Today I installed a Wossner top end kit into my 2004 CR125 and during the run in procedure after approximately 10-15 minutes it siezed quite badly. I had finished the first break in session which I took a good 5 mins to warm the bike up and then rode very conservatively for a few more minutes and then allowed the bike to cool for a few hours. Upon the second break in session I repeated a good warm up of the bike and rode up to 3rd gear using about 1/3 to 1/2 of the throttle varying the revs up and down when it suddenly locked up.I was careful during the rebuild to check the ring end gap and replace all the gaskets and seals etc and I think I did all of the necessary things for a successful rebuild. The previous piston I installed (a Wossner) had just reached the 30 hour mark and when I pulled it out it still looked in good condition. I have done about 10 top end rebuilds on two strokes and they have all been successful so I was just wondering if anyone had any knowledge as to what the problem might be and where to look. Was the piston too tight for the bore? I ordered one to fit a "B" sized cylinder. The ring is still free and didn't melt into the gap and both piston clips are still intact and in place.The cylinder was replaced about 60 hours ago and the crank was done about 30 hours ago. The jetting has remained unchanged since the last piston and seems to run clean. All other maintenance such as air filters etc is done regularly, and the bike has run fine for quite a while. The cylinder has a bit of pickup from the piston which will need removing somehow but will hopefully be all right and won't need replating (fingers crossed) But as for the piston I'll need to get a new one - Please see attached photos of the worst score mark.Any help would be much appreciated.