In terms of Manufacture:

1.The quality of the cylinder is not good or the surface is rough;

2.The connecting rod and bent axle is not vertical;

3.The piston center and the end face is not vertical;

4.The ring groove of the piston is askew;

5.The piston ring’s elasticity is too big pr the surface’s rigidity is too high;

6.The piston seat center amd the piston center is not vertical;

7.The end gap of the bent axle  is too big;

8.The piston ring’s open amounts at work is too small;

9.The gap between the piston and the cylinder is too small.