On the end of the bolt that came thru the socket you put the washer on first and then the nut. Line up everything carefully again and tighten the nut til the gudgeon just passes the the snap ring groove.
Works for me all the time. Just do it all carefully!