一机手购买了一台１２０型拖拉机使用几个月后发现发动机没劲压缩不良冒黑烟起动困难机手认定缸筒活塞磨损严重拆开后发现缸筒已磨出台阶活塞和活塞环磨损超过极限于是机手更换了缸筒活塞及活塞环可仅几个月又出现了上述现象于是机手将车开到县... A tractor bought a 120 tractor  after a few months  found the engine stunted  poor compression  black smoke  start difficulties  hand identified cylinder  severe piston wear  disassembled cylinder found to have been worn out The steps  Pistons and piston rings wear out of limits  The hands were replaced by the cylinder  Pistons and piston rings  only a few months there have been the above phenomena  so the hand to drive to the county ...