Motorcycle Repair: Kawasaki 1989 kdx200, kawasaki 1989, mechanic friend

Question
My husband 's bike wouldnt start while on a riding weeken,  he was told he needed to replace the piston and rings.  He did this with a little help from a mechanic friend and when they started the bike it ran for about 30 second and suddenly wne bang and stopped... When they pulled the barrel appart the top of the piston had a huge hole in it on the side and the top ring was bentupwards.  He took the piston, rings and barrell back to the bike shop and no-one seemed to know what had gone wrong. They sleeved the barrell and gave him another pistons and they fitted the rings.  he attempted to assemble the bike again and sthe sam thin happend again.  Do you have any idea as to what he may be doing wrong or what could be causing this to happen,  its getting quite expensive and my husband is getting quite frustrated.   Thanks   Emajo Barthelson

Answer
been there done that. my problem was a gear 1/4 turn or less off its got to be something stupidly overlooked like that. the timing of the movement it out of whack so go back and retrace every thing and before you actually put electronical starting power to it toyou roll itv through by hand slowly abnd watch how everything is moving and if its hitting
also see that you dopnt have too long of s park plug in there