Sounds to me like its a stock piston in a punched out sleeve.. Same thing happened to me with my old YZ400, The guy told me nothing about it being .040 over and when I ordered the new piston and everything I ordered a stock size and it was the worst sounding thing ever.. luckily I only paid 100$ for the bike and 40$ for the piston, It lasted a complete summer of hard core ripping on it. Now she is up for her winter rebuild.