My mate has bought an ns250r of a 1984 vintage, its been sat in a barn for 12 years since being imported..
Anyway, finally got it registered yesterday, and we went out for a spin.
Unfortunately, we got a bit carried away and ended up going a little bit too quick for the bike seeing as it had dont 60 miles in 12 years
well I was flat out on my tdr250 and the ns was still gaining distance on me! I came through a cloud of blue smoke, which smelt rubbery, and when we got back it turned out the bike had locked up 
Quick check, still running, no unusual knocks or noises but not very happy on the rear cylinder...
Barrel off today, seems to have gotten a bit hot in there, the spark plug electrode had melted a little on the tip
sure signs of overheating inside, a little 'dish' in the centre of the piston and a 5mm piece of piston vanished off the crown on the exhaust port side. Plus the usual seizure marks, luckily bore seems good!
Anyway... any ideas why this happened? The bike is bog stock, runs fine, 8000kms on the clock.
Also, where can we get pistons/gaskets from? Dave silver didnt seem to want to help much!
Thanks!
Paul