On a cold day the piston expands but the barrel less so. On japanese trail bikes the clearance was a touch extra to allow for cold seizures. For instance when one goes through a puddle the barrel can shrink onto the piston. Normally with a light seizure one can rub off the high spots with fine emery and clear any jamming ring lands and aluminium from the barrel and, after washing, put the piston back in [with a new circlip if one has disconnected the piston from the rod]

Hi thanks for your input chaps, I will check all the items meantioned by you all soon. Are you into trail riding Jez? I have a Honda crf 230

Yeh I've got a few twinshock DR400s, a couple of twinshock KLX 250s
An Ossa MAR frame with Honda 185 engine and running gear, and a Dick Mann framed TT540 mark 4, possibly the only running one in existence. Oh and a Series One 80 inch Landrover and an M21 and the recently acquired 16H. Most of the stuff runs, some of it is even road legal. Live in the welsh marches and occasionally compete, at a very low level.