It's just metal expansion. Metal expand at higher temps and this is normal. The sleeve os made from a brass/chrome and the piston is made from a forged aluminum. The brass sleeve with the inside of the sleeve being chrome expands more then the piston does. Dont be worried with with just over a gallon of fuel through it you have some time till you start to loose compression if you broke in the engine properly. I have over 7 gallons through my one and it's still going good and has a ton of compression. So you still have a while till you start to loose compression to the point where you may consider a rebuild.