Well following on my previous post.. I decided to take the cylinder off to check what I had to work with.. ie if the piston and cylinder can be re-used or do I need to replace stuff. Knowing the size of the current piston would help sourcing a replacement. If it was standard I could go to the next bigger size etc. It's pretty easy on this bike to remove the cylinder as you only need to undo the exhaust collar followed by the spark plug lead and the 4 bolts holding the head on.

Well its not good news.. the cylinder and piston are pretty worn with deepish scratches down the side of the cylinder and piston...ouch!

What is worse.. the piston is marked 1.25. This is the size relative to the original bore in mm. The 1st oversize is usually 0.25 followed by 0.50 for 2nd oversize. I would imagine that this is the max as the spigot of the cylinder is already razor thin.. see the area circled in yellow below!

Size marked on the piston.

I'll probably clean it up and re-install the bits while I hunt up for a replacement cylinder or barrel.

Watch this space!