If the new ring is oval, place the large part of the ring to the bottom of the cylinder. I recommend that you order and replace the cylinder and the piston ring. We offer a kit that has both the parts. The part number for the kit is K-0650. When installing the new ring make sure it is seated onto the top of the piston and use a thread lock on the screw of the piston. I have also included an image of how to properly install the counter balance.
The parts needed can be ordered from the link.
Sears Parts Direct
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services