How to replace piston rings

If your piston rings are compressed, you can try replacing them by using retaining-ring-pliers or snap-ring pliers.

Are you sure you need to replace your piston rings? You should replace your piston rings when there is significant slop between the piston and cylinder, or when you are re-honing the cylinders and putting in an oversized piston, as the distance between the piston and cylinder will change with different diameter pistons.

Since you mentioned that you are working on a lawnmower engine, I would highly suggest reconsidering piston ring replacement unless your cylinder walls are relatively undamaged and your piston rings are compressed.

Most, it not all lawn mower engines produced after ~1988 use silicon impregnated aluminum cylinder walls, making rehoning impossible as the silicon is necessary for proper engine operation.

If you notice significant scoring on the cylinder walls of your engine there is nothing you can do to fix it. Unfortunately, you will have to buy a new engine. A lot of vertical scratches in the cylinder walls of your engine will cause oil leaking problems.

Lawnmower engines used to be cast iron blocks that can be rehoned quite a number of times. Unfortunately, that is not the case anymore. If you notice significant scratches in your cylinder walls, I would suggest engine replacement.

Keep in mind that most lawnmower engines require oil replacement every 6-8 hours. A lot of lawnmower owners never change the oil. The chances of your cylinder walls being heavily scored is very high, and it may be in your best interests to find a replacement engine block or a lightly used lawnmower. Again, please remember to regularly change the oil in your lawn mower. Good luck!