So after my seize at Nogaro - I was hoping that there would just be a touch due to shutting the throttle at the end of the straight, but hey, no such luck! I took off the exhaust pipes to get a look at each piston, and both rear pistons have suffered. After removing the rear head, both rear pistons have been eaten away on the exhaust side which is a typical sympton of running lean. really annoying since I went to the trouble of doing a plug chop - if I had left it one or two more laps I think I would have seen it on the plug chop - so I live and learn the expensive way.
Both pistons on cylinders 3 & 4 have melted around the exhaust port, on number 4 the molten piston has spread itself over the inside of the cylinder causing the piston to seize against the cylinder. Both pistons are dead, but I think the cylinders are probably OK, after a ream out, however the big issue I have now is sourcing the same type of piston. These were ProX CR125 pistons, and that model no longer exists.
My choices are twofold :- Either I go back to Suzuki pistons, in which case it is probably prudent to change all 4 pistons at once, or I can buy a couple of more recent CR125 pistons, which are different from the old ones (only one ring), and this is probably dodgy as the friction will be different from the front crank to the back, so it will unbalance things.
I will probably go to the Suzuki pistons as I don't have to get them machined on the inside, and at least I will have no doubts about the quality of them. They are also offest to the original specifications, i.e. there is a back set and a front set - so this will bring things back to standard, and in the future I can source my pistons from any Suzuki dealer instead of going through Mark Dent.
In any case I will need to send off my cylinders for a check up, and we will see if a ream will clear out the aluminium debris or whether I need to rebore to the next size up.
On top of that I will need to clean out the crankcase, and up jet (a bit late now!)